cli
What is the Node.js Command-line API (CLI)?
The Node.js CLI is a set of commands you can run in the terminal or command prompt to:
Debug programs
Execute scripts
Configure runtime settings
How to use the Node.js CLI:
Open your terminal or command prompt and type node.
Basic Commands:
node my-script.js: Executes a JavaScript file named
my-script.js.node --version: Displays the Node.js version.
node --help: Lists all available CLI commands.
Debugging Options:
node --inspect: Starts the Node.js debugger, allowing you to step through code.
node --trace-warnings: Prints warnings as they occur.
node --trace-deprecation: Prints deprecation warnings.
Multiple Script Execution:
node -e "console.log('Hello World!')": Executes JavaScript code inline.
node first.js second.js third.js: Executes multiple JavaScript files in sequence.
Runtime Options:
node --max-memory=512: Sets the maximum memory usage to 512 MB.
node --trace-events-enabled: Enables event tracing for debugging.
Real-World Applications:
Debugging Large Applications: Use the debugger to pinpoint errors in complex code.
Automating Tasks: Create scripts that can be executed from the command line, automating repetitive tasks.
Customizing Runtime Settings: Optimize performance or enable advanced debugging features by modifying runtime options.
Example Code:
Node.js CLI
Synopsis:
Options:
1. Entry Point:
Specifies the JavaScript file or script to run.
Can be a file path, a module name, or an inline script (using
-e).
Example:
2. Arguments:
Parameters to be passed to the script.
Similar to command-line arguments in other programs.
Example:
V8 Options:
Specifically for fine-tuning the V8 JavaScript engine.
Not typically used by beginners.
Other Options:
1. Inspect:
Starts a debugger session that allows you to inspect the running script.
Useful for debugging and troubleshooting.
Example:
2. Help:
Displays a list of available options.
Example:
Real-World Applications:
1. Running Programs:
Write JavaScript programs and execute them from the command line.
2. Interactive REPL:
Use the Node.js REPL as an interactive shell for quick prototyping.
3. Debugging:
Use the
--inspectoption to debug scripts and identify errors.
4. Automation:
Create scripts to automate tasks such as file processing, data manipulation, etc.
5. Development Tools:
Use Node.js CLI as a core component in development tools like Electron and Node-RED.
Program Entry Point
In Node.js, when you run a script, the first thing that happens is the program entry point is determined. This is the starting point of your script.
Resolving the Entry Point
The program entry point is specified when you run the script. It can be an absolute path (e.g., /home/user/my-script.js) or a relative path (e.g., ./my-script.js). If you don't specify the full path, Node.js will look for the file in the current working directory.
Module Loaders
Once the entry point is found, Node.js uses a module loader to load the script. There are two main module loaders in Node.js:
CommonJS Module Loader: Used for scripts that follow the Node.js CommonJS module system (e.g.,
require()andmodule.exports).ES Module Loader: Used for scripts that follow the ECMAScript (ES) module system (e.g.,
importandexport).
Choosing a Module Loader
The module loader used is determined by several factors:
Command-line flags (e.g.,
--import)File extension (e.g.,
.mjsfor ES modules)Presence of a
"type": "module"field in thepackage.jsonfile of the parent directory
Loading Script
After the module loader is selected, it loads the script. If the script is an ES module, the ES module loader is used. Otherwise, the CommonJS module loader is used.
Real-World Applications
CommonJS Modules: Widely used in traditional Node.js applications, especially for interacting with native Node.js APIs.
ES Modules: Modern approach for writing JavaScript modules, offering better encapsulation and improved performance.
Example
To run a script using the CommonJS module loader:
To run a script using the ES module loader:
ECMAScript Modules Loader Entry Point Caveat
When loading a program, the ES module loader (used in Node.js) has specific requirements for the file extension of the entry point:
Files with
.js,.mjs, or.cjsextensions are accepted.Files with
.wasmextensions are accepted when the--experimental-wasm-modulesflag is enabled.Files with no extension are accepted when the
--experimental-default-type=moduleflag is passed.
Here's a simplified explanation:
ES module loader: A tool that loads and executes JavaScript modules in Node.js.
Entry point: The main file that is executed when running a Node.js program.
File extension: The suffix at the end of a file name (.js, .mjs, etc.).
Real-World Examples:
Potential Applications:
Modular Code: Breaking down code into smaller, reusable modules (e.g., entry-point.js, utils.js, etc.).
Code Organization: Allowing different types of modules (e.g., JavaScript, WebAssembly) to be used in the same program.
Performance Optimizations: Using
.mjsfiles can enable certain performance optimizations in V8 (the JavaScript engine that Node.js uses).
Options in Node.js CLI
1. Option Format
You can use either dashes (-) or underscores (_) to separate words in option names. Both --pending-deprecation and --pending_deprecation refer to the same option.
2. Single-Value Options
If an option expects only one value (like --max-http-header-size), you can specify it multiple times. However, the last value provided will be used.
3. Precedence
Options specified on the command line (e.g., node --option value) take priority over options set through the NODE_OPTIONS environment variable.
Example:
Let's set the --max-http-header-size option both on the command line and in the NODE_OPTIONS variable:
In this case, the value set on the command line (8192) will be used because it has higher precedence.
Real-World Applications:
Setting HTTP header size: You can adjust the maximum size of HTTP headers sent by your Node.js application to improve performance or meet specific security requirements.
Enabling debugging features: Options like
--inspectand--inspect-brkallow you to debug Node.js applications remotely or break into the debugger on startup.Fine-tuning performance: Various options, such as
--max-old-space-sizeand--expose-gc, provide granular control over memory allocation and garbage collection behavior, which can impact the performance of your application.
Simplified Explanation:
- Option
The - option is like a shortcut to tell a Node.js script to read your script from the command line instead of a file. It's similar to how you use - (dash) in other tools like cat or grep.
Detailed Explanation:
The - option has the following functionality:
Stdin: It specifies that the script should be read from the standard input (stdin). Stdin is a stream of data that can be typed in from the command line or piped from another command.
Alias for stdin: Essentially,
-is an alias for stdin. When you use-, it's like you're saying, "Hey, Node.js script, read this script from my keyboard or wherever I'm typing it."Passing Options: After
-, you can still pass options to the script just like you would if you were specifying a file. These options will be passed to the script when it's read from stdin.
Real-World Example:
Let's say you have a script called my-script.js:
You can run this script using - in different ways:
Interactive Mode:
Type
node -at the command line.Paste the script (copy the code above) into the command line and press Enter.
You should see "Hello World!" printed to the console.
Piped Mode:
Run another command that generates output, such as
echo "Hello World!".Pipe the output of this command to your script using
-:
You should still see "Hello World!" printed to the console.
Potential Applications:
Interactive Scripting: Use
-for quick prototyping or testing scripts in an interactive way.Piping Output: Combine multiple commands by piping their output to a script using
-.Unit Testing: Simplify unit testing by passing test data to a script using stdin.
--: End of Node Options
Simplified Explanation:
When you run a Node.js script, you can pass options to the Node.js command (node). To indicate that you're done specifying options and want to start the script, use the -- (double dash).
In-depth Explanation:
Node.js options start with a single dash, for example:
When you pass the -- option, it tells Node.js that you've finished specifying options and want to start running the script. Anything after the -- will be treated as part of the script.
Real-World Example:
Let's say you have a Node.js script called my-script.js. You can run this script with options like this:
This command increases the memory limit for the script to 8192 MB.
Potential Applications:
Using -- is useful when you want to pass options to Node.js, but also need to pass arguments to the script. For example, you might use -- to specify debugging options, while passing additional command-line arguments to the script.
Improved Code Snippet:
When you run this script with options and arguments, you can see the -- separator in the process.argv array:
Output:
--abort-on-uncaught-exception Flag
Imagine your JavaScript program is like a car racing on a track. Normally, if the car crashes (encounters an error it doesn't handle), it will just stop racing and exit the track.
However, when you use the --abort-on-uncaught-exception flag, it's like you're putting a safety net under the track. If the car crashes, instead of just stopping, it will slam into the net, causing the entire track (your program) to crumble.
This is useful for debugging because it creates a "core file," which contains information about the crash that you can use with tools like lldb, gdb, or mdb to investigate what went wrong.
Example:
In this example, if the app.js program encounters an unhandled exception, Node.js will create a core file for debugging purposes.
Potential Applications:
Using the --abort-on-uncaught-exception flag can be helpful for:
Finding bugs: The core file can help you identify the specific line of code that caused the crash and debug the issue.
Testing stability: You can use this flag to check how your program handles unexpected errors and make it more robust.
Security analysis: The core file can also be used by security researchers to analyze potential vulnerabilities in your program.
What is the --allow-addons flag?
When you use Node.js with the "Permission Model" enabled, by default your Node.js process cannot use native addons.
Native addons are code libraries that are written in a different language (like C++) and compiled into a binary file that can be loaded by Node.js. They are often used to extend the functionality of Node.js, for example to access hardware devices or to perform complex calculations.
If you try to use a native addon in a Node.js process with the Permission Model enabled, you will get an error message saying "Cannot load native addon because loading addons is disabled."
The --allow-addons flag tells Node.js to allow the process to use native addons, even if the Permission Model is enabled.
Why would you use the --allow-addons flag?
There are several reasons why you might want to use the --allow-addons flag:
You need to use a native addon in your Node.js process.
You are developing a native addon and you need to test it in a Node.js process with the Permission Model enabled.
You are running a Node.js application in a production environment where the Permission Model is enabled, but you need to use a native addon for performance or other reasons.
How to use the --allow-addons flag
To use the --allow-addons flag, simply pass it to the Node.js command when you start your process:
Real-world example
One real-world example of where you might use the --allow-addons flag is if you are using a Node.js application to control a hardware device. You might need to use a native addon to access the device's low-level hardware interface. In this case, you would pass the --allow-addons flag to the Node.js command when you start your process.
Potential applications
The --allow-addons flag can be used in a variety of real-world applications, including:
Developing and testing native addons
Running Node.js applications in production environments where the Permission Model is enabled
Accessing hardware devices from Node.js
Performing complex calculations in Node.js
--allow-child-process
--allow-child-processPurpose:
By default, in Node.js's Permission Model, a process cannot create child processes without explicit permission. This flag allows you to override this restriction.
How it Works:
When you start Node.js with the --allow-child-process flag, the process is granted the ability to create child processes. This means that any child processes created by this process will not be restricted by the Permission Model.
Example:
Run without --allow-child-process:
Run with --allow-child-process:
Real-World Applications:
Spawning child processes: Launching external programs or scripts from your Node.js application.
Piping data: Connecting the output of one process to the input of another process.
Concurrent tasks: Running multiple tasks simultaneously by creating multiple child processes.
Parallel computations: Distributing computational tasks across multiple cores or machines using child processes.
--allow-fs-read Flag in Node.js
What is it?
This flag allows your Node.js program to read files from the file system.
How does it work?
Node.js has a security feature called the Permission Model that restricts access to certain resources like the file system. By default, your program cannot read files unless you explicitly grant it permission.
The --allow-fs-read flag tells Node.js to allow your program to read files from a specific path.
How to use it:
To use the --allow-fs-read flag, simply pass it to your Node.js script followed by the path to the directory or file you want to read.
For example, to allow your program to read files from the /tmp directory, you would run:
Multiple Paths:
If you need to allow your program to read files from multiple paths, you can specify them using multiple --allow-fs-read flags.
For example, to allow your program to read files from both the /tmp and /var/tmp directories, you would run:
Security Considerations:
Always remember that granting permissions to your program can have security implications. Only grant permissions that are absolutely necessary for your program to function.
Real-World Applications:
The --allow-fs-read flag can be used in a variety of real-world applications, such as:
Reading user data from a file
Processing files for data analysis
Copying files from one location to another
Creating backup archives of files
Improved Example:
Here is an improved version of the example from the original documentation:
--allow-fs-write flag:
Explanation:
This flag lets you control which files and directories your Node.js script can write to on your computer's file system.
How it works:
When you run your script, Node.js asks the operating system for permission to write to certain files or directories. By default, Node.js has limited permissions and can only write to certain default locations. If you want your script to write to other files or directories, you need to grant it permission using the --allow-fs-write flag.
Arguments:
*: Allows your script to write to any file or directory on your computer. This is the most permissive setting and should be used with caution./path/to/directory: Allows your script to write to the specified directory and any subdirectories within it./path/to/file: Allows your script to write to the specified file only.
Examples:
Real-world applications:
Writing logs to a file
Generating reports that are stored on the file system
Creating or updating configuration files
Saving user preferences
Potential applications in the real world:
A server application that generates daily logs and saves them to a file on the server's file system.
A desktop application that allows users to create and save text files.
A build tool that creates and updates configuration files based on user input.
Summary
The --allow-worker flag allows you to create worker threads in a Node.js process that is using the Permission Model. By default, worker threads are not allowed due to security concerns.
How it Works
When you use the Permission Model, you can restrict access to certain APIs and features in your Node.js process. This includes the ability to create worker threads. To create a worker thread, you must explicitly pass the --allow-worker flag when starting your Node.js process.
Example
Here's an example of how to create a worker thread with the --allow-worker flag:
Code Snippet
Real-World Applications
Worker threads can be used to perform tasks in parallel, which can improve the performance of your Node.js application. For example, you could use worker threads to process data, perform calculations, or handle I/O operations.
Potential Applications
Image processing
Video encoding
Data mining
Scientific simulations
Financial modeling
Simplified Explanation
Imagine your Node.js process as a house. By default, the house has a strict security system that prevents you from opening any windows or doors (creating worker threads). However, the --allow-worker flag gives you a special key that unlocks the door, allowing you to open it and create worker threads.
Building Snapshots
What is a snapshot?
A snapshot is a file that contains the current state of your Node.js application. It's like a "frozen" copy of your application at a specific point in time.
Why use snapshots?
Snapshots can be used to:
Speed up application startup time: When you load your application from a snapshot, it will start up much faster than if you were to load it from source code.
Reduce memory usage: Snapshots can help to reduce memory usage by sharing common code between multiple instances of your application.
Improve reliability: Snapshots can help to improve reliability by ensuring that your application always starts up in a consistent state.
How to build a snapshot
To build a snapshot, you can use the --build-snapshot flag when you run your Node.js application. For example:
This will generate a snapshot file called snapshot.blob.
Loading a snapshot
To load your application from a snapshot, you can use the --snapshot-blob flag when you run your Node.js application. For example:
This will load your application from the snapshot.blob file.
Real-world applications
Snapshots can be useful in a variety of real-world applications, such as:
Web servers: Snapshots can be used to speed up the startup time of web servers. This can be especially useful for servers that are used to serve a large number of requests.
Microservices: Snapshots can be used to reduce the memory usage of microservices. This can be especially useful for microservices that are deployed on a limited amount of hardware.
Serverless functions: Snapshots can be used to speed up the startup time of serverless functions. This can be especially useful for functions that are used to handle a large number of events.
Potential applications
Here are some potential applications for snapshots:
Creating a read-only cache of data: You can use a snapshot to create a read-only cache of data that is frequently accessed by your application. This can improve the performance of your application by reducing the number of times that it needs to load the data from disk.
Creating a backup of your application: You can use a snapshot to create a backup of your application. This can be useful in case your application is damaged or lost.
Distributing your application: You can use a snapshot to distribute your application to other users. This can be useful if you want to share your application with others, or if you want to deploy your application to a server.
Specifying Snapshot Configuration
Imagine your code as a blueprint for constructing a building. A snapshot is like a pre-assembled version of this building, which can be deployed quickly and easily. However, sometimes you need specific adjustments before that blueprint can be used.
The --build-snapshot-config option lets you tweak these adjustments through a special configuration file:
Builder: Acts like a foreman, directing the assembly of the building (snapshot). It's a script name that tells the system how to prepare the code.
WithoutCodeCache: Decides whether to include a "code cache" in the snapshot. This cache makes future assemblies faster but also makes the snapshot larger and harder to move to other systems.
Example Configuration File:
Usage:
Run the command:
Real-World Applications:
Code Optimization: Use the builder script to optimize the code before creating the snapshot, resulting in faster execution.
Selective Code Cache: Exclude the code cache to reduce snapshot size, making it more portable to different environments.
Preprocessing: Preprocess your code using the builder script to make it easier to debug or maintain.
-c, --check
Simplified Explanation:
It's like a grammar checker for your JavaScript code. It checks for syntax errors without actually running the code.
Detailed Explanation:
Syntax errors are when there's something wrong with how your code is written, like missing brackets or incorrect spelling. Syntax check mode lets you find these errors before running your code, saving you time and frustration.
Code Example:
Real-World Applications:
Catching syntax errors early: Detect and fix errors right away, preventing your code from crashing.
Improving code quality: Ensure your code is readable and error-free before sharing it with others.
Automating code checks: Integrate syntax checking into your build process to automatically detect errors.
What is Bash Completion?
When you type a command in your terminal, your shell tries to complete the command automatically based on what you've already typed. Bash completion is a feature that helps you complete Node.js commands more easily.
How to Enable Bash Completion for Node.js
To enable bash completion for Node.js, run the following command:
This will create a file called node_bash_completion in your current directory. Add the following line to your .bashrc file:
This will load the bash completion script every time you open a new terminal window.
How to Use Bash Completion for Node.js
Once you have bash completion enabled, you can start using it to complete Node.js commands. For example, if you type the following command:
Your terminal will automatically complete the command to:
This can save you a lot of time when you're typing long or complex commands.
Real-World Applications
Bash completion can be used in a variety of real-world applications. For example, you can use it to:
Quickly complete long or complex commands
Discover new Node.js commands and options
Automate tasks that require repetitive command typing
Code Example
Here is a complete code example that shows how to use bash completion for Node.js:
This example will enable bash completion for Node.js and load the completion script every time you open a new terminal window.
-C condition, --conditions=condition
This option allows you to enable experimental support for custom conditions when resolving exports in Node.js modules.
---
Simplified Explanation:
Imagine you have a special condition called "development" that you want to use to load different versions of your module depending on whether you're running in development or production mode.
---
Detailed Explanation:
Conditions: Conditions are like filters that determine which versions of a module to load based on specific criteria. For example, the "development" condition might tell Node.js to load the development version of your module if you're running in development mode.
Custom Conditions: By default, Node.js has predefined conditions like "node" and "import," but you can create your own custom conditions.
Enable Custom Conditions: To enable your custom conditions, use the
-Cor--conditionsflag followed by the name of your condition.Examples:
---
Real-World Applications:
Development/Production Modes: You can use custom conditions to load different versions of your module depending on whether you're developing or running in production.
Feature Flags: You can use custom conditions to enable or disable specific features in your module based on runtime conditions.
Custom Build Configurations: You can create custom conditions to build different versions of your module for different platforms or environments.
--cpu-prof
--cpu-profSimplified Explanation:
This option tells Node.js to start collecting data about how your program is using the computer's processor (CPU) when you run it. It saves this information to a file on your computer so that you can later analyze it to see where your program is spending its time.
Potential Applications:
Identifying performance bottlenecks in your program
Optimizing your program to run faster
Debugging errors related to CPU usage
Real World Complete Code Implementation:
The following command starts collecting CPU profile data and writes it to a file named my-profile.cpuprofile in the current directory:
Improved Code Snippet:
This code snippet uses the util.profile() function to start and stop the CPU profiler. The profile object contains the collected profile data, which you can then print to the console or save to a file.
--cpu-prof-dir
--cpu-prof-dirSimplified Explanation:
Tell Node.js where to save the files that track your application's CPU usage.
Detailed Explanation:
The --cpu-prof command-line option lets you generate CPU profiles, which show how your application is using the CPU. By default, these profiles are saved in the directory specified by the --diagnostic-dir option.
However, you can use the --cpu-prof-dir option to specify a different directory where the CPU profiles will be saved. This can be useful if you want to organize your CPU profiles or keep them separate from other diagnostic data.
Code Example:
This command will run the my_script.js script and generate CPU profiles. The profiles will be saved in the my_cpu_profiles directory.
Real-World Applications:
Performance optimization: CPU profiles can help you identify bottlenecks in your application's CPU usage. This information can then be used to improve the performance of your application.
Debugging: CPU profiles can help you understand how your application is using the CPU, which can be useful for debugging performance issues or memory leaks.
--cpu-prof-interval
--cpu-prof-intervalSimplified Explanation:
When you use --cpu-prof in Node.js, it generates a CPU profile that shows how your code is spending time. By default, the profile is taken every 1000 microseconds (a microsecond is one millionth of a second). Using --cpu-prof-interval, you can change this interval, specifying how often you want the profile to be taken.
Code Example:
This command will generate a CPU profile every 2000 microseconds while running my-script.js.
Real-World Applications:
Performance Optimization: By examining the CPU profile, you can identify bottlenecks in your code and optimize it for better performance.
Debugging: When encountering performance issues, the CPU profile can help pinpoint the specific areas causing slowdowns, enabling faster debugging.
Note:
--cpu-prof-interval is an experimental feature and may change in future versions of Node.js.
--cpu-prof-name
--cpu-prof-nameStability: 1 - Experimental
Allows you to specify the file name of the CPU profile generated by --cpu-prof.
Simplified Explanation:
--cpu-prof-name lets you choose the name and location of the file where the CPU profile information will be saved.
Syntax:
Example:
This will generate a CPU profile named my-cpu-profile.cpuprofile.
Real-World Application:
Performance profiling: You can use
--cpu-prof-nameto save CPU profiling data to a specific location for further analysis and optimization.
--diagnostic-dir=directory
--diagnostic-dir=directorySimplified Explanation:
You can use this option to specify a specific folder where Node.js will save any files containing diagnostic information (like performance data, warnings, and error logs).
Default Behavior:
If you don't use this option, Node.js will save these files in the current folder where you're running the command.
Usage:
To use this option, simply specify the path to the desired folder after the --diagnostic-dir flag. For example:
Example:
Let's say you want to save diagnostic output from a script called my-script.js to a folder called diagnostics on your desktop. You would use the following command:
Applications:
Debugging: You can use the diagnostic output files to troubleshoot issues with your Node.js applications.
Performance Monitoring: You can analyze the CPU and heap profile files to identify performance bottlenecks.
Error Logging: You can redirect warnings and errors to a specific file for easier tracking and analysis.
Disable Specific Process Warnings Using --disable-warning=code-or-type
Explanation:
When you run a Node.js program, the process may emit warnings to notify you of potential issues. You can disable specific warnings by using the --disable-warning command-line option.
How to Use:
To disable a warning, specify its code or type after the --disable-warning option. For example:
This will disable the DEP0025 deprecation warning.
Warning Codes and Types:
Warnings have specific codes and types. You can find a list of Node.js core warning codes and types here.
Examples:
Disable
DEP0025warning:
Disable Experimental Warnings:
Potential Applications:
Silencing outdated warnings in legacy code.
Temporarily suppressing warnings while debugging to focus on other problems.
Customizing warning behavior for specific scripts or applications.
--disable-proto
Purpose: Disables access to the Object.prototype.__proto__ property in JavaScript.
Explanation:
In JavaScript, every object has a hidden property called __proto__ that points to its prototype object. The prototype object defines default properties and methods that are inherited by the object.
However, __proto__ can be modified, which can lead to unexpected behavior. This option allows you to disable access to __proto__ to prevent such issues.
Modes:
delete: Deletes the__proto__property from all objects.throw: Throws an error (with codeERR_PROTO_ACCESS) when accessing__proto__.
Code Snippet:
Real-World Applications:
Security: Disabling
__proto__can help prevent malicious code from modifying object prototypes and compromising your application.Code Maintenance: Enforces consistent object structures by preventing accidental modifications to prototypes.
Performance: Deleting
__proto__can slightly improve performance by removing the need to check it for every object property access.
Example:
Overview:
--disallow-code-generation-from-strings is a Node.js flag that prevents certain built-in functions from generating code from strings.
Simplifying the Content:
Purpose:
This flag makes it safer to run Node.js code by preventing malicious code from being executed.
Affected Functions:
eval(): Evaluates a string as JavaScript code and runs it.new Function(): Creates a new function object from a string of code.
Behavior with the Flag:
When the flag is enabled, these functions will throw an exception if they try to generate code from a string.
Example Use:
Real-World Applications:
Securing server-side code: Prevents hackers from injecting malicious code into Node.js applications.
Enhancing code reliability: Reduces the risk of unexpected errors caused by code generated from strings.
Enforcing security policies: Enforces a policy to prevent code generation from strings, improving security posture.
dns-result-order
Plain English Explanation
The dns-result-order option allows you to control the order in which DNS lookup results are returned.
Technical Explanation
When performing a DNS lookup, multiple results may be returned. For example, if you look up the domain example.com, you may get an IPv4 address and an IPv6 address.
The dns-result-order option specifies whether the results should be returned in IPv4-first order or verbatim order.
IPv4-first: The results will be returned in IPv4 order first, followed by IPv6 results.
Verbatim: The results will be returned in the order they were received from the DNS server.
Code Snippet
Real-World Applications
The dns-result-order option can be useful in a number of situations, such as:
Prioritizing IPv4 results: If you want to make sure that IPv4 results are returned first, even if IPv6 results are available, you can use the
ipv4firstoption.Preserving result order: If you need to ensure that the results are returned in the same order they were received from the DNS server, you can use the
verbatimoption.
--enable-fips
--enable-fipsSimplified Explanation:
This option tells Node.js to use special encryption methods that meet a government standard called FIPS (Federal Information Processing Standard) when it starts up. This makes Node.js more secure, but it requires Node.js to be built with a special version of OpenSSL that supports FIPS.
Detailed Explanation:
FIPS is a set of standards for encryption that are used by the U.S. government and other organizations. These standards are designed to protect sensitive information from unauthorized access. When you enable FIPS in Node.js, it will use these special encryption methods to protect data that is sent and received over the network.
Code Snippet:
Real-World Applications:
FIPS-compliant encryption is often used in applications that handle sensitive data, such as financial transactions, medical records, and government secrets. By enabling FIPS in Node.js, you can help protect your data from unauthorized access.
Potential Applications:
Financial institutions: Banks and other financial institutions can use FIPS to protect customer data, such as account numbers, balances, and transaction history.
Healthcare providers: Hospitals and other healthcare providers can use FIPS to protect patient data, such as medical records, prescriptions, and insurance information.
Government agencies: Government agencies can use FIPS to protect sensitive information, such as classified documents, intelligence reports, and personal information of government employees.
--enable-network-family-autoselection
Explanation:
By default, Node.js automatically selects the best network protocol (IPv4 or IPv6) to use based on the system's configuration and the network connections available. However, you can disable this behavior using connection options.
This flag enables the network family autoselection algorithm, even if the connection options explicitly disable it.
Example:
In this example, the server will listen on all IPv4 addresses (0.0.0.0) and port 8080. The enableNetworkFamilyAutoselection flag ensures that the server will use the best network protocol (IPv4 or IPv6) based on the system's configuration and network connections available.
Real-World Applications:
Network family autoselection can be useful in environments where both IPv4 and IPv6 are supported. It allows applications to handle connections from both types of addresses without having to manually specify the network protocol. This can simplify development and improve performance by ensuring that the most efficient protocol is used for each connection.
Imagine you're writing code in JavaScript, but you're using a special tool called a transpiler, like TypeScript. This tool takes your code and converts it into a different form that's easier for computers to understand.
One problem with using a transpiler is that when there's an error in your code, the error message shows the location of the transpiled code, not the original code you wrote.
To fix this, you can use the --enable-source-maps flag when you run your code. This flag tells the tool to keep track of the mapping between the original code and the transpiled code. When an error occurs, it will use this mapping to show you the error location in the original code.
Here's an example:
When you run this code without --enable-source-maps, the error message will show the location in the transpiled code:
With --enable-source-maps, the error message will show the location in the original code:
This makes it much easier to debug errors, because you can see exactly where the problem occurred in your original code.
Potential applications include:
Debugging errors in code that has been transpiled
Improving the accuracy of error messages
Making it easier to develop and maintain code
--env-file=config
--env-file=configSimplified Explanation:
What it does: Loads environment variables from a file into the Node.js process.
How it works:
Reads a file with key-value pairs of environment variable names and values.
Adds these variables to the
process.envobject, which is accessible to your Node.js applications.
Benefits:
Keeps your environment variables separate from your code, making them easier to manage and share.
Allows you to easily set different environment variables for different environments (e.g., development, production).
Real-World Example:
file.txt:
JavaScript Code:
Potential Applications:
Managing configuration settings for different environments.
Storing sensitive information, such as API keys and passwords.
Providing default values for environment variables that are not explicitly set.
-e, --eval "script"
-e, --eval "script"Explanation:
The -e or --eval option allows you to execute a JavaScript code snippet directly from the command line. It's like running a small script on the spot.
Usage:
This command will print "Hello, world!" to the console.
Examples:
Calculate the sum of two numbers:
Create a simple function:
Real-World Applications:
Quickly testing code snippets without creating a separate file.
Automating simple tasks, such as generating data or processing files.
Debugging JavaScript code on the fly.
Simplified Explanation:
Purpose:
The --experimental-default-type option lets you control how Node.js handles certain types of JavaScript files. In previous versions, Node.js would treat files with no extension or ending in .js as CommonJS modules (a type of module system). However, with this option, you can switch the default behavior to ECMAScript (ES) modules.
Module Systems:
CommonJS: An older module system that's been used for a long time in Node.js.
ES Modules: A newer module system that's part of the JavaScript language itself.
What it Does:
When you use --experimental-default-type=module, Node.js will:
Treat files with no extension or ending in
.jsas ES modules unless you explicitly specify otherwise with--input-type.Ignore the
"type"field inpackage.jsonfiles when determining the module type.Still treat files in
node_modulesfolders as CommonJS modules for backward compatibility.
Real-World Example:
Suppose you have a .js file without a package.json file. By default, Node.js would treat it as a CommonJS module. However, if you use --experimental-default-type=module, it will be treated as an ES module. This can be useful if you're working with a project that uses ES modules.
Potential Applications:
Developing new applications using ES modules: Using
--experimental-default-type=modulecan streamline development by eliminating the need to specify module types explicitly.Migrating existing projects to ES modules: By gradually switching to
--experimental-default-type=module, you can start using ES modules in your projects while maintaining compatibility with older code innode_modules.
Node.js Experimental Detect Module
This feature helps Node.js figure out if a file or script is an ES module (a newer type of module in JavaScript) or a CommonJS module (an older type of module).
How does it work?
When you run a file or script, Node.js checks to see if it has a .js extension or no extension. If it does, and there's no package.json file in the same folder (or the package.json file doesn't say what type of module it is), Node.js tries to guess if it's an ES module.
Node.js looks for special words in the file, like import and export. If it finds these words, it decides that the file is an ES module.
Why is this useful?
It's useful because it helps Node.js understand how to run your code. ES modules and CommonJS modules have different ways of working, so Node.js needs to know which one it's dealing with.
Real-world example
Imagine you have a folder with two files: main.js and helper.js. main.js uses a library called "useful-stuff" that has an ES module version.
helper.js is a CommonJS module that also uses "useful-stuff".
When you run main.js, Node.js uses the --experimental-detect-module feature to recognize that it's an ES module and runs it correctly. Without this feature, Node.js might try to run it as a CommonJS module, which would cause an error.
Potential applications
This feature is especially helpful when working with code that uses both ES modules and CommonJS modules, or when you're not sure what type of module a file is. It ensures that Node.js runs your code correctly, without any surprises.
Simplified Explanation:
import.meta.resolve() is a JavaScript function that lets you find the absolute path to a module or file from your current script.
--experimental-import-meta-resolve is a command-line flag for Node.js that enables an experimental feature that allows you to specify a "parent URL" when resolving modules. The parent URL is the URL of the script that is importing the module being resolved.
Example:
Suppose you have a script named main.js that imports a module named foo.js:
To resolve the absolute path to foo.js using import.meta.resolve() with the --experimental-import-meta-resolve flag, you would do the following:
This would output the absolute path to foo.js.
Real-World Applications:
Dynamically Loading Modules: You can use
import.meta.resolve()to dynamically load modules based on user input or runtime conditions.Resolving Relative Paths: The
parentURLargument can be useful for resolving relative paths correctly in cases where the current script is loaded from a different URL than the imported module.
Complete Code Implementation:
Potential Applications:
Modularizing a complex application: You can use
import.meta.resolve()to dynamically load modules based on the current context, making it easier to modularize a complex application.Customizing module resolution: The
parentURLargument allows you to customize how modules are resolved, which can be useful for supporting custom module loading mechanisms.
--experimental-loader=module
--experimental-loader=moduleThis flag specifies the module that contains exported [module customization hooks][].
Module customization hooks are functions that allow you to customize how Node.js loads and executes modules. For example, you could use a module customization hook to:
Load modules from a custom location
Transpile modules before executing them
Modify the exports of a module
To use the --experimental-loader flag, you must specify the path to the module that contains the module customization hooks. For example:
This would load the module customization hooks from the custom-loader.js module and then execute the my-script.js script.
Note: The --experimental-loader flag is discouraged and may be removed in a future version of Node.js. Instead, you should use the [--import flag with the register() function][module customization hooks: enabling].
Real-world applications
Potential applications of module customization hooks include:
Loading modules from a custom location
This could be useful if you want to load modules from a private repository or from a local file system location.
Transpiling modules before executing them
This could be useful if you want to use a newer version of JavaScript that is not supported by the current version of Node.js.
Modifying the exports of a module
This could be useful if you want to add or remove properties from the exports of a module.
Example
Here is an example of a simple module customization hook that loads modules from a custom location:
To use this module customization hook, you would run the following command:
This would load the my-script.js script from the /my-modules directory.
Simplified Explanation
This command allows you to use the https: protocol in the import statement. Normally, only the file: protocol is supported, meaning you can only import code from local files. With this option, you can also import code from websites using the https: protocol.
Code Snippet
This code snippet imports a function named example from a file called file.js located at the website https://example.com.
Real-World Application
This option is useful if you want to import code from a shared library or repository hosted on the web. For example, you could import a module that provides common utility functions or a machine learning library.
Potential Applications
Sharing code libraries with others
Accessing open-source code on the web
Quickly adding new features to your application by importing modules from the web
Simplified Explanation of --experimental-permission:
The --experimental-permission flag allows you to control which actions the current Node.js process is allowed to perform.
Topics in Detail:
File System Permissions:
Default: The process cannot access the file system.
--allow-fs-read: The process can read files and directories.--allow-fs-write: The process can write to files and directories.
Child Process Permissions:
Default: The process cannot create child processes.
--allow-child-process: The process can create child processes.
Worker Threads Permissions:
Default: The process cannot create worker threads.
--allow-worker: The process can create worker threads.
Real-World Applications:
Secure Environments: In environments where security is paramount, you can use the
--experimental-permissionflag to restrict the actions that a Node.js process can perform.Sandboxing: You can create a sandboxed environment where a Node.js process is isolated from the rest of the system.
Complete Code Implementations:
--experimental-policy
Simplified explanation: This flag lets you specify a file containing a set of rules that control access to your Google Cloud resources. It's like a security guard that checks whether requests to access your resources are allowed or not.
Real-world example: Let's say you have a Cloud Storage bucket with sensitive data. You can use a policy file to restrict access to this bucket only to users who are in a specific group or have a certain role.
Code implementation: You can specify the policy file using the --experimental-policy flag when you create the bucket:
Potential applications:
Enforcing fine-grained access control to your resources
Ensuring regulatory compliance
Protecting sensitive data from unauthorized access
Simplified Explanation:
The --experimental-sea-config flag allows you to bundle your Node.js application and its dependencies into a single executable file. This makes your application easy to deploy and distribute, and can improve performance by reducing startup time.
Topics:
Single Executable Application (SEA): A self-contained application that includes all its dependencies.
Blob: A binary file that contains the configuration for the SEA.
Injection: Inserting the blob into the Node.js binary to create the SEA.
Real-world Applications:
Simplified Deployment: No need to manage separate Node.js and dependency installations.
Faster Startup: The application can start up more quickly because all the dependencies are already loaded.
Improved Security: The application is less vulnerable to security breaches by isolating it from the underlying system.
Code Example:
To generate the blob:
To inject the blob into the Node.js binary:
To run the SEA:
Potential Applications:
Cloud Functions: Creating self-contained functions that can be deployed with minimal setup.
Edge Devices: Distributing applications to devices with limited resources.
Enterprise Applications: Improving security and simplifying deployment in large organizations.
--experimental-shadow-realm
--experimental-shadow-realmWhat it does:
This flag turns on a feature called ShadowRealm.
How it works:
ShadowRealm is a new way to isolate JavaScript code in Node.js. It creates a separate "realm" for your code, which means that any errors or exceptions that occur in that code won't affect the rest of your application.
Benefits:
Using ShadowRealm can make your code more stable and reliable. It can also help you to isolate different parts of your application, so that they don't interfere with each other.
Real-world applications:
Here are some real-world applications for ShadowRealm:
Isolating different components of a complex application, so that they don't interfere with each other.
Creating a safe environment for running untrusted code, such as code from third-party libraries.
Improving the stability and reliability of your application by preventing errors and exceptions from propagating throughout the codebase.
Example:
Here is an example of how to use ShadowRealm:
In this example, the createShadowRealm() function is used to create a new shadow realm. The evaluate() method is then used to create a function in the shadow realm. The fn() function is then called, and the result is printed to the console.
What is --experimental-test-coverage?
When you use the node:test module, you can add the --experimental-test-coverage flag to your command to generate a report that shows how much of your code was covered by your tests.
How to use it:
To use it, just run your tests with the --experimental-test-coverage flag:
What will happen:
If any tests are run, a coverage report will be generated as part of the output. The report will show which parts of your code were executed during the tests and which parts were not.
Why is this useful?
This can be useful for several reasons:
To identify areas of your code that are not being tested. This can help you improve your test coverage and ensure that your code is being thoroughly tested.
To track the amount of coverage over time. This can help you measure the effectiveness of your tests and identify any areas where coverage is declining.
To compare the coverage of different versions of your code. This can help you identify changes that have affected the coverage of your code.
Real-world examples:
A developer wants to improve the test coverage of their code. They use the
--experimental-test-coverageflag to generate a report that shows which parts of their code are not being tested. They then write new tests to cover the untested code.A team wants to track the amount of coverage over time. They use the
--experimental-test-coverageflag to generate reports on a regular basis. They then track the coverage percentage over time to identify any trends.A company wants to compare the coverage of different versions of their code. They use the
--experimental-test-coverageflag to generate reports on each version. They then compare the reports to identify any changes in coverage that could potentially impact the stability of their code.
Topic: --experimental-vm-modules
Simplified Explanation:
This option allows you to try out a new feature in the node:vm module that supports modules written using the ES Module syntax.
Detailed Explanation:
The node:vm module provides a way to execute JavaScript code in a separate sandbox environment. By default, this sandbox environment only supports the CommonJS module syntax, which is the older module system used in Node.js.
The --experimental-vm-modules option enables support for ES Modules in the node:vm sandbox. This means you can now use the newer ES Module syntax when writing code to be executed in the sandbox.
Real-World Example:
Imagine you have a script that uses a third-party library written using the ES Module syntax. You want to test this script in a sandbox environment to ensure it doesn't interfere with your own code.
Normally, you wouldn't be able to do this because the node:vm sandbox doesn't support ES Modules by default. However, by using the --experimental-vm-modules option, you can enable ES Module support and test your script safely.
Code Implementation:
Potential Applications:
Testing code that uses ES Modules in a sandboxed environment
Debugging code that uses ES Modules to isolate issues
Isolating code that could potentially interfere with other code in the application
--experimental-wasi-unstable-preview1
Simplified Explanation:
This option turns on the WebAssembly System Interface (WASI) feature, which is still in early development and should be used with caution. WASI lets JavaScript programs run on the web just like native code, enabling them to interact with the operating system's files, processes, and devices.
Potential Applications:
Running JavaScript applications natively: WASI can be used to run JavaScript programs on the file system instead of just in a web browser. This can speed up JavaScript execution and open up new possibilities for using JavaScript in areas like server-side scripting and desktop applications.
Interacting with system resources: WASI enables JavaScript programs to access files, create processes, and use system devices, like webcams and microphones. This allows JavaScript to control hardware and peripherals in a way that was previously impossible.
Developing portable WASM modules: WASI provides a standard way for WASM modules to interact with different operating systems, making it easier to develop and deploy WASM-based applications across multiple platforms.
Real-World Example:
Let's say you have a JavaScript program that needs to interact with a file on your computer. Here's how you would do it with WASI:
This example uses the fs module to read a file from the file system using WASI. The readFileSync method reads the entire contents of the file and converts it to a string, which is then printed to the console.
Note:
WASI is still under development and could change significantly in the future. It's recommended to use it with caution and be aware of its limitations.
--experimental-wasm-modules
This flag enables experimental support for WebAssembly (Wasm) modules. Wasm is a binary format for running code on the web, similar to JavaScript but with better performance and security. By enabling this flag, you can use Wasm modules in your Node.js applications.
Real-world example:
Imagine you're building a Node.js application that processes large amounts of data. You could use a Wasm module to perform complex calculations on the data, significantly improving performance compared to using pure JavaScript.
Complete code implementation:
Potential applications:
High-performance computing: Wasm can be used to offload computationally intensive tasks from JavaScript to Wasm modules, improving performance.
Security: Wasm modules are sandboxed, making them more secure than running native code.
Portability: Wasm modules can be written in a variety of languages, making them portable across different platforms and environments.
Topic: Node.js Add-ons
Add-ons are libraries that extend the functionality of Node.js. They can be written in C++, Rust, or other compiled languages, and they allow you to access features that are not natively supported by JavaScript.
Topic: Context-Aware Add-ons
Context-aware add-ons are add-ons that are able to detect the current execution context of Node.js. This is important because Node.js can run in different contexts, such as the main thread, a worker thread, or a REPL (Read-Eval-Print-Loop) session.
--force-context-aware Option
The --force-context-aware option forces Node.js to only load context-aware add-ons. This can be useful for debugging purposes, or if you want to ensure that your add-ons are compatible with all execution contexts.
Real-World Example
Here's a real-world example of how you might use the --force-context-aware option:
In this example, we're using the --force-context-aware option to ensure that our script only loads context-aware add-ons. This could be useful if we're experiencing problems with a particular add-on that is not context-aware.
Potential Applications
The --force-context-aware option can be useful in a variety of situations, including:
Debugging add-ons to ensure that they are compatible with all execution contexts.
Deploying Node.js applications in environments where you want to restrict the use of non-context-aware add-ons.
Improving the performance of Node.js applications by only loading add-ons that are necessary for the current execution context.
--force-fips
--force-fipsPurpose: Enforce the use of FIPS-compliant (Federal Information Processing Standard) cryptography on startup.
Simplified Explanation:
Imagine FIPS as a special rulebook for cryptography, like a secret code. --force-fips makes your Node.js application follow this rulebook strictly.
Usage Example:
Requirements:
Your operating system must support FIPS-compliant cryptography.
Use a FIPS-approved Node.js version, such as:
Node.js 18 with FIPS enabled:
node --enable-fips --versionNode.js 16 with FIPS enabled:
n 16 --enable-fips
Real-World Applications:
Government and healthcare: Sensitive data must be protected using FIPS-compliant methods.
Financial institutions: Cryptography is crucial for secure transactions. FIPS ensures compliance with industry regulations.
Cloud computing: FIPS helps ensure the security of cloud-based services and data.
--force-node-api-uncaught-exceptions-policy
--force-node-api-uncaught-exceptions-policySimplified Explanation
By default, Node.js doesn't require add-ons (native C++ code) to handle exceptions thrown in their asynchronous callbacks. This can lead to the entire Node.js process crashing.
The --force-node-api-uncaught-exceptions-policy flag forces add-ons to handle these exceptions and ensures that the process doesn't crash.
Detailed Explanation
Asynchronous Callbacks
Node.js uses asynchronous callbacks to handle events and I/O operations without blocking the event loop. This allows Node.js to handle multiple requests concurrently, making it highly efficient.
Add-ons
Add-ons are native C++ code that can extend Node.js with additional functionality. Add-ons can execute asynchronous callbacks just like JavaScript code.
Uncaught Exceptions
In JavaScript, uncaught exceptions (exceptions not handled by a try-catch block) cause the Node.js process to crash. This can happen if an error occurs in an asynchronous callback.
Add-ons and Uncaught Exceptions
By default, Node.js doesn't require add-ons to handle uncaught exceptions in their asynchronous callbacks. This means that if an error occurs in an add-on's callback, the Node.js process can crash.
--force-node-api-uncaught-exceptions-policy Flag
The --force-node-api-uncaught-exceptions-policy flag forces add-ons to handle uncaught exceptions in their asynchronous callbacks. If an exception occurs, the add-on's callback is responsible for handling it and preventing the Node.js process from crashing.
Real-World Applications
The --force-node-api-uncaught-exceptions-policy flag is useful for ensuring that add-ons don't crash the Node.js process. This is especially important for add-ons that execute long-running asynchronous tasks or that interact with external resources (e.g., hardware devices).
For example, an add-on that reads data from a hardware device could handle exceptions that occur during data transfer. This would prevent the Node.js process from crashing and allow the add-on to continue operating.
Code Example
To enable the --force-node-api-uncaught-exceptions-policy flag when starting a Node.js application, use the following command:
Summary
The --force-node-api-uncaught-exceptions-policy flag ensures that add-ons handle uncaught exceptions in their asynchronous callbacks. This prevents the Node.js process from crashing and allows add-ons to continue operating even when errors occur.
Simplified Explanation:
The --frozen-intrinsics flag in Node.js allows you to experiment with a feature that freezes built-in objects like Array and Object.
What are Intrinsics?
Intrinsics are built-in objects that are part of the JavaScript language. They include things like Array, Object, and Math.
What does Freezing Intrinsics Mean?
Freezing intrinsics means that they can no longer be modified or replaced. This is in contrast to the normal behavior where you can, for example, add new properties to the Array object.
Why Freeze Intrinsics?
Freezing intrinsics can help prevent unexpected behavior and errors in your code. For example, if you freeze the Array object, you can be sure that any code that uses it will always access the same set of methods and properties.
How to Use the Flag:
To use the --frozen-intrinsics flag, simply add it to the command when you start Node.js:
Potential Applications:
Security: Freezing intrinsics can make it harder for attackers to exploit vulnerabilities in your code.
Performance: Freezing intrinsics can improve performance by preventing unnecessary modifications to built-in objects.
Code consistency: Freezing intrinsics can help ensure that code behaves the same way across different environments.
Real-World Example:
Let's say you have a function that relies on the Array object having a specific set of methods. If you freeze the Array object, you can be sure that the function will always work as expected, even if other code tries to modify the Array object.
Note:
The --frozen-intrinsics flag is still experimental and may not work as expected in all cases. Use it with caution and be prepared for unexpected behavior.
What is --heap-prof?
It's like a "snapshot" of what's happening inside your Node.js application's memory. It helps you understand how your application uses memory and identify any potential memory leaks.
How to use --heap-prof:
Just add --heap-prof when you start your Node.js application.
What does --heap-prof do?
It starts a special tool called the "heap profiler" which continuously monitors your application's memory usage. When you exit your application, the heap profiler saves a report to a file.
Where can I find the report?
The report will be saved in the same directory where you ran your application. It will have a name like Heap.${date}.${time}.${pid}.${tid}.${seq}.heapprofile, where:
dateis the date when the report was createdtimeis the time when the report was createdpidis the process ID of your applicationtidis the thread ID of the heap profilerseqis a sequence number
How do I use the report?
You can open the report using a tool like Chrome DevTools. DevTools will show you a detailed breakdown of your application's memory usage, including:
Which objects are taking up the most memory
How objects are connected to each other
Whether there are any memory leaks
Potential Applications:
Memory Leak Detection: Identify and fix memory leaks that can cause your application to slow down or crash.
Performance Optimization: Determine which parts of your application are consuming the most memory and optimize them for better performance.
Code Analysis: Understand how your code interacts with memory and make changes to improve memory efficiency.
--heap-prof-dir
--heap-prof-dirImagine your computer is like a big house with many rooms. Each room has different things in it, like furniture, clothes, and toys. The --heap-prof option is like a special camera that takes a snapshot of one of these rooms, showing you what's in it at that moment.
The --heap-prof-dir option lets you choose where this snapshot should be saved. By default, it's saved in a special room called "--diagnostic-dir," which is like a place where your computer keeps important information. But you can use this option to tell the computer to save the snapshot in a different room, such as on your desktop.
Here's an example:
In this example, the --heap-prof-dir option tells the node command to save the snapshot in a folder called "heap-profiles" on your desktop.
Potential Applications
Debugging memory leaks: You can use the snapshots to see if there are any unused objects in your program, which can help you find and fix memory leaks.
Optimizing performance: You can see how much memory your program is using and where it's being used. This can help you identify bottlenecks and optimize your program's performance.
--heap-prof-interval
--heap-prof-intervalWhen profiling your Node.js application, you can generate heap snapshots to analyze memory usage. The --heap-prof-interval option lets you control how often these snapshots are taken.
By default, heap snapshots are taken every 512KB of memory allocated. You can change this interval by specifying a different value after the --heap-prof-interval option. For example, to take heap snapshots every 1MB of memory allocated, you would use the following command:
Real-world application:
Heap profiling can be used to identify memory leaks or other memory-related issues in your application. By taking heap snapshots at regular intervals, you can track how memory usage changes over time and identify any potential problems.
Example:
This command will take heap snapshots every 512KB of memory allocated. The heap snapshots will be saved to files in the ./heap-snapshots directory.
--heap-prof-name
--heap-prof-nameStability: 1 - Experimental
Purpose: Specify the file name of the heap profile generated by --heap-prof.
Simplified Explanation:
Imagine your code is using too much memory. To find out why, you can use Node.js's --heap-prof option to generate a detailed report on how memory is being used.
By default, this report is saved to a file called heapsnapshot.heapdump. However, you can use --heap-prof-name to specify a different file name.
Code Sample:
This command will generate a heap profile report and save it to the file my-heap-profile.heapdump.
Real-World Applications:
Debugging memory leaks in your code
Optimizing your code to use less memory
Understanding how your code allocates and deallocates memory over time
Topic: --heapsnapshot-near-heap-limit
Explanation: This option helps you capture snapshots of the V8 heap memory when the heap usage is close to its limit. It's like taking a picture of the memory status when it's almost full. The snapshots can be used to analyze what objects are being allocated and why the heap is filling up.
How it Works: When you specify a number (e.g., --heapsnapshot-near-heap-limit=3), Node.js will write up to that number of snapshots to a file. As the heap usage increases towards the limit, Node.js will trigger garbage collection to free up some memory. If the heap limit is still being approached after garbage collection, a snapshot will be written.
Benefits:
Helps identify memory leaks or excessive object allocation.
Provides insights into memory usage patterns.
Code Example:
Real-World Application:
Debugging memory-related issues in Node.js applications.
Optimizing memory usage and performance.
Heap Snapshot Signal
This option enables the Node.js process to write a heap dump when a specified signal is received.
Signal: A signal is a notification sent to a process that can cause it to stop, pause, or perform a specific action.
Usage:
To use this option, you need to specify a valid signal name when you start the Node.js process. For example:
This command will enable the signal handler that writes a heap dump when the SIGUSR2 signal is received.
Real-World Application:
Heap dumps are useful for debugging memory issues in your Node.js application. By sending a signal to the process, you can generate a heap dump at any time, even when the application is running in production.
Complete Code Implementation:
How to Use:
Start the Node.js process with the
--heapsnapshot-signal=SIGUSR2option.Send the
SIGUSR2signal to the process using thekillcommand:
The heap dump will be written to the current directory.
Potential Applications:
Identifying memory leaks
Debugging performance issues
Analyzing memory usage patterns
Simplified Explanation:
-h, --help is a built-in command in Node.js that displays a summary of the command-line options available for the node command. This information can be helpful when you want to understand what options you can use to customize the behavior of Node.js when running scripts or applications.
Improved Code Snippet:
Real-World Implementation:
The following example shows how to use the --help option to display the command-line options for the node command:
Potential Applications:
The --help option can be used in a variety of real-world applications, such as:
Debugging: You can use the
--helpoption to display a list of available command-line options that can be used to customize the behavior of Node.js when debugging scripts or applications.Learning: You can use the
--helpoption to learn about the different command-line options available for thenodecommand and how to use them.Customization: You can use the
--helpoption to understand the different command-line options available for thenodecommand and how to use them to customize the behavior of Node.js to meet your specific requirements.
Simplified Explanation:
Purpose:
--icu-data-dir is used to specify the location of the ICU (International Components for Unicode) data files. ICU is a library for handling internationalization tasks such as language detection, currency formatting, and date formatting.
How It Works:
Normally, Node.js loads ICU data from its default location. However, you can override this default by using the --icu-data-dir option to specify a different location.
Usage:
To use --icu-data-dir, run Node.js with the following command:
where <path-to-icu-data> is the full path to the directory containing the ICU data files.
Benefits:
Using --icu-data-dir allows you to:
Use a custom ICU data directory, if the default location is not accessible.
Set a specific ICU data directory for testing or development scenarios.
Ensure that your application uses the correct version of ICU data.
Real-World Applications:
Localization: For applications that handle multiple languages and locales, customizing the ICU data directory allows for more precise language detection and formatting.
Testing: When testing different versions of ICU data,
--icu-data-direnables you to quickly switch between versions without modifying the codebase.Specific Locale: If your application requires specific locale data not available in the default ICU data, you can use
--icu-data-dirto specify a directory with the necessary data.
--import=module
--import=moduleSimplified Explanation:
Imagine Node.js like a machine that runs your JavaScript code. When you start the machine, you can tell it to load certain JavaScript modules or programs into memory before it starts running your code. These modules can help your code run faster or give it extra features.
Preloading Modules:
The --import=module flag lets you preload a specific JavaScript module into memory before Node.js starts running your code. Modules are like small programs that add functionality to your JavaScript code, like a calculator module that can add numbers or a database module that can manage data.
Execution Order:
If you're using the --import flag multiple times, each module will be loaded in the order you specify them. So, if you have three modules, moduleA, moduleB, and moduleC, and you use --import=moduleA --import=moduleB --import=moduleC, Node.js will load moduleA first, then moduleB, and finally moduleC.
ECMAScript Module Resolution:
Node.js uses specific rules to find and load JavaScript modules. These rules are called "ECMAScript module resolution rules." When you use --import, Node.js will follow these rules to find the module you want to load.
Loading CommonJS Modules:
CommonJS modules are another type of JavaScript module that's different from ECMAScript modules. To load a CommonJS module, you use the --require flag instead of --import.
Real-World Example:
Let's say you have a JavaScript program that needs to access a database. You can use the --import flag to preload a database module before running your program. This will make your program run faster because the module will already be loaded into memory when your program starts.
Potential Applications:
Preloading commonly used modules to improve performance
Loading modules that are required by external libraries or frameworks
Simplifying the loading of complex dependencies in large applications
Simplified Explanation
When you use Node.js to run a JavaScript program from the command line, you can specify the type of input you want to provide. By default, Node.js expects the input to be in the CommonJS module format. However, you can also use the ES module format.
CommonJS vs. ES Module
CommonJS: A module format that uses the
require()andmodule.exportssyntax to import and export modules.ES Module: A newer module format that uses the
importandexportsyntax to import and export modules.
--input-type Option
The --input-type option allows you to specify the type of input you want to provide. Valid values are "commonjs" or "module".
--input-type=commonjs: Node.js will interpret the input as a CommonJS module.--input-type=module: Node.js will interpret the input as an ES module.
Default Value
The default value of the --input-type option is "commonjs". This means that if you don't specify the option, Node.js will assume that the input is a CommonJS module.
When to Use ES Modules
ES modules are generally preferred over CommonJS modules for several reasons:
They are more concise and easier to read.
They are better supported by modern JavaScript tools and frameworks.
They are more secure.
However, ES modules are not fully compatible with older versions of Node.js. If you need to support older versions of Node.js, you may need to use CommonJS modules.
Real-World Examples
Example 1: Running a CommonJS Module
To run this CommonJS module, you would use the following command:
Example 2: Running an ES Module
To run this ES module, you would use the following command:
Potential Applications
The --input-type option is useful in a variety of scenarios, including:
Developing and testing new JavaScript code: You can use the
--input-typeoption to experiment with different module formats.Integrating JavaScript code with other languages: You can use the
--input-typeoption to specify the module format that is compatible with the other language.Creating custom scripts and tools: You can use the
--input-typeoption to specify the module format that is most appropriate for your needs.
Insecure HTTP Parser Flag
What is it?
It's an option that makes the HTTP parser in Node.js more lenient.
What does it mean for the HTTP parser to be lenient?
It means that the parser will be less strict about following the rules of the HTTP protocol. This can be useful in some cases, but it can also open up your application to security vulnerabilities.
What are the specific things that the insecure HTTP parser flag allows?
Invalid HTTP headers values (e.g.,
Content-Type: text/html5)Invalid HTTP versions (e.g.,
HTTP/1.5)Messages containing both
Transfer-EncodingandContent-LengthheadersExtra data after a message when
Connection: closeis presentExtra transfer encodings after
chunkedhas been provided(space) to be used as a token separator instead of (tab)
not to be provided after a chunk
Spaces to be present after a chunk size and before
Why is it called insecure?
Because it can expose your application to request smuggling or poisoning attacks. These attacks can allow an attacker to send malicious requests that your application will interpret as legitimate.
When should I use the insecure HTTP parser flag?
Only if you are absolutely sure that you need to. In most cases, it is better to follow the HTTP protocol strictly.
Code snippet:
Real-world example:
You might use the insecure HTTP parser flag if you are trying to interoperate with a non-conformant HTTP implementation. For example, some older web servers may not implement the HTTP protocol strictly. By enabling the insecure HTTP parser flag, you can allow your application to work with these servers.
Potential applications:
Interoperating with legacy web servers
Debugging HTTP traffic
Writing HTTP fuzzers
Simplified Explanation of --inspect Option:
What is it?
--inspect is a flag for the Node.js command line that lets you use tools like Chrome DevTools and IDEs to debug and inspect your Node.js programs.
How it works:
When you use --inspect, Node.js starts a special server that listens for a connection from a debugging tool. Once the tool connects, it can send commands to Node.js to:
Set breakpoints
Step through code
Examine variables
Profile performance
How to use it:
To use --inspect, simply add it to your Node.js command like this:
By default, the inspector server listens on port 9229 on your local machine. You can specify a different host and port if needed, like this:
Real-World Examples:
Debugging: If your Node.js program crashes or behaves unexpectedly,
--inspectcan help you identify the source of the problem.Profiling:
--inspectcan help you track down performance bottlenecks in your code.Code collaboration: Multiple developers can connect to the inspector server to debug and collaborate on changes.
Testing: Some testing frameworks use
--inspectto set breakpoints and control the execution of tests.
Potential Applications:
Building and testing complex applications
Troubleshooting problems with Node.js programs
Optimizing performance for demanding workloads
Facilitating code collaboration in development teams
Warning: Binding Inspector to a Public IP:Port Combination
Problem:
Exposing the Node.js inspector to the public internet is dangerous because it allows anyone on the internet to access your application and potentially execute malicious code on it.
Solution:
To prevent this, make sure that either:
The inspector is only accessible from a private network: Do not expose it to the public internet.
A firewall blocks unauthorized access to the inspector's port: Configure your firewall to only allow connections from trusted sources.
Real-World Example:
This code should not be used in production because it exposes the inspector to the public internet.
This code is more secure because the inspector is only accessible from localhost, which is a private IP address.
Potential Applications:
Debugging an application in production: Connect a debugger to the inspector to identify and fix issues.
Profiling performance: Collect data about application performance and identify bottlenecks.
Remote code execution: Allow external tools to execute code on the application for debugging or testing purposes.
Simplified Explanation:
Imagine you have a car and want to check its engine while it's running. To do this, you have two options:
Option 1: --inspect
Like using a regular toolbox, you can inspect the engine while the car is running. But you can't stop the car and make any changes.
In Node.js,
--inspectallows you to check the code while it's running, but you can't pause it or modify it.
Option 2: --inspect-brk
Like using a mechanic's tools, you can inspect the engine and stop the car to make changes.
In Node.js,
--inspect-brkallows you to check the code while it's running, and you can also pause it and make changes.
Real-World Example:
You have a Node.js script that runs a website. The script has a bug that causes the website to crash. You can use --inspect-brk to:
Start the script with
node --inspect-brk my-script.js.Open a debugger tool (e.g., Chrome DevTools) and connect to the script at
chrome://inspect.Step through the code and find the bug.
Pause the code at the point of failure and make changes.
Resume the code and check if the bug is fixed.
Potential Applications:
Debugging code: Find and fix bugs in your Node.js scripts.
Testing code: Run unit tests and inspect the results.
Profiling code: Analyze performance and identify bottlenecks.
Code collaboration: Share debugging sessions with other developers.
Simplified Explanation
--inspect-port Option
This option allows you to specify the port on which the Node.js debugger (inspector) will be accessible. This is useful if you want to connect to the debugger remotely or from a different host.
Usage:
Parameters:
host(optional): The hostname or IP address where the debugger will be accessible. By default, it's127.0.0.1(localhost).port: The port number where the debugger will listen.
Examples:
To listen on port 9229 on the local host:
To listen on port 9230 on the host with IP address 192.168.1.100:
Security Warning:
Be cautious when specifying the host parameter, as allowing connections from external hosts can be a security risk.
Applications:
Debugging Node.js applications remotely or from another machine.
Profiling and analyzing application performance using the inspector's features.
Setting breakpoints and inspecting variables during debugging sessions.
--inspect-publish-uid
Simplified Explanation:
This option lets you specify where the URL for the Chrome DevTools inspector will be shown when you start your Node.js application in debug mode.
Topics in Detail:
Inspector Web Socket URL: This is the URL that you can use to connect to the DevTools inspector with tools like the Chrome browser or Visual Studio Code.
Exposure Methods: You can choose how the URL is exposed:
stderr: The URL will be printed in the command line output of your application.
http: The URL will be available at a specific endpoint on your application (e.g.,
/json/list).
Code Snippet:
Real-World Example:
Developers typically use the inspector URL to debug their Node.js applications. By default, the URL is only available in the command line output, which can be inconvenient. By specifying --inspect-publish-uid=http, you can easily access the URL from a browser or IDE.
Potential Applications:
Remote Debugging: Allow other developers or support teams to remotely debug your application without sharing the source code.
Improved Error Reporting: Log the inspector URL in your error reports to help users quickly identify and resolve issues.
Automating Testing: Use DevTools inspectors to automate testing and verification processes.
Interactive Mode
When you run a Node.js script from the command line, it typically executes the script and exits. However, sometimes you may want to interact with your script in a more interactive way.
The -i or --interactive flag allows you to open an interactive REPL (Read-Eval-Print Loop) environment, even if you're not running your script from a terminal window.
With the REPL, you can type in commands and have them executed immediately. This can be useful for debugging, testing, or experimenting with your code.
Example:
Real-World Applications:
Debugging: You can use the REPL to step through your code line by line and inspect the values of variables.
Testing: You can use the REPL to quickly test different scenarios and verify the output.
Interactive development: You can use the REPL to experiment with different code snippets and get immediate feedback.
Simplified Explanation:
When JavaScript code is run in Node.js, it is typically compiled into faster machine code (executable memory) at runtime. This process is called "Just-in-time (JIT) compilation."
--jitless Flag:
The --jitless flag disables JIT compilation and forces Node.js to run JavaScript code in its slower, interpreted form. This means that code will not be compiled into executable memory, which can improve security and reduce the attack surface of your application. However, it will also result in a significant performance decrease.
Real-World Example:
Suppose you have a Node.js application that processes large amounts of data and needs to execute code very quickly. Using the --jitless flag would not be beneficial in this scenario because it would slow down the application's performance.
When to Use --jitless:
The --jitless flag should only be used in situations where security is paramount and performance can be sacrificed. For example:
If your application handles sensitive data and you are concerned about the risk of memory corruption attacks.
If you are deploying your application on a platform that has strict security requirements.
Improved Example:
Potential Applications:
Security-sensitive applications: Financial transactions, healthcare systems, and government applications.
Applications deployed on constrained environments: IoT devices, embedded systems, and cloud functions with limited resources.
What is --max-http-header-size?
When you send an HTTP request or response, it contains a header that includes information about the request or response. The --max-http-header-size option allows you to specify the maximum size of this header, in bytes.
Default value: 16 KiB
Why is this useful?
HTTP headers can sometimes become very large, especially when they contain a lot of cookies or other data. This can slow down the server's processing of the request or response. By setting a maximum size for the header, you can prevent the server from spending too much time processing large headers.
How to use --max-http-header-size:
You can use the --max-http-header-size option when starting the node.js server. For example:
This will set the maximum size of the HTTP header to 32 KiB.
Real-world example:
A real-world example of where you might use the --max-http-header-size option is if you are running a website that receives a lot of traffic from mobile devices. Mobile devices often have smaller screens and less memory than desktop computers, so they may not be able to handle large HTTP headers. By setting a maximum size for the header, you can ensure that mobile devices can still access your website without any problems.
Potential applications:
Preventing denial-of-service attacks that use large HTTP headers to overwhelm the server
Improving the performance of servers that process a lot of HTTP traffic
Reducing the bandwidth usage of HTTP traffic
The --napi-modules option in the Node.js CLI is a remnant of older versions of Node.js and is now a no-op, meaning it has no effect.
No-op
In computer science, a no-op is an operation that does nothing. It's like pressing a button that has no function. In the case of --napi-modules, the option was once used to enable the use of Node.js Add-on Modules for Native Interface (N-API), but it is now redundant as N-API is now the default way to create Node.js add-on modules.
Real-world example
Since --napi-modules is a no-op, there are no real-world examples or applications for it.
Simplified example
Here's a simplified example of what the --napi-modules option would look like in a command:
In this example, the --napi-modules option is used before running the my-script.js script. However, since it is a no-op, it will have no effect on the execution of the script.
Improved code snippet
Since --napi-modules is a no-op, there is no need to use it in your code. Here's an example of how to run a Node.js script without using the --napi-modules option:
This will run the my-script.js script without any additional options.
--no-addons
--no-addonsBy default, Node.js can use native C++ addons to extend its functionality. However, these addons can sometimes cause problems or be incompatible with certain systems.
The --no-addons flag disables the use of native addons, which can be useful for troubleshooting problems or ensuring compatibility.
When --no-addons is specified, the following things happen:
The
process.dlopen()function, which is used to load native addons, will throw an error.Any attempts to require a native C++ addon will also throw an error.
Here's an example of how to use the --no-addons flag:
This will run the script my-script.js without loading any native addons.
Real-world applications
The --no-addons flag can be useful in the following situations:
Troubleshooting problems with native addons
Ensuring compatibility with systems that do not support native addons
Running Node.js in a sandboxed environment
--no-deprecation
--no-deprecationSimplified Explanation:
This option tells Node.js to ignore warnings about features that are planned to be removed in future versions.
Detailed Explanation:
Node.js sometimes includes features that are marked as "deprecated." This means that these features are still available for use, but they are likely to be removed in future versions. To help users prepare for these changes, Node.js prints deprecation warnings when these features are used.
The --no-deprecation option can be used to suppress these warnings. This can be useful if you are using deprecated features and do not want to be bothered by the warnings. However, it is important to remember that if you ignore the warnings, you may encounter errors when using the deprecated features in future versions of Node.js.
Example:
To disable deprecation warnings in your Node.js code, add the following line to the beginning of your script:
Real-World Applications:
The --no-deprecation option can be useful in the following situations:
When you are working with a large codebase that uses many deprecated features, and you do not have time to update all of the code before the next major version of Node.js is released.
When you are using a third-party library that uses deprecated features, and you do not want to have to update the library immediately.
When you are running Node.js in a production environment, and you do not want to deal with deprecation warnings that could be confusing for users.
Potential Applications:
Here are some examples of how the --no-deprecation option can be used in real-world applications:
A company's website might use a third-party library that relies on a deprecated feature in Node.js. The company could use the
--no-deprecationoption to disable the deprecation warnings and avoid having to update the website immediately.A developer might be working on a large codebase that contains many deprecated features. The developer could use
--no-deprecationoption to suppress the deprecation warnings and focus on other tasks until they have time to update the code.A system administrator might be running Node.js in a production environment where stability is important. The administrator could use the
--no-deprecationflag to prevent deprecation warnings from appearing in the logs.
Simplified Explanation
The --no-experimental-fetch flag in Node.js disables the Fetch API, a newer and more powerful way to make requests over the internet, from being available globally (on all web pages).
What is the Fetch API?
Imagine you're a kid at a lemonade stand. When someone orders lemonade, you would go to your fridge, get a cup, and fill it up with lemonade.
The Fetch API is like a special employee at the lemonade stand. When someone orders lemonade, instead of you going to the fridge to get it, you just ask the employee, and they fill up the cup for you. It's a faster and more convenient way of getting lemonade.
Why would you want to disable it?
Some websites may not work properly if the Fetch API is enabled globally. It's like having a new employee at the lemonade stand who doesn't know how to make the lemonade correctly.
Real-World Code Example
This command runs the JavaScript file my-script.js with the Fetch API disabled.
Potential Applications
Websites that rely on older techniques for making requests
Websites that have not yet been updated to work with the Fetch API
Websites that may encounter compatibility issues with the Fetch API
--no-experimental-global-customevent
Simplifies as:
Disable CustomEvent Web API on Global Scope
Explanation:
Imagine you have a website with a button that triggers a custom action when clicked. In JavaScript, this action is represented by a CustomEvent.
Normally, you would use the CustomEvent Web API to define and trigger this custom event. However, sometimes you don't want these custom events to be accessible globally on your website.
This is where the --no-experimental-global-customevent flag comes in. It disables the exposure of the CustomEvent Web API on the global scope, making it inaccessible to any scripts that are not explicitly defined to use it.
Real-World Example:
Let's say you have a website with multiple pages, each with its own custom events. By disabling global exposure of CustomEvent, you can prevent conflicts between events on different pages.
Complete Code Implementation:
Potential Applications:
Isolating custom events to specific modules or pages
Preventing conflicts between events defined in different scripts
Enhancing security by limiting access to custom events
--no-experimental-global-navigator
By default, Node.js does not expose the Navigator API on the global scope. This means that you cannot access the navigator object in your code.
However, you can enable the experimental --no-experimental-global-navigator flag to expose the Navigator API on the global scope. This can be useful for debugging purposes, but it is not recommended for production use.
Real-world example
The following code enables the experimental --no-experimental-global-navigator flag:
Once this flag is enabled, you can access the navigator object in your code:
Potential applications
The Navigator API can be used to access information about the user's browser, such as the user agent string, the language, and the cookie settings. This information can be useful for debugging purposes, or for customizing the user experience based on the user's browser.
Conclusion
The --no-experimental-global-navigator flag is a useful tool for debugging purposes, but it is not recommended for production use. If you need to access the Navigator API in your code, you should use the navigator module instead.
--no-experimental-global-webcrypto
Explanation:
The Web Crypto API is a set of tools that allow websites to perform cryptographic operations, such as encryption and decryption, securely in the browser.
By default, Node.js exposes these tools on the global scope, making them accessible to all parts of your code. However, if you use the --no-experimental-global-webcrypto flag, you can disable this behavior.
Why disable it?
Disabling the global Web Crypto API can improve security by limiting the potential for malicious code to access these sensitive cryptographic operations. It also helps prevent accidental use of the Web Crypto API, which could lead to bugs or vulnerabilities.
Example:
To disable the global Web Crypto API, you can use the following command when starting Node.js:
Real-world applications:
Secure messaging: Encrypting messages before sending them over the network to prevent eavesdropping.
Data protection: Encrypting sensitive data at rest to prevent unauthorized access.
Digital signatures: Generating digital signatures to verify the authenticity of messages and documents.
--no-experimental-repl-await
--no-experimental-repl-awaitThis flag disables the experimental REPL (read-eval-print loop) await feature.
Simplified Explanation:
Imagine you're playing with code in a REPL, like a Python shell. Normally, you have to type in code and hit Enter to run it. With --no-experimental-repl-await, you can write code that looks like it's running immediately, even though it's actually running later.
For example:
Without this flag, you would have to write:
Real-World Example:
You're developing a server that uses asynchronous operations (e.g., database queries). You want to test some code in the REPL that depends on these operations. With this flag, you can write your code to look synchronous, making it easier to debug.
Applications:
Debugging asynchronous code in REPL
Writing REPL code that looks more like synchronous code
Making REPL code more readable and easier to understand
--no-experimental-websocket
This flag is used to disable experimental WebSocket support in Node.js. WebSocket is a protocol that allows for real-time communication between a web client and a server. It is often used for features such as chat, live updates, and multiplayer games.
By default, WebSocket support is enabled in Node.js, but it is still considered experimental. This means that it may not be as stable or reliable as other features, and it may change in future versions of Node.js.
If you are using WebSocket in your application, you can use this flag to disable experimental support. This may help to improve the stability and reliability of your application.
Here is an example of how to use the --no-experimental-websocket flag:
This will disable experimental WebSocket support in your script.
Here is a real-world example of where you might use the --no-experimental-websocket flag:
You are developing a production application that uses WebSocket. You want to ensure that your application is as stable and reliable as possible, so you disable experimental WebSocket support.
Potential Applications:
WebSocket can be used in a variety of real-world applications, including:
Chat: WebSocket can be used to create real-time chat applications. This allows users to send and receive messages in real time, without having to reload the page.
Live updates: WebSocket can be used to send live updates to users. This is often used for features such as stock tickers and news feeds.
Multiplayer games: WebSocket can be used to create multiplayer games. This allows players to interact with each other in real time, without having to wait for the server to send updates.
Simplified Explanation:
By default, when a serious error (known as a "fatal exception") occurs and causes your Node.js program to exit, Node.js displays a lot of extra information, such as the error stack trace.
The --no-extra-info-on-fatal-exception flag is used to hide this extra information. This can be useful if you want to keep your console output clean or if you're not interested in the details of the error.
Real-World Code Example:
To use the --no-extra-info-on-fatal-exception flag, simply pass it as an argument to the Node.js command when starting your program:
Applications in the Real World:
Debugging: When you're debugging a program, you may want to see the extra information displayed by default. However, once you've identified the source of the error, you can use the
--no-extra-info-on-fatal-exceptionflag to hide the extra information and keep your console output clean.Production Environments: In production environments, where stability is critical, you may want to use the
--no-extra-info-on-fatal-exceptionflag to minimize the amount of information displayed in case of an error. This can help prevent sensitive information from being leaked.
Topic: --no-force-async-hooks-checks
Explanation:
Async Hooks: A mechanism that allows Node.js to monitor and control asynchronous operations.
Runtime Checks: Tests that ensure async hooks are working properly.
Purpose of this Flag:
By default, Node.js performs runtime checks for async hooks when the
async_hooksmodule is enabled.This flag disables these runtime checks, making Node.js start faster.
Simplified Explanation:
Think of your Node.js program as a car.
Async hooks are like a mechanic under the hood, checking if the car is running smoothly.
Runtime checks are like additional safety checks that the mechanic performs.
This flag allows you to disable the extra safety checks, which makes the car start up faster, but may increase the risk of issues.
Real-World Example:
Suppose you have a large Node.js application that takes a long time to start.
You can use this flag to disable the runtime checks for async hooks, which may reduce the startup time.
However, be aware that disabling these checks could lead to problems later on, such as memory leaks or crashes.
Potential Applications:
Optimizing startup time for performance-critical applications.
Debugging issues related to async hooks.
Experimenting with different async hook implementations.
--no-global-search-paths
By default, Node.js searches for modules in global paths like $HOME/.node_modules and $NODE_PATH.
Using --no-global-search-paths tells Node.js not to search these paths, which can help speed up module resolution.
This is useful if you're developing a project that uses a lot of local modules, and you don't want Node.js to waste time searching global paths for modules that don't exist.
For example, if you have a project with the following directory structure:
And you run the following command:
By default, Node.js will search the following paths for the utils module:
./node_modules/utils../node_modules/utils$HOME/.node_modules/utils$NODE_PATH/utils
If you don't want Node.js to search the global paths, you can run the following command:
This will tell Node.js to only search the local node_modules directory for the utils module.
--no-network-family-autoselection
--no-network-family-autoselectionSimplified Explanation:
This option tells Node.js to not automatically choose which type of network (IPv4 or IPv6) to use when making connections. Instead, you will have to explicitly specify the network family in your connection options.
Explanation in Detail:
Network family refers to the type of network protocol used for communication, such as IPv4 or IPv6.
By default, Node.js automatically selects the network family based on the availability and preference of the operating system.
This option disables the autoselection algorithm, forcing you to manually specify the network family.
Real-World Complete Code Implementation:
Potential Applications in Real World:
Enforcing Network Family Preference: You may want to explicitly use IPv6 or IPv4 for security or performance reasons.
Troubleshooting Network Issues: Disabling autoselection can help identify network connectivity issues related to specific network families.
Optimizing Network Performance: In certain cases, manually selecting the network family can improve latency or throughput.
--no-warnings
--no-warningsThis option silences all process warnings, including deprecation warnings.
Example:
Explanation:
When you run a Node.js script, it may output warnings to the console. These warnings can be helpful for debugging, but they can also be annoying if you don't care about them. The --no-warnings option silences all warnings, so you won't see them in the console.
Real-world applications:
If you're running a Node.js script in a production environment, you may want to use the
--no-warningsoption to silence any warnings that might be output to the console.If you're writing a Node.js script that you plan to share with others, you may want to use the
--no-warningsoption to ensure that your users don't see any warnings when they run your script.
Simplified Explanation:
--node-memory-debug Flag:
This flag is like a detective for memory problems in Node.js.
Real World Implementation and Examples:
You work for Node.js and you're trying to find a memory leak in Node.js itself.
You turn on the
--node-memory-debugflag and run your tests.The flag helps you track down the source of the leak so you can fix it.
Potential Applications:
Debugging and fixing memory leaks in Node.js internals.
Improving the performance and stability of Node.js.
Simplified Explanation:
When you use Node.js, it connects to the OpenSSL library for secure communication. By default, Node.js uses its own preset OpenSSL configuration. However, you can customize OpenSSL's behavior by providing your own configuration file.
Topics and Simplified Explanations:
OpenSSL Configuration File:
A file that contains settings for how OpenSSL operates, such as enabling stronger encryption algorithms or FIPS compliance.
FIPS (Federal Information Processing Standards) Compliance:
A government standard for secure encryption. FIPS-compliant OpenSSL provides more secure encryption, but it can be slower than non-FIPS OpenSSL.
Real-World Applications:
Enhancing Security: You can use an OpenSSL configuration file to enable FIPS compliance, which is necessary for certain government and healthcare applications.
Customizing Cryptographic Algorithms: You can specify which encryption algorithms OpenSSL should use, which is useful for compatibility with older systems or for specific security requirements.
Example Code:
Assuming you have an OpenSSL configuration file named "openssl.cnf," you can use it with Node.js by running the following command:
Complete Implementation:
Here is a complete implementation using the example configuration file:
Potential Applications:
Government Systems: FIPS compliance is required for many government systems that handle sensitive data.
Healthcare Systems: Healthcare systems often require FIPS compliance for protecting patient data.
Financial Institutions: FIPS compliance can enhance security for financial transactions and data storage.
--openssl-legacy-provider
What it does:
Allows you to continue using the old version of OpenSSL (1.1) in Node.js, even though Node.js now supports the new version (3.0).
Why you might need it:
Some programs or libraries might still require OpenSSL 1.1. By enabling the legacy provider, you can keep using these older programs and libraries.
How to use it:
When starting Node.js, use the --openssl-legacy-provider flag:
Example:
Let's say you have a program that uses OpenSSL 1.1:
When you run this program in Node.js 18 or later, you might get an error like this:
This is because Node.js 18 now uses OpenSSL 3.0, which does not support the SHA-256 algorithm directly.
To fix this, you can enable the legacy provider:
This will allow the program to use OpenSSL 1.1 and run successfully.
Potential applications:
Maintaining compatibility with legacy programs and libraries that require OpenSSL 1.1.
Migrating programs from older versions of Node.js that used OpenSSL 1.1 to newer versions that use OpenSSL 3.0.
--openssl-shared-config
--openssl-shared-configSimplified Explanation:
When you use OpenSSL with Node.js, you can load the default OpenSSL configuration from a file called openssl.cnf. This file contains settings that control various aspects of how OpenSSL works.
By default, Node.js uses a separate configuration specific to Node.js called nodejs_conf. However, you can choose to share the default OpenSSL configuration instead by enabling the --openssl-shared-config option.
Real-World Example:
Suppose you want to configure OpenSSL to use a specific cipher suite for encrypting communications. You can do this by editing the openssl.cnf file and adding the following line:
If you then run Node.js with the --openssl-shared-config option, it will load this configuration and use the specified cipher suite.
Potential Applications:
Sharing the OpenSSL configuration can be useful in situations where you need to ensure consistency between OpenSSL applications. For example, if you have multiple Node.js applications that use OpenSSL, you can use the shared configuration to apply the same settings to all of them.
Usage:
To enable the --openssl-shared-config option, simply run Node.js with the following command:
Note:
It is generally recommended to use the Node.js-specific configuration, nodejs_conf, unless you have a specific need to share the OpenSSL configuration.
--pending-deprecation
--pending-deprecationSimplified Explanation:
Imagine your code uses a feature that will be removed in a future update. Normally, you wouldn't get any warnings until the update. However, with --pending-deprecation, you can get warnings now so that you can fix the code before it breaks.
Topics in Detail:
Pending Deprecation:
A pending deprecation is like a warning that a feature is going to be removed in the future. It's like getting a notice from the library you use saying, "Hey, this part will stop working soon, update your code."
Command-Line Flag:
The --pending-deprecation command-line flag tells Node.js to show pending depreciation warnings. You can use it when you run your code using node.
Environment Variable:
You can also set the NODE_PENDING_DEPRECATION environment variable to 1 to enable pending deprecation warnings. This is useful if you want to enable warnings even when you're not using the command-line flag.
Benefits of Pending Deprecation:
Early Warning: You get notified about potential problems before they break your code.
Grace Period: It gives you time to fix your code and avoid unpleasant surprises.
Real-World Example:
Let's say you're using a library that contains a function called calculateDistance(). In a future update, this function will be renamed to computeDistance(). If you don't update your code, it will break when the update happens.
With --pending-deprecation, you'll get a warning when you run your code now, telling you that calculateDistance() will be deprecated. This gives you the opportunity to change your code to use computeDistance() before the update, preventing a runtime error.
Code Implementation:
To enable pending deprecation warnings, you can use:
Or set the environment variable:
--policy-integrity=sri
Simplified Explanation:
Imagine you have a strict security policy for your code. This flag tells Node.js to check if the code you're running has the right 'security fingerprint' before allowing it to run. If the fingerprint doesn't match, it's like a red flag and Node.js will not run the code.
Technical Explanation:
This flag sets a policy for checking the integrity of code using Subresource Integrity (SRI). SRI is a way of ensuring that the code you're loading comes from a trusted source. It works by attaching a "fingerprint" to the code, which is a unique identifier for the code. When the code is loaded, the browser checks the fingerprint against the expected value. If the fingerprints don't match, the code will not be loaded.
Code Snippet:
Real-World Applications:
Prevent malicious code from running on your website or application.
Ensure that only trusted code is loaded from third-party sources.
Verify the authenticity of code updates.
What is the --preserve-symlinks flag?
When Node.js loads a module, it usually tries to find the real path to the module (the path without any symbolic links). This is done to avoid loading the same module multiple times if it is linked in multiple places.
However, sometimes you might want Node.js to use the symbolic link path instead of the real path. This can be useful if you are using symbolic links to share modules between different projects, or if you are using a module that relies on being linked in a specific location.
The --preserve-symlinks flag tells Node.js to use the symbolic link path for modules, instead of the real path. This can be useful in the following situations:
Sharing modules between projects: If you have two projects that use the same module, you can create a symbolic link to the module in each project. This way, both projects will use the same copy of the module, and you won't have to maintain two separate copies.
Using modules that rely on being linked in a specific location: Some modules may rely on being linked in a specific location in order to work properly. For example, a module may need to access a file that is located next to the module. If you use the
--preserve-symlinksflag, Node.js will use the symbolic link path for the module, which will allow the module to access the file it needs.
Real-world example
Let's say you have two projects, projectA and projectB, that both use the same module, moduleC. You can create a symbolic link to moduleC in each project, like this:
Now, both projects will use the same copy of moduleC, and you won't have to maintain two separate copies.
Potential applications
The --preserve-symlinks flag can be useful in a variety of situations, including:
Developing and testing modules: When you are developing and testing a module, you may want to use the
--preserve-symlinksflag to link the module to your project directory. This will allow you to test the module without having to install it globally.Sharing modules between projects: As mentioned above, the
--preserve-symlinksflag can be used to share modules between different projects. This can be useful if you have multiple projects that use the same set of modules.Using modules that rely on being linked in a specific location: As mentioned above, the
--preserve-symlinksflag can be used to use modules that rely on being linked in a specific location. This can be useful if you are using a module that needs to access a file that is located next to the module.
Code snippet
This will run the script my-script.js with the --preserve-symlinks flag enabled.
--preserve-symlinks-main
This flag tells the JavaScript engine to keep track of symbolic links when it's loading the main file of your program (the file that you run with node).
What are symbolic links?
Symbolic links are like shortcuts on your computer. They point to another file or directory, but they look like they're a regular file or directory themselves.
Why would you want to preserve symbolic links?
Let's say you have a file called main.js that you run with node. Inside main.js, you have this code:
If data.txt is a symbolic link to another file, the fs.readFileSync function will follow the link and read the contents of the other file.
However, if you run node with the --preserve-symlinks-main flag, the fs.readFileSync function will not follow the link. Instead, it will read the contents of the symbolic link itself.
When would you want to use --preserve-symlinks-main?
You might want to use this flag if you're running a script that needs to keep track of symbolic links. For example, a script that manages files and directories might need to know whether a file is a symbolic link or not.
Real-world example:
Let's say you have a script that backs up files to a remote server. You want the script to follow symbolic links so that it can back up all of the files that are linked to.
To do this, you would run the script with the --preserve-symlinks-main flag:
Potential applications:
Managing files and directories
Backing up data
Creating and managing symbolic links
-p, --print
The -p or --print flag is used with the node REPL (Read-Eval-Print-Loop) to print the result of the executed code.
How it Works:
When you use the -p flag, the REPL will execute the code you enter and print the result to the console. This is different from the default behavior of the REPL, which only executes the code.
Example:
In this example, the -p flag is used to print the result of the expression 1 + 2, which is 3.
Real-World Applications:
The -p flag can be useful for quickly checking the result of a code snippet without having to create a separate script. It can also be used to print the value of a variable or expression for debugging purposes.
Improved Code Example:
You can use the -p flag with more complex code snippets to evaluate the result. For example, you could use it to calculate the average of a list of numbers:
In this example, the -p flag is used to evaluate the expression ([1, 2, 3, 4, 5].reduce((a, b) => a + b, 0)) / 5, which calculates the average of the numbers in the list.
--prof
Purpose:
Helps you analyze the performance of your Node.js application by generating a profile that contains information about the code's execution.
How it works:
When you run your application with the --prof flag, Node.js will record a snapshot of the program's execution. This snapshot contains details about:
Functions called and how often they were executed
Time spent in each function
Memory usage
How to use it:
This will generate a file named your_script.prof in the current directory. You can open this file in a profiler tool, such as Google Chrome's Developer Tools, to analyze the performance data.
Real-world applications:
Identifying bottlenecks in your code
Optimizing performance by reducing the time spent in certain functions
Detecting memory leaks
Example:
Let's say you have a Node.js script that takes a long time to process a large dataset:
You can run this script with the --prof flag to generate a profile:
Opening the generated profile file in a profiler tool will show you details about the execution, including:
The
readFileSyncfunction is called once and takes a significant amount of time.The loop runs for a long time, suggesting that the
dataarray is large.
Based on this information, you can optimize the script by loading the dataset incrementally or using a more efficient algorithm for processing the data.
--prof-process
Simplified Explanation:
Imagine V8 as the engine that powers JavaScript in Node.js. When you use the --prof option while running V8, it generates a special file that contains information about how your JavaScript code is executing. The --prof-process option allows Node.js to read and process this file, giving you insights into your code's performance.
In-Depth Explanation:
Profiler Output: Using
--profwhen running V8 generates a CPU profile that records the time spent in different parts of your code. This file contains information about function calls, memory usage, and other performance metrics.Processing:
--prof-processtakes this profile and processes it, creating a summary report that highlights performance issues and bottlenecks in your code. It provides detailed statistics and visualizations of the profile data, making it easier to understand and optimize your application.
Real-World Implementation:
Consider the following simple JavaScript function:
To generate a profile for this function, run:
This will create a file named output.cpuprofile. Now, to process the profile and get insights:
Output (simplified):
This output shows that the function sumNumbers is the most time-consuming, with most of the time spent in the loop (line 7). It also provides detailed information about each line of code, including the percentage of time spent, total time taken, and the script name.
Applications:
Performance Tuning: Identify performance issues and optimize code for better speed and efficiency.
Code Profiling: Get insights into how different parts of your code are used and where bottlenecks occur.
Efficiency Assessment: Evaluate the performance of different algorithms or code structures to determine the most efficient approach.
--redirect-warnings=file
--redirect-warnings=fileSimplified Explanation:
This option tells Node.js to send all warning messages to a specific file instead of printing them on the screen.
Detailed Explanation:
When you run a Node.js program, any warning messages that occur during execution are printed to the standard error output (stderr), which is usually your console window. The --redirect-warnings option allows you to redirect these warnings to a file.
By specifying a filename after the option, Node.js will create the file if it doesn't exist or append to it if it does. If the file cannot be written to, the warnings will still be printed to stderr.
Code Snippet:
This command will run the script.js file and redirect all warnings to the warnings.txt file.
Real-World Applications:
Preserving Warning Messages: Redirecting warnings to a file allows you to keep a record of them for future reference or troubleshooting.
Centralized Logging: You can combine this option with other logging tools to centralize all warning messages in one location, making it easier to monitor and analyze.
Filtering Warnings: Some logging systems allow you to filter warnings based on severity or source, making it easier to focus on specific types of warnings.
Simplified Explanation:
Imagine your Node.js application generates reports that give you information about how it's running.
By default, these reports are displayed in a human-readable format, with multiple lines and clear labels for each piece of information. But sometimes, you may want these reports to be easier for computer systems to process, rather than for humans to read.
That's where the --report-compact flag comes in. When you use this flag, your reports will be generated in a simplified, single-line JSON format. This makes it much easier for log processing systems or other automated tools to parse and analyze the information in your reports.
Real-World Example:
Let's say you have a Node.js application that tracks user behavior and generates reports about how often certain features are used. By default, you might see a report like this:
With the --report-compact flag, you would get the same information in a single-line JSON format:
This simplified format makes it much easier for your system to analyze and track user behavior data, without needing to parse through multiple lines of text.
Applications:
Using --report-compact can be beneficial in situations where you need to:
Integrate report data into automated systems or processes
Feed report data into other analysis tools or dashboards
Store report data in a compact and easily searchable format
--report-dir=directory, report-directory=directory
--report-dir=directory, report-directory=directorySimplified Explanation:
Imagine you're doing a test run and want to see a report of all the tests you've run, along with their results. This option lets you choose where you want that report to be saved. You can specify a specific directory (folder) on your computer.
Technical Explanation:
This option sets the directory where the test report will be generated. The report typically includes information about the tests that were run, their results, and any errors or warnings encountered.
Code Example:
This command will generate a test report in the "test-reports" directory.
Real-World Application:
Test Reporting: Used in automated testing to create a report of the tests that were run and their results.
Code Coverage: Used to generate reports that show which parts of your code were executed during testing.
Error Analysis: Helps developers identify and fix errors and warnings encountered during testing.
Topic: --report-filename Option
Explanation:
This option allows you to specify the file name where a report about the command execution will be saved.
Usage:
Real-World Example:
Suppose you want to run a Node.js script and save a report of its execution in a file called "report.txt". You can use the following command:
This will generate a report file with information about the script execution, such as the time it took to run, any errors that occurred, and other details.
Potential Applications:
Debugging and troubleshooting: The report can help you identify any issues or problems with the script.
Performance analysis: The report can provide insights into the performance of the script, allowing you to optimize it for efficiency.
Compliance and auditing: The report can be used as evidence of the script's execution and its compliance with certain standards or requirements.
Simplified Explanation:
--report-on-fatalerror Flag:
This flag lets you get detailed information about any fatal errors that crash your Node.js application. Fatal errors are problems that happen inside Node.js itself, like running out of memory.
How it Helps:
The report provides valuable clues about the crash, such as:
The state of the heap memory
What your code was doing at the time of the crash
How the event loop was running
How much memory and other resources were being used
Real-World Use Case:
You're running a Node.js web server that's getting lots of traffic. One day, the server crashes with a fatal error. You can use the "--report-on-fatalerror" flag to find out why the crash happened so you can fix the problem.
Code Example:
To use the flag, just add it to the command you use to start your Node.js application:
Improved Example:
Let's say you're running a Node.js app that uses a database. You're getting a fatal error that looks like this:
You can use the "--report-on-fatalerror" flag to get more details about the error:
This report will show you how much memory was being used, what objects were allocated, and other details that can help you figure out why you're running out of memory.
--report-on-signal Option
--report-on-signal OptionThe --report-on-signal option in the Node.js CLI allows you to generate a report when the Node.js process receives a specific signal. This option is particularly helpful for debugging and troubleshooting issues in your Node.js application.
Usage:
Where:
[signal](optional): Specifies the signal to trigger the report. If not provided, the default signal isSIGUSR2.
Real-World Application:
Suppose you have a Node.js application that crashes intermittently due to an unknown error. To debug the issue, you can use the --report-on-signal option to generate a report when the application crashes. This report can provide valuable information about the state of the application at the time of the crash, helping you identify and fix the underlying issue.
Complete Code Implementation:
Upon receiving the SIGUSR2 signal, the Node.js process will generate a report and exit. The report can be found in the default output directory (usually the current directory). The report contains detailed information about the state of the application, including:
Stack traces of all threads
Heap snapshot
CPU profile
Performance timeline
By analyzing the report, you can gain insights into the application's behavior and identify potential issues that may have caused the crash.
--report-signal=signal
--report-signal=signalThis option allows you to set or reset the signal that triggers report generation. By default, the signal is SIGUSR2, but you can change it to any signal you want.
Simplified explanation:
Imagine you have a running Node.js application. You can send a signal to the application to tell it to generate a report. By default, the signal is SIGUSR2, but you can change it to something else if you want.
Real-world example:
Let's say you have a Node.js application that generates reports on a daily basis. You could use the --report-signal option to set the signal to SIGTERM (which is typically sent when a program is terminated). This would allow you to generate a report by sending a SIGTERM signal to the application.
Code example:
This would set the report signal to SIGTERM for the application my-app.js.
Potential applications:
Generating reports on demand
Triggering report generation from other processes or scripts
Automating report generation
Simplified Explanation:
--report-uncaught-exception:
This flag helps you investigate errors in your JavaScript code. When an unhandled exception occurs, the process usually crashes and exits. With this flag, Node.js will generate a report before exiting. This report includes the JavaScript stack trace, which can be useful for debugging.
Real-World Example:
Suppose you have a JavaScript program that crashes due to an unhandled exception. Without the --report-uncaught-exception flag, you might not know exactly where the error occurred. By adding the flag, Node.js will generate a report that includes the following information:
JavaScript stack trace
Native stack trace
C++ crash site
Operating system details
Node.js version
Using this report, you can easily identify the line of code that caused the error and fix it.
Potential Applications:
Debugging production errors: This flag is especially useful for investigating errors that occur in production environments, where you don't have access to the console logs.
Understanding native crashes: If your JavaScript code crashes within a native module, the native stack trace can help you understand the underlying issue.
Improving error reporting: You can use the report generated by this flag to create custom error reporting systems that provide more detailed information to your users.
Improved Code Example:
To use this flag, simply add it to the command line when running your Node.js program:
If an uncaught exception occurs, Node.js will generate a report in a file named exception.txt in the current directory.
Simplified Explanation:
The -r or --require option in Node.js allows you to load a specific module before your main program starts running.
Detailed Explanation:
Preloading Modules: A module is a piece of code that provides specific functionality, like reading a file or connecting to a database. By using
-ror--require, you can tell Node.js to load a module at startup, so it's ready to use when your program needs it.Module Resolution: Node.js follows certain rules to find the module you want to load. It first checks if the module name you provide is a path to a file. If not, it treats it as a name of a module installed in your system or in the current project directory.
CommonJS Modules: Node.js currently supports CommonJS modules, which use the
require()function to load modules.
Code Snippet:
In this example, Node.js will load the my-module.js module before running the main.js script.
Real-World Applications:
Preloading a module that establishes a database connection to make it available for all pages of a web application.
Loading a module that provides logging functionality to track errors and debug issues during development.
Advantages of Preloading Modules:
Improved performance since the module is loaded once at startup, instead of every time it's needed.
Reduced code duplication if the module is used multiple times in the program.
Easier maintenance as module initialization is done in one place.
What is a Secure Heap?
A secure heap is like a special safe place in your computer's memory. It stores secret information securely, like the password to your favorite website. This way, if someone tries to peek at your secrets, they won't find them in the regular memory, but in this special protected place.
How Does it Work?
The secure heap is like a locked box. When you put secrets in it, they are encrypted, which means they are scrambled so that no one can read them without the right key. When you need to use your secrets, the secure heap unlocks them with the key and gives them to you.
Why is it Important?
Sometimes, computer programs have bugs that can allow people to peek at memory where secrets might be stored. If the secrets are stored in the regular memory, the peekers might be able to see them. But if the secrets are stored in the secure heap, they will be safe, even if the program has bugs.
How to Use it in Node.js
To use the secure heap in Node.js, you need to tell Node.js that you want to use it by adding this line to your code:
This line creates a secure heap of 16384 bytes. You can choose a different size if you need more or less space.
Real-World Example
Imagine you have a website that stores your credit card information. You want to make sure that your credit card information is not stolen if someone hacks your website. One way to do this is to store the credit card information in a secure heap. This way, even if the hackers find a way to hack your website, they won't be able to see your credit card information.
Potential Applications
Secure heaps can be used to store any type of secret information, such as:
Passwords
Credit card numbers
Social Security numbers
Health information
Business secrets
Benefits of Using Secure Heaps
Protects sensitive information from hackers
Prevents memory leaks
Improves program security
--secure-heap-min Flag
--secure-heap-min FlagExplanation:
Node.js has a special type of memory called "secure heap" that is used to store sensitive data, like passwords and encryption keys.
The
--secure-heap-minflag lets you specify the minimum amount of secure heap memory that Node.js should allocate.This helps protect your sensitive data from being accessed by other parts of your program or by attackers.
Real-World Implementation:
Potential Applications:
Securing sensitive data in web applications, such as user passwords or financial information.
Protecting encryption keys used to encrypt data stored in databases or files.
Preventing sensitive information from being leaked or stolen by malicious code.
Simplified Explanation:
Snapshot Blob:
A snapshot blob is a file that stores the saved state of a running Node.js application. It's like a photograph of the application at a specific point in time.
Building a Snapshot Blob:
You can use the --build-snapshot command-line option to create a snapshot blob. The --snapshot-blob option specifies where to save the blob file. For example:
This will save the snapshot blob to the file my-snapshot.blob in the current directory.
Restoring from a Snapshot Blob:
To restore an application from a snapshot blob, use the --snapshot-blob option without --build-snapshot:
This will load the saved state from my-snapshot.blob and start the application.
Compatibility Checks:
When loading a snapshot, Node.js checks that:
The version of Node.js that built the snapshot blob matches the version you're using.
The architecture (32-bit or 64-bit) and platform (Windows, macOS, Linux) match.
The V8 flags and CPU features are compatible.
If any of these checks fail, Node.js will not load the snapshot and will exit with an error message.
Real-World Applications:
Fast Startup: Snapshot blobs can significantly speed up the startup time of Node.js applications by preserving the application's state instead of having to reload all modules and data from scratch.
Resiliency: Snapshot blobs can help make applications more resilient against crashes by allowing them to be quickly restored to a known good state.
Debugging: Snapshot blobs can be used for debugging purposes, as they provide a way to inspect the state of an application at a specific point in time.
--test flag in Node.js CLI
--test flag in Node.js CLISimplified Explanation:
The --test flag tells Node.js to run your test scripts using the built-in test runner. It's a convenient way to run and see the results of your tests from the command line.
Usage:
To use the --test flag, type the following in your terminal:
Example:
Let's say you have a test script named test.js in the following directory:
To run this test script using the --test flag, type:
This will start the test runner and execute your tests. The results will be displayed in the terminal window.
Benefits of Using --test:
Convenience: Allows you to run tests easily from the command line without having to set up a separate testing environment.
Test Automation: Helps automate the testing process, making it easier to run tests regularly and check for errors.
Test Visibility: Provides a clear overview of test results, making it easy to identify failures.
Potential Applications:
Continuous Integration (CI): Automate testing as part of a CI pipeline to ensure code changes don't break existing functionality.
Test-Driven Development (TDD): Use the
--testflag to run tests while writing code to catch errors early on.Regression Testing: Regularly run tests using
--testto verify that previous fixes don't introduce new problems.
--test-concurrency explained:
--test-concurrency explained:In simple terms:
It's like having a team of helpers running your tests. This option sets how many helpers can work at the same time. By default, it's set to the number of available processors on your computer, minus one.
Detailed explanation:
The --test-concurrency option in the test-runner CLI controls how many test files are executed concurrently.
Test files: Each
.test.jsor.test.tsfile in your project is considered a test file.Concurrency: Running tests concurrently means that multiple test files can be executed at the same time.
Default value: The default value for
--test-concurrencyisos.availableParallelism() - 1. This means that the test runner will attempt to use all available processors on your computer, minus one.
Benefits of increasing concurrency:
Faster testing: Running more tests in parallel can make your testing process faster.
Considerations when increasing concurrency:
Resource usage: Running more tests in parallel can use more memory and CPU resources.
Test stability: Some tests may not be designed to run concurrently and may fail when executed in parallel.
Real-world applications:
Large test suites: If you have a large number of test cases, increasing concurrency can significantly reduce the testing time.
Fast feedback loops: Increasing concurrency can make it easier to get quick feedback on code changes, as tests run faster.
CI/CD pipelines: Optimizing test concurrency can improve the efficiency of CI/CD pipelines.
Simplified example code:
Complete example code:
--test-name-pattern
--test-name-patternSimplified Explanation:
Imagine you have a lot of tests in your project. You can use this option to only run the tests that you're interested in.
In Plain English:
It's like when you're at a party and you only want to talk to people whose names start with "A". You can use this option to do the same thing with your tests.
Code Snippet:
Real World Application:
You can use this option to run a specific group of tests, such as all the tests in a particular file or all the tests for a particular feature.
This can be useful when you're debugging or when you're working on a specific part of your project and you only want to run the tests that are relevant to what you're working on.
--test-only
--test-onlyIf you want to run only the tests that have the only option set, you can use the --test-only flag with the test command. This will ensure that only the tests that you want to run are executed.
Example:
This will run only the tests that have the only option set.
Potential applications in real world:
When you are debugging a test and you only want to run a specific test, you can use the
--test-onlyflag to specify the test that you want to run.When you are working on a large test suite and you only want to run a specific set of tests, you can use the
--test-onlyflag to specify the tests that you want to run.
Simplified Explanation:
Test Reporter
When you run tests in Node.js, you can use a test reporter to display the results in a specific format. Reporters can make the test results more readable, organized, and easy to understand.
Example:
To use a test reporter, type this command in the terminal:
Potential Applications:
Quickly troubleshooting failed tests
Monitoring test progress
Creating custom reports for team collaboration
Improving code coverage and quality
--test-reporter-destination
This option allows you to specify where the output of a test reporter should be sent. This is useful if you want to redirect the output to a file or another location.
Example:
This will send the output of the test reporter to the file test-results.txt.
Real-world application:
This option can be useful if you want to save the output of a test reporter for later analysis or if you want to send the output to a specific location, such as a remote server.
Test Sharding
What is it?
Test sharding is a way to divide your test suite into smaller parts, so that you can run them in parallel. This can make your tests run faster, especially if you have a large test suite.
How to use it?
To use test sharding, you need to specify the --test-shard option when you run your tests. The format of the option is <index>/<total>, where:
<index>is the index of the shard you want to run<total>is the total number of shards
For example, to divide your test suite into three parts and run the first part, you would use the following command:
Real-world example
Let's say you have a test suite with 100 tests. Running all of these tests in a single process could take a long time.
By using test sharding, you could divide your test suite into 10 shards and run them in parallel. This would reduce the time it takes to run your tests by a factor of 10.
Potential applications
Test sharding is useful for any project with a large test suite. It can be especially beneficial for projects that use continuous integration (CI) to run their tests automatically.
Simplified explanation
Imagine you have a big puzzle with 100 pieces. It would take a long time to put the puzzle together all at once.
But if you divide the puzzle into 10 smaller groups of 10 pieces each, it would be much easier to put together.
Test sharding works the same way. By dividing your test suite into smaller parts, you can run them in parallel and make your tests run faster.
Improved code snippet
Simplified explanation of --test-timeout:
Purpose:
--test-timeout is a flag that you can use when running tests with Node.js's testing framework. It sets a maximum time limit for each test to run.
How it works:
If a test takes longer than the specified time limit, the test will fail. This is useful to prevent tests from running indefinitely and hanging your system.
Default value:
By default, the time limit for tests is set to Infinity, which means that tests can run for as long as they need to.
How to use it:
You can specify the --test-timeout flag when running tests using the npm test command. For example:
This will set the time limit for all tests to 5 seconds.
Real-world applications:
--test-timeout is useful for:
Preventing tests from hanging your system indefinitely.
Ensuring that tests run quickly and efficiently.
Identifying tests that are taking too long to run and may need to be optimized.
--throw-deprecation Option
--throw-deprecation OptionThe --throw-deprecation option in the Node.js CLI module causes the CLI to throw errors when it encounters deprecated code. This can be useful for identifying and fixing deprecated code in your projects.
Here is a simple example of using the --throw-deprecation option:
If the my-script.js file contains any deprecated code, the CLI will throw an error like the following:
You can fix the deprecated code by updating it to use the recommended syntax. In this case, you would update the code to use console.error().
Potential Applications
The --throw-deprecation option can be useful in several situations:
Identifying deprecated code: You can use the option to identify deprecated code in your projects. This can help you to update your code to use the latest recommended syntax.
Enforcing code standards: You can use the option to enforce code standards in your projects. For example, you could require all developers to use the latest recommended syntax by using the
--throw-deprecationoption in your project's build scripts.Improving code quality: Using the latest recommended syntax can help to improve the quality of your code. This can make your code easier to read, maintain, and debug.
--title=title
--title=titleThis sets the title of the running Node.js process, which is displayed in process monitoring tools like the Activity Monitor on macOS or the Task Manager on Windows.
Simplified Example
Imagine you have a Node.js script named my-script.js that you want to run with a specific title. You can use the --title flag to set the title:
Now, when you run the my-script.js script, the process title will be displayed as "My Awesome Script" in the process monitoring tool.
Real-World Application
This flag can be useful for tracking and identifying Node.js processes, especially when running multiple scripts or services on a system. By setting custom titles, you can easily distinguish between different processes and their purposes.
Simplified Explanation:
The --tls-cipher-list option lets you choose which encryption methods (called "ciphers") your TLS connection will use. TLS is a security protocol that keeps your data private when you connect to a server over the internet.
Topics in Detail:
TLS Cipher List:
A list of encryption methods that your TLS connection can use. Different ciphers provide different levels of security.
Alternative Default Cipher List:
By default, your TLS connection uses a specific set of ciphers. This option lets you change that default list to one that you specify.
Node.js with Crypto Support Required:
This option only works if your version of Node.js was built with "crypto" support enabled. This is the default setting, so you usually don't need to worry about it.
Code Snippet:
To use this option, append it to the node command when you run your script. For example:
This will force your script to use only the AES128-SHA256 cipher for its TLS connections.
Real-World Applications:
In the real world, this option can be used to:
Improve security: By specifying a stronger cipher list, you can make your TLS connections more resistant to attacks.
Fix compatibility issues: Some servers may not support all ciphers. By specifying a cipher list that the server supports, you can ensure that your connection will work.
Meet security regulations: Some industries have specific regulations regarding which ciphers can be used. This option allows you to comply with those regulations.
--tls-keylog=file
--tls-keylog=fileSimplified Explanation
When you establish a secure connection over the internet, the TLS protocol is used to keep your data private. TLS uses encryption keys to protect the information being sent.
This flag allows you to log the encryption keys used in your TLS connections to a file. These key logs can be used by special tools to decode the encrypted traffic and analyze its contents.
Code Example
Real-World Applications
Key logs can be useful for:
Troubleshooting TLS connections
Analyzing network traffic
Debugging security issues
Spying on encrypted data (with appropriate permissions)
Topic: --tls-max-v1.2
Explanation:
Imagine you have a castle with a drawbridge that allows people to enter. In this castle, you have different versions of the drawbridge: TLSv1.2 and TLSv1.3.
TLSv1.2: This is an older version of the drawbridge that is still secure. It's like a wooden drawbridge that is strong but not as strong as the newer one.
TLSv1.3: This is a newer version of the drawbridge that is even more secure. It's like a metal drawbridge that is harder to break into.
By default, your castle allows people to enter using both the wooden and metal drawbridges. But sometimes, you might want to only allow people to use the metal drawbridge for extra security. This is what --tls-max-v1.2 does.
Usage:
To enable this setting, you would use the following command:
Real-World Applications:
This setting is useful when you want to enforce the use of the most secure TLS version possible. For example, if you are running a website that handles sensitive data, you might want to disable support for TLSv1.2 to prevent potential attacks.
Code Example:
The following code demonstrates how to use the --tls-max-v1.2 setting:
In this example, the HTTPS server is configured to accept connections using TLSv1.2 as the maximum version. Any connections attempting to use TLSv1.3 will be rejected.
--tls-max-v1.3
--tls-max-v1.3This flag sets the default value of the tls.DEFAULT_MAX_VERSION property to 'TLSv1.3'. By default, TLSv1.3 is not enabled in Node.js. This flag allows you to enable it.
TLSv1.3 is the latest version of the TLS protocol. It offers improved security and performance over previous versions. By enabling TLSv1.3, you can take advantage of these improvements.
Example usage:
Real-world applications:
Enabling TLSv1.3 can improve the security of your Node.js applications.
TLSv1.3 can also improve the performance of your applications by reducing the amount of time it takes to establish a secure connection.
Explanation:
The --tls-min-v1.0 option in Node.js's CLI module sets the minimum TLS version that your application will accept. TLS (Transport Layer Security) is a protocol that provides secure communication over a network. By default, Node.js supports TLS versions 1.2 and 1.3. However, some older clients or servers may only support TLS version 1.0. This option allows you to enable compatibility with these older systems.
Simplified Explanation:
Imagine you're sending a secret message to your friend. You want to make sure that no one else can read your message, so you use a special code to encrypt it. The code you use is like a key that only you and your friend know.
TLS is like that code. It encrypts your communication so that only the people who are supposed to read it can do so. By using this option, you're setting the minimum level of encryption that your application will accept. This means that any devices that use an older, less secure version of TLS will not be able to communicate with your application.
Code Snippet:
Real-World Example:
This option is useful in situations where you need to support legacy devices or systems that may not have been updated to use the latest TLS versions. For example, if you have an old web server that only supports TLS 1.0, you can use this option to allow your Node.js application to communicate with it securely.
Potential Applications:
Enabling compatibility with legacy systems
Ensuring secure communication with devices that use older TLS versions
Protecting sensitive data from eavesdropping
--tls-min-v1.1
Simplified Explanation:
This option sets the minimum version of TLS that Node.js will use when making or accepting secure connections. By default, Node.js uses TLSv1.2, which is a more secure and modern version of TLS. However, if you need to connect to older systems that only support TLSv1.1, you can use this option to enable compatibility.
Technical Details:
TLS (Transport Layer Security) is a protocol that encrypts data sent over a network, making it secure from eavesdropping.
TLSv1.1 is an older version of TLS that is still used by some systems.
TLSv1.2 is a more secure version of TLS that is the default in Node.js.
Code Snippet:
Real-World Example:
You need to connect to a server that only supports TLSv1.1.
You have a legacy application that requires TLSv1.1 for compatibility.
Potential Applications:
Connecting to older devices or systems that use TLSv1.1.
Ensuring compatibility with legacy applications that require TLSv1.1.
Simplified Explanation:
The --tls-min-v1.2 option sets the minimum version of TLS that your Node.js application will accept for secure connections. TLS (Transport Layer Security) is a protocol that encrypts data sent over the internet, ensuring its privacy and integrity.
What is TLS and Why is it Important?
TLS is like a secret code that scrambles data sent over the internet, making it difficult for anyone else to read it. It's important because it protects sensitive information like passwords, credit card numbers, and private messages from getting into the wrong hands.
What does --tls-min-v1.2 do?
TLS has different versions, with newer versions being more secure. By default, Node.js accepts TLS versions 1.0 and above, which are outdated and could be vulnerable to attacks. The --tls-min-v1.2 option forces Node.js to only accept TLS version 1.2 or higher, which is more secure.
Why use --tls-min-v1.2?
Using --tls-min-v1.2 makes your Node.js applications more secure by preventing them from using weak TLS versions. This is especially important for applications that handle sensitive data or connect to external services over the internet.
Real-World Example:
Consider an e-commerce website that collects customer information and payment details. By setting --tls-min-v1.2, the website ensures that the data sent between the customer's browser and the website's server is encrypted using a strong TLS version, protecting it from eavesdropping or interception.
Code Example:
Potential Applications:
The --tls-min-v1.2 option can be used in a variety of applications that require secure data transmission, including:
E-commerce websites
Online banking systems
Cloud computing platforms
Social networking applications
Healthcare systems
Simplified Explanation:
Imagine the internet as a road with different levels of security, like TLS versions. TLSv1.2 is like an older, less secure road, while TLSv1.3 is like a newer, more secure road.
This option sets the default security level for your app to be TLSv1.3, which is the most secure option. By doing so, your app avoids using the less secure TLSv1.2 level.
Code Snippet:
To use this option, add it to the command you use to start your Node.js app:
Real-World Applications:
This option is essential for apps handling sensitive data or that need to meet high-security standards. By ensuring that your app only uses TLSv1.3, you protect it against potential attacks and data breaches that exploit vulnerabilities in older TLS versions.
Summary:
What it does: Sets the default security level for your app to be the most secure option, preventing it from using less secure TLS versions.
Why it's useful: Protects your app against potential attacks and data breaches that exploit vulnerabilities in older TLS versions.
--trace-atomics-wait
--trace-atomics-waitSimplified Explanation:
This flag helps you track how Atomics.wait() is used in your Node.js program. When enabled, it prints information about when and why Atomics.wait() is called to the console.
Detailed Explanation:
What is Atomics.wait()?
Atomics.wait() is a method used in Node.js to pause the execution of a thread until a shared memory location changes to a specified value or a timeout occurs. It's used for synchronization in multithreaded programs, where multiple threads need to coordinate their actions.
How does --trace-atomics-wait work?
When you run Node.js with --trace-atomics-wait, it prints summary information about each call to Atomics.wait(). These summaries include:
The thread that made the call
The memory location being waited on
The expected value of the memory location
The timeout specified (or "inf" if there was no timeout)
The reason why the wait ended (e.g., timeout, value mismatch, or being woken up by another thread)
Real-World Example:
When to use --trace-atomics-wait:
You might use --trace-atomics-wait if you're having issues with synchronization in your multithreaded program. It can help you identify if Atomics.wait() is being used correctly and if there are any potential problems, such as deadlocks or race conditions.
Potential Applications:
Debugging multithreaded programs: Identify synchronization issues
Performance optimization: Track wait times to find performance bottlenecks
Understanding thread behavior: See how threads are interacting with shared memory
Simplified Explanation:
--trace-deprecation Option:
When you use certain features in Node.js that are no longer recommended (known as "deprecations"), the --trace-deprecation option will print out a stack trace (a list of where the feature was used in your code). This helps you identify where you need to update or remove the deprecated features.
Real-World Applications:
When updating older code to use newer versions of Node.js, you may see deprecation warnings.
By using
--trace-deprecation, you can easily find the source of the deprecation and fix it accordingly.This helps ensure your code is using the latest and most secure practices.
Improved Example:
If your_script.js uses a deprecated feature, the console output will look like this:
The stack trace will show you the specific line of code that caused the deprecation:
This helps you easily locate the deprecated code and replace it with the recommended alternative.
--trace-event-categories is a command-line option that allows you to specify which categories of events should be traced when trace event tracing is enabled. Trace event tracing is a tool that can be used to record and analyze performance and debugging information about your application.
By default, trace event tracing is not enabled. To enable it, you can use the --trace-events-enabled command-line option. Once trace event tracing is enabled, you can use the --trace-event-categories option to specify which categories of events should be traced.
Each category corresponds to a different type of event that can occur in your application. For example, there is a category for network events, a category for disk events, and a category for user interface events.
By specifying which categories of events should be traced, you can control the amount of data that is recorded by trace event tracing. This can be useful if you are only interested in tracing a specific type of event.
Here is an example of how to use the --trace-event-categories option:
This command will enable trace event tracing and will record all network and disk events that occur in your application.
Trace event tracing can be a valuable tool for debugging and performance analysis. By using the --trace-event-categories option, you can control the amount of data that is recorded and focus on the events that are most relevant to your investigation.
Simplified Explanation:
--trace-event-file-pattern is a way of telling Node.js where to find the trace data it should use to debug your code. It's like a pattern that matches the filename of the trace data file.
Topics:
Template String: It uses a special string that allows you to include variables in the filename pattern.
Variables: The variables it supports are:
${rotation}: The number of times the trace data file has been rotated (i.e., new files are created as the old ones fill up).${pid}: The process ID (PID) of the Node.js process generating the trace data.
Example:
Suppose you have a Node.js process with PID 1234 that generates trace data files named trace-1234-0.json. The following pattern would match these files:
Real World Applications:
Debugging: You can use trace data to identify performance bottlenecks or other issues in your code.
Profiling: Trace data can help you understand how your code is executing and where it's spending most of its time.
Example Implementation:
To enable trace event recording with the specified file pattern, you can use the following code:
--trace-events-enabled
--trace-events-enabledEnables the collection of trace event tracing information.
Simplified Explanation
Imagine your program as a car. Trace events are like breadcrumbs that you drop along the way as your program runs. These breadcrumbs help you trace the path that your program took, and identify any bottlenecks or issues that may have slowed it down.
Real-World Example
Suppose you have a program that takes a long time to load a large file. By enabling trace events, you can see exactly where the time is being spent, and identify any potential optimizations.
Code Implementation
Potential Applications
Debugging performance issues
Identifying bottlenecks
Optimizing code
Understanding program behavior
--trace-exit
--trace-exitThis option prints a stack trace whenever an environment is exited proactively, i.e. invoking process.exit().
Imagine you have a script that runs a long-running process. If the script exits unexpectedly, you may not know why it happened. The --trace-exit option can help you debug this issue by printing a stack trace when the script exits.
Here's an example:
If the script exits with an error, the stack trace will be printed to the console:
This stack trace shows that the script exited with an error code of 1. The error was caused by the ChildProcess object emitting an exit event.
The --trace-exit option can be helpful for debugging scripts that exit unexpectedly. It can also be used to track the execution of long-running processes.
--trace-sigint
--trace-sigintThis flag is used to print a stack trace when the user presses Ctrl+C (SIGINT). This can be helpful for debugging purposes, as it can show where the program was when the user interrupted it.
For example, the following command will run the node program with the --trace-sigint flag:
If the user then presses Ctrl+C, the following output will be printed:
This output shows that the program was in the run_main_module function when the user pressed Ctrl+C.
Real-world applications
The --trace-sigint flag can be useful for debugging programs that crash or hang when the user presses Ctrl+C. By printing a stack trace, it can help the developer to identify the source of the problem.
For example, if a program crashes when the user presses Ctrl+C, the developer can use the --trace-sigint flag to see where the program was when it crashed. This can help the developer to fix the bug that is causing the crash.
Topic: Synchronous I/O
Simplified Explanation:
Imagine a waterpark with a set of slides. Water slides are like your program's tasks that need to be done. Synchronous I/O is like a kid who insists on going down a specific slide only and will wait until it's their turn. This can slow down the entire waterpark (or your program).
Technical Explanation:
Synchronous I/O means that a program pauses and waits for a particular input or output operation to complete before continuing. This can be inefficient, especially in a Node.js application where the main event loop is responsible for handling all asynchronous tasks efficiently.
--trace-sync-io Option:
This option prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop. In other words, it helps you identify which parts of your code are using synchronous I/O and potentially slowing down your program's execution.
Code Snippet:
This will print a stack trace similar to the following whenever synchronous I/O is detected:
Real-World Applications:
Identifying and removing synchronous I/O from your code can improve the performance and responsiveness of your Node.js applications, especially when handling multiple requests concurrently. For example:
In a web server, synchronous I/O can cause requests to become backed up and slowed down.
In a data processing pipeline, synchronous I/O can create bottlenecks that limit the overall throughput.
By using the --trace-sync-io option, you can quickly identify and address sources of synchronous I/O in your codebase, helping to optimize your applications and improve their performance.
Topic: Tracing TLS Packets
Simplified Explanation: When you connect to a website using HTTPS, your computer and the website exchange information securely using a protocol called TLS. TLS ensures that your data is protected from eavesdroppers and hackers.
The --trace-tls flag lets you see a detailed record of all the TLS packets exchanged during the connection. This can be useful for debugging problems with TLS, such as certificate errors or connection failures.
Real-World Example: Suppose you're trying to access a website, but you're getting a certificate error. You could use the --trace-tls flag to see exactly what TLS packets are being exchanged and identify the source of the problem.
Code Snippet:
Potential Applications:
Debugging TLS connection problems
Verifying the security of a TLS connection
Analyzing TLS traffic patterns
Simplified Explanation:
What is --trace-uncaught?
It is a command-line option for Node.js that makes it show more information about errors that happen during a program's execution.
Why is this useful?
When an error occurs, Node.js usually only shows the error message and the line of code where the error was created. --trace-uncaught also shows the line of code where the error was actually thrown, which can be helpful for debugging.
Real-World Example:
Imagine the following code:
When you run this code without --trace-uncaught, you will only see the following error message:
This error message tells us that an error occurred in the divide function, but it doesn't tell us where the error was actually thrown.
If we run the code with --trace-uncaught, we will see the following:
As you can see, the stack trace now includes an additional line:
This line tells us that the error was actually thrown in the REPL (Read-Eval-Print-Loop) environment, on line 6. This additional information can be very helpful for debugging, as it can point us to the specific line of code that caused the error.
Potential Applications:
--trace-uncaught is a valuable tool for debugging Node.js programs, especially when dealing with errors that are difficult to track down. It can help you quickly identify the source of the error and resolve it.
--trace-warnings
--trace-warningsIf enabled, any warning or deprecation emitted by the Node.js process will include the stack trace of where the warning or deprecation was triggered. This can be useful for debugging purposes, as it can help to identify the exact location in the code where the warning or deprecation was generated.
To use --trace-warnings, simply add it to the command line when starting your Node.js application:
Example:
In this example, the --trace-warnings flag has been used to print the stack trace of a deprecation warning that was emitted by the fs.exists() method. The stack trace shows that the deprecation was triggered on line 3 of the my-script.js file.
Potential Applications
--trace-warnings can be useful for debugging purposes, as it can help to identify the exact location in the code where a warning or deprecation was generated. This can be helpful for understanding why a warning or deprecation is being emitted, and for taking steps to address the issue.
Node.js CLI Module Option: --track-heap-objects
Explanation:
This option tells Node.js to keep track of all objects allocated on the JavaScript heap. This information is used to create heap snapshots, which can help you identify memory leaks and performance issues.
Simplified Example:
Imagine your computer's memory is like a museum filled with different exhibits (objects). --track-heap-objects acts like a tour guide that records every new exhibit added to the museum. Later, you can use the tour guide's notes (heap snapshots) to find any exhibits that are not in use and can be removed to free up space (memory).
Code Snippet:
You can use the --track-heap-objects option when starting Node.js:
Real-World Applications:
Memory Leak Detection: Identify objects that are not being used anymore but still taking up memory.
Performance Optimization: Find bottlenecks in your code by tracking how objects are allocated and released.
Code Debugging: Trace object creation and destruction to identify potential issues.
Potential Applications:
Web development: Improve the performance and stability of web applications.
Server-side programming: Optimize memory usage and prevent memory leaks in server applications.
Data analysis: Track object allocations to analyze memory patterns and improve efficiency.
Unhandled Rejections
When a JavaScript program has an error that isn't handled, it's called an "unhandled rejection." In Node.js, we can control how these errors are handled using the --unhandled-rejections flag.
Modes
The --unhandled-rejections flag lets us choose how unhandled rejections should be treated:
throw: This is the default setting. If an unhandled rejection occurs, it's thrown as an uncaught exception. If there's an
unhandledRejectionevent listener, it's called before the exception is thrown.strict: This setting is stricter than "throw." An unhandled rejection is always thrown as an uncaught exception. Even if there's an
unhandledRejectionevent listener, it's called after the exception is thrown.warn: This setting prints a warning message when an unhandled rejection occurs. However, it doesn't throw an exception or call the
unhandledRejectionevent listener.warn-with-error-code: This setting is similar to "warn," but it also sets the process exit code to 1. This means that if an unhandled rejection occurs, the program will exit with an error code.
none: This setting silences all warnings and exceptions. Unhandled rejections are ignored.
Examples
Real-World Applications
Preventing Uncaught Exceptions: The "throw" and "strict" modes ensure that unhandled rejections are always caught and handled properly. This can prevent the program from crashing or behaving unexpectedly.
Debugging: The "warn" and "warn-with-error-code" modes provide a way to track down and debug unhandled rejections.
Silencing Warnings: The "none" mode can be used to silence warnings about unhandled rejections in cases where they are not relevant or are causing clutter in the output.
Topic: Managing Certificate Authorities (CAs) for TLS/SSL connections
TLS/SSL connections are used to secure communication between devices, such as a web browser and a server. To establish these secure connections, devices rely on a trusted third party called a Certificate Authority (CA).
CAs issue digital certificates that verify the identity of a device and allow it to communicate securely.
nodejs's --use-bundled-ca and --use-openssl-ca options allow you to choose between two methods for managing CAs:
1. --use-bundled-ca:
Uses a snapshot of the Mozilla CA store that is packaged with Node.js.
This store is fixed at the time Node.js is released and is the same for all platforms.
It provides a consistent and secure CA store that is not affected by external changes.
Example:
Applications:
Suitable for applications that require a consistent and reliable CA store that is not dependent on external updates.
Can be used for secure connections in environments where external CA management is not feasible or desirable.
2. --use-openssl-ca:
Uses the default CA store managed by the OpenSSL library.
This store can be modified by system administrators or distribution maintainers.
It allows for flexibility and customization of the CA store, as updates and modifications can be applied as needed.
Example:
Applications:
Suitable for applications that require fine-tuned control over the CA store or need to use the most up-to-date CAs.
Can be used in environments where external CA management is necessary or preferred.
Real-World Code Example:
In this example, the first HTTPS request uses the bundled CA store by specifying a certificate file. The second request uses the OpenSSL CA store by leaving the ca option unset.
What is Large Page Mapping?
Imagine your computer's memory as a bookshelf. Regular memory pages are like small shelves, each holding a few words of your program. Large memory pages are like giant shelves, each holding a whole chapter or more.
Why Use Large Pages?
If your program is large, it can take a long time to search through all the small shelves to find the words it needs. By using large pages, your program can quickly flip through the giant shelves and find what it needs faster.
--use-largepages Option
The --use-largepages option allows you to tell Node.js to use large memory pages if the operating system supports it. This can improve the performance of Node.js applications that require large amounts of memory.
Values for mode
The mode parameter can have three values:
off: Do not use large memory pages.on: Attempt to use large memory pages. Print an error message if mapping fails.silent: Attempt to use large memory pages. Do not print an error message if mapping fails.
Real-World Applications
Large memory pages can be beneficial for applications that:
Process large datasets in memory
Run complex simulations
Perform image processing
Host large websites with many visitors
Example Usage
To run a Node.js application with large memory pages, use the following command:
Potential Benefits
Using large memory pages can provide:
Improved application performance
Reduced memory fragmentation
Increased memory efficiency
--v8-options
--v8-optionsPrints V8 command-line options.
Simplified Explanation:
V8 is the JavaScript engine that powers Node.js. The --v8-options flag allows you to print the command-line options that are supported by V8. These options can be used to tune the performance and behavior of the JavaScript engine.
Code Snippet:
Output:
The output will be a list of all the V8 command-line options, along with their descriptions.
Real-World Applications:
Performance Tuning: You can use the
--v8-optionsflag to identify and tweak V8 options that can improve the performance of your JavaScript applications.Debugging: The output of the
--v8-optionsflag can help you troubleshoot issues related to the V8 JavaScript engine.Experimentation: You can use the
--v8-optionsflag to experiment with different V8 options and see how they affect your applications.
Simplified Explanation:
The --v8-pool-size option in Node.js allows you to control how many background tasks the V8 JavaScript engine can run simultaneously.
Detailed Explanation:
When Node.js runs JavaScript code, it uses the V8 engine. V8 can run multiple tasks concurrently using a thread pool. The size of this thread pool, which is the number of threads available to run tasks, can affect the performance of your application.
Default Behavior:
If you don't specify the --v8-pool-size option, Node.js will automatically determine an appropriate size for the thread pool based on the estimated amount of parallelism available on your machine. Parallelism refers to the ability of multiple tasks to run simultaneously.
Setting a Custom Pool Size:
You can override the default pool size by setting --v8-pool-size to a specific number. For example:
This will set the V8 thread pool size to 8 threads.
Potential Applications:
1. Optimizing for Parallelism:
If your application performs many parallel tasks, setting a larger thread pool size can improve performance by allowing more tasks to run concurrently.
2. Isolating Tasks:
Running tasks in separate threads helps isolate them from each other, reducing the risk of a single thread blocking the execution of other tasks.
3. Reducing Latency:
A larger thread pool can help reduce latency for time-consuming tasks by allowing them to be processed more quickly.
Real-World Code Example:
In this example, we create a thread pool with 8 threads and use it to read multiple files concurrently. By using a larger thread pool, we can potentially reduce the time required to read all the files.
-v, --version
-v, --versionPrints the version of Node.js that is currently running.
Example:
Real World Use Case:
This can be useful if you need to check the version of Node.js that you are using for compatibility reasons or if you are debugging an issue.
--watch Flag
--watch FlagWhat it does:
The --watch flag makes Node.js monitor specified files for changes. When a change occurs, Node.js automatically restarts the running script.
How it works:
By default, --watch monitors the file you're running with Node.js and any files it imports or requires.
You can also specify specific paths to watch using the --watch-path flag. For example:
This will monitor index.js and any files in the ./src directory.
Benefits:
Using --watch can save time during development because you don't have to manually restart Node.js every time you make a change to a file.
Real-World Applications:
Rapid development: Quickly test and iterate on changes without having to manually restart Node.js.
Debugging: Easily track down errors by watching file changes and seeing which ones trigger the issue.
Example:
Consider a script called app.js:
To run this script in watch mode:
Now, any changes you make to app.js will automatically restart Node.js and print the updated result.
For example, if you change the add() function:
The result will immediately be recalculated and printed:
Simplified Explanation
The --watch-path flag tells Node.js to keep an eye on specific folders or files. If anything changes in those locations, Node.js will restart itself.
Detailed Explanation
What it does:
Enables "watch mode" for Node.js.
Specifies specific paths (folders or files) to monitor for changes.
If any changes occur in the watched paths, Node.js will automatically restart.
Why it's useful:
You don't have to manually restart Node.js each time you make a code change.
Makes it easier to develop and test code by quickly seeing the results of changes.
How to use it:
./srcand./testsare the folders you want to watch.index.jsis the main script file you want to run.
Limitations:
Only works on macOS and Windows.
Can't be used with certain other command-line options:
--check--eval--interactive--testREPL
Real-World Example:
You're developing a web application and want to test it in real-time. By using --watch-path to monitor your source code and test files, you can:
Make changes to your code.
Node.js will automatically restart and run the tests.
You can immediately see if your changes have broken anything.
Potential Applications:
Web development: Monitoring source code and test files for real-time testing.
File system monitoring: Watching for changes in specific directories or files (e.g., for logging or backup purposes).
Automation: Automatically triggering actions (e.g., scripts or commands) when specific files or directories change.
--watch-preserve-output
--watch-preserve-outputWhen you run a Node.js program with the --watch option, the program will automatically restart whenever you make changes to the source code. By default, the console output from the previous run is cleared before the program restarts.
The --watch-preserve-output option disables this behavior, so that the console output from the previous run is preserved when the program restarts. This can be useful if you want to see the full history of the program's output, or if you are using a tool that captures the console output for analysis or debugging.
Here is an example of how to use the --watch-preserve-output option:
This will run the test.js program in watch mode, and the console output from the previous run will be preserved when the program restarts.
Here are some potential applications of the --watch-preserve-output option:
Debugging: The preserved output can be helpful for debugging, as it allows you to see the full history of the program's output, including any errors or warnings that were generated.
Monitoring: The preserved output can be used to monitor the program's behavior over time, by capturing the output and analyzing it for trends or patterns.
Logging: The preserved output can be used to create a log file of the program's output, which can be useful for troubleshooting or auditing purposes.
--zero-fill-buffers
--zero-fill-buffersSimplified Explanation:
When you create a buffer (a space to store data), sometimes it might contain leftover data from previous uses. This option automatically fills these buffers with zeros to ensure a clean and secure starting state.
Detailed Explanation:
Buffers are used to store data in Node.js. When you create a new buffer, it might contain leftover data from previous uses, which can compromise security or cause unexpected behavior. The --zero-fill-buffers option automatically fills these buffers with zeros, ensuring that they are clean and secure from the beginning.
Code Example:
Real-World Applications:
Security: Zero-filling buffers prevents sensitive data from being leaked or accessed by unauthorized sources.
Reliability: It ensures that buffers are in a consistent and predictable state, reducing the likelihood of data corruption or unexpected behavior.
Performance: Zero-filling can improve performance in certain scenarios, such as when working with large buffers or when initializing buffers with specific values.
Environment variables are like special containers that store data that your programs can use. They can be set by the operating system, by other programs, or even by you.
Node.js has a built-in module called process that provides access to the environment variables. You can use the process.env object to get and set environment variables.
Setting an environment variable is as simple as assigning a value to the corresponding property of the process.env object. For example:
Getting an environment variable is just as easy:
Here are some real-world applications of environment variables:
Configuration: You can use environment variables to configure your applications without having to change the code. For example, you could use an environment variable to specify the database connection string.
Secrets management: You can use environment variables to store sensitive information, such as passwords and API keys. This is more secure than storing the information in your code.
Debugging: You can use environment variables to enable or disable debugging features in your applications.
Here is a complete code implementation of how to use environment variables in Node.js:
In this example, we use the process.env.MY_VARIABLE environment variable to store a message. We then use the res.send() function to send the message to the user.
You can run this example by creating a file called app.js and pasting the code into it. Then, open a terminal window and run the following command:
This will start the Node.js application. You can then visit http://localhost:3000 in your browser to see the message.
FORCE_COLOR Environment Variable
FORCE_COLOR Environment VariableThe FORCE_COLOR environment variable is used to control whether or not stdout and stderr are colorized. It's often used in development environments and tools to make it easier to read and debug output.
How it Works
The value of FORCE_COLOR determines the level of colorization that will be used:
1,true, or the empty string (''): Enables basic 16-color support.2: Enables 256-color support.3: Enables 16 million-color support.
If FORCE_COLOR is not set or is set to any other value, colorization will be disabled.
Use Cases
FORCE_COLOR is useful in situations where you want to ensure that colorization is enabled, even when other environment variables (such as NO_COLOR or NODE_DISABLE_COLORS) are set to disable it.
For example, if you're writing a development tool that outputs colorized text, you can use FORCE_COLOR to make sure that the colors are always shown, regardless of the user's environment settings.
Real-World Example
Here's an example of how to use FORCE_COLOR in a Node.js script:
In this example, if the FORCE_COLOR environment variable is set to any supported value, the console.log output will be colorized. Otherwise, the output will not be colorized.
Potential Applications
The FORCE_COLOR environment variable has a wide range of potential applications, including:
Debugging and troubleshooting tools
Development environments
Command-line tools
Interactive shells
Any application that outputs text to stdout or stderr and wants to ensure that colors are always shown
NO_COLOR=<any>
NO_COLOR=<any>Simplified Explanation:
NO_COLOR is like a secret password that tells some programs, like the Node.js command line, not to use colors in their output.
Detailed Explanation:
When you type commands into the Node.js command line, it sometimes prints colored text, like red errors or green success messages. This is because Node.js has a feature called "colors" that it uses to make its output more readable.
However, sometimes you might want to turn off these colors, for example if you're piping the output to another program that doesn't like colors. You can do this by setting the NO_COLOR environment variable to any value.
Code Snippet:
Real-World Application:
One potential application of NO_COLOR is when you're using Node.js to automate tasks in a script. For example, you might have a script that runs a series of tests and then sends the results to a reporting tool.
If the reporting tool doesn't like colors, you can set the NO_COLOR environment variable to ensure that the script's output is compatible.
NODE_DEBUG=module[,…]
NODE_DEBUG=module[,…]NODE_DEBUG is an environment variable that you can set to enable debugging output for specific core modules.
Simplified explanation:
Imagine you have a cake and you want to know what ingredients are inside. Instead of taking a bite out of the whole cake, you can use NODE_DEBUG to take a "bite" out of specific modules and see what's going on inside.
Code snippet:
To enable debugging for the fs module, you can set NODE_DEBUG like this:
Real world example:
Suppose you're having trouble writing to a file using fs. You can use NODE_DEBUG to see exactly what's happening:
This output shows you that the error is caused by trying to write to a file that doesn't exist.
Potential applications:
Debugging errors in core modules
Understanding how core modules work
Detecting performance issues
NODE_DEBUG_NATIVE=module[,…]
NODE_DEBUG_NATIVE=module[,…]This environment variable allows you to enable debug logging for specific C++ modules in Node.js.
Simplified Explanation:
Imagine your Node.js program as a big machine with lots of different parts. These parts are called "modules" and they do specific tasks. By setting NODE_DEBUG_NATIVE, you're telling Node.js to print extra information about how certain modules are working.
Code Snippet:
This code will run the script.js file and enable debug logging for the "http" and "fs" modules.
Real-World Application:
Debugging memory leaks or performance issues in a specific module.
Understanding the internal workings of a module to troubleshoot errors.
Optimizing code by identifying bottlenecks and improving module efficiency.
NODE_DISABLE_COLORS=1
NODE_DISABLE_COLORS=1Simplified Explanation:
When you type commands in Node.js's interactive mode (REPL), the output will normally show in different colors to make it easier to read. For example, errors might show in red, while successful commands show in green.
Setting the NODE_DISABLE_COLORS environment variable to 1 turns off this color highlighting. This can be useful if you're redirecting the output to a file or if you have trouble viewing the colors correctly on your system.
Real-World Example:
Suppose you want to save the output of the REPL to a text file for later reference. You can do this by redirecting the output to a file, like so:
However, if you leave the color highlighting enabled, the output in the file will include the color codes. This can make the file difficult to read later. To avoid this, you can set the NODE_DISABLE_COLORS environment variable before redirecting the output:
This will produce a text file with the output from the REPL, but without the color highlighting.
NODE_EXTRA_CA_CERTS=file
NODE_EXTRA_CA_CERTS=fileThis environment variable lets you add extra certificates to the list of trusted certificates that Node.js uses to verify SSL/TLS connections. Normally, Node.js comes with a set of pre-installed certificates that it trusts. However, you may need to add additional certificates if you are trying to connect to a server that uses a self-signed certificate or a certificate that is not signed by a well-known Certificate Authority (CA).
To use this environment variable, simply set it to the path of a file that contains the extra certificates in PEM format. For example:
Once you have set the environment variable, Node.js will automatically load the extra certificates and use them to verify SSL/TLS connections.
This variable is ignored if Node.js is running as root or if it has Linux file capabilities set.
Real-world applications:
Connecting to a server that uses a self-signed certificate
Connecting to a server that uses a certificate that is not signed by a well-known CA
Testing SSL/TLS connections to a server
Improved code example:
NODE_ICU_DATA=file
Explanation:
Node.js uses a library called ICU to handle internationalization and localization tasks. ICU provides data for different locales, such as language and number formats. By default, Node.js includes a small amount of ICU data in its binary.
However, you can specify a custom location for ICU data using the NODE_ICU_DATA environment variable. This is useful if you need more complete ICU data or if you want to use a specific version of ICU.
Usage:
To specify a custom location for ICU data, set the NODE_ICU_DATA environment variable to the path of the ICU data directory. For example:
Example:
The following example shows how to use the NODE_ICU_DATA environment variable to specify a custom location for ICU data:
Real-World Applications:
The NODE_ICU_DATA environment variable is useful in the following scenarios:
Customizing the language formats: You can use a custom ICU data directory to provide specific language formats. For example, you can install an ICU data directory that includes support for a rare language that is not supported by the default ICU data.
Updating the ICU version: If you need to use a specific version of ICU, you can install it and set the
NODE_ICU_DATAenvironment variable to the location of that version.Reducing the Node.js binary size: By providing a custom ICU data directory, you can reduce the size of the Node.js binary. This is useful if you need to deploy Node.js in a constrained environment.
NODE_NO_WARNINGS=1
NODE_NO_WARNINGS=1When set to 1, process warnings are silenced.
Explanation:
Node.js generates warnings during program execution.
These warnings can be helpful for debugging and identifying potential issues.
Setting
NODE_NO_WARNINGSto1prevents Node.js from displaying these warnings.
Example:
Real-World Applications:
Suppressing unnecessary warnings: This is useful when a program generates numerous warnings that can clutter output and make it difficult to identify important information.
Preventing warnings from interrupting execution: Warnings can cause interruptions during program execution, so suppressing them can ensure smooth runtime operation.
NODE_OPTIONS Environment Variable
The NODE_OPTIONS environment variable allows you to specify a list of options that will be passed to Node.js when it starts. These options can be used to configure Node.js's behavior, enable experimental features, and more.
How to Use NODE_OPTIONS
To use NODE_OPTIONS, simply set the environment variable to a space-separated list of options. For example, to enable the experimental --experimental-modules feature, you would set NODE_OPTIONS like this:
You can also use NODE_OPTIONS to set multiple options at once. For example, the following command would enable the --experimental-modules and --inspect options:
Node.js Options
The following Node.js options can be set using NODE_OPTIONS:
--allow-addons: Allow addons to be loaded.--allow-child-process: Allow child processes to be spawned.--allow-fs-read: Allow files to be read.--allow-fs-write: Allow files to be written.--allow-worker: Allow worker threads to be created.--conditions: Specify a set of conditions that must be met before Node.js will start.--diagnostic-dir: Specify a directory where diagnostic information will be written.--disable-proto: Disable the__proto__property.--disable-warning: Disable warning messages.--dns-result-order: Specify the order in which DNS results will be returned.--enable-fips: Enable Federal Information Processing Standard (FIPS) mode.--enable-network-family-autoselection: Enable automatic selection of network families.--enable-source-maps: Enable source maps.--experimental-abortcontroller: Enable theAbortControllerclass.--experimental-default-type: Set the default type of imported modules.--experimental-detect-module: Enable the detection of module types.--experimental-import-meta-resolve: Enable the resolution ofimport.meta.resolve.--experimental-json-modules: Enable the loading of JSON modules.--experimental-loader: Enable the experimental loader.--experimental-modules: Enable the experimental modules system.--experimental-network-imports: Enable the importing of network resources.--experimental-permission: Enable the experimental permission system.--experimental-policy: Enable the experimental policy system.--experimental-shadow-realm: Enable the experimental shadow realm system.--experimental-specifier-resolution: Enable the experimental specifier resolution system.--experimental-top-level-await: Enable the experimental top-level await syntax.--experimental-vm-modules: Enable the experimental VM modules system.--experimental-wasi-unstable-preview1: Enable the experimental WASI unstable preview1 system.--experimental-wasm-modules: Enable the experimental WASM modules system.--force-context-aware: Force all scripts to run in a sandboxed, context-aware environment.--force-fips: Force FIPS mode to be enabled.--force-node-api-uncaught-exceptions-policy: Force the Node.js API uncaught exceptions policy to be applied to all scripts.--frozen-intrinsics: Freeze the intrinsic objects.--heapsnapshot-near-heap-limit: Trigger a heap snapshot when the heap is near its limit.--heapsnapshot-signal: Trigger a heap snapshot when the specified signal is received.--http-parser: Specify the HTTP parser to use.--icu-data-dir: Specify the directory where ICU data is stored.--import: Import a module.--input-type: Specify the input type for stdin.--insecure-http-parser: Disable HTTP parser security checks.--inspect-brk: Enable breakpoint debugging.--inspect-port,--debug-port: Specify the port to use for remote debugging.--inspect-publish-uid: Enable the publishing of debugging information over a Unix domain socket.--inspect: Enable remote debugging.--max-http-header-size: Specify the maximum size of HTTP headers.--napi-modules: Enable the loading of N-API modules.--no-addons: Disallow addons to be loaded.--no-deprecation: Disable deprecation warnings.--no-experimental-fetch: Disable the experimental fetch API.--no-experimental-global-customevent: Disable the experimental globalCustomEventconstructor.--no-experimental-global-navigator: Disable the experimental globalnavigatorobject.--no-experimental-global-webcrypto: Disable the experimental globalWebCryptoobject.--no-experimental-repl-await: Disable the experimentalawaitsyntax in the REPL.--no-experimental-websocket: Disable the experimental WebSocket API.--no-extra-info-on-fatal-exception: Disable the printing of extra information on fatal exceptions.--no-force-async-hooks-checks: Disable forcing of async hooks checks.--no-global-search-paths: Disable the use of global search paths for module loading.--no-network-family-autoselection: Disable automatic selection of network families.--no-warnings: Disable all warnings.--node-memory-debug: Enable memory debugging.--openssl-config: Specify the OpenSSL configuration file to use.--openssl-legacy-provider: Enable the legacy OpenSSL provider.--openssl-shared-config: Enable the shared OpenSSL configuration file.--pending-deprecation: Enable pending deprecation warnings.--policy-integrity: Enable policy integrity.--preserve-symlinks-main: Preserve symbolic links when loading the main module.--preserve-symlinks: Preserve symbolic links when loading modules.--prof-process: Enable process profiling.--redirect-warnings: Redirect warning messages to stderr.--report-compact: Generate a compact report on fatal errors.--report-dir,--report-directory: Specify the directory where reports on fatal errors will be written.--report-filename: Specify the filename to use for the report on fatal errors.--report-on-fatalerror: Generate a report on fatal errors.--report-on-signal: Generate a report on the specified signal.--report-signal: Specify the signal to generate a report on.--report-uncaught-exception: Generate a report on uncaught exceptions.--require,-r: Require a module.--secure-heap-min: Specify the minimum size of the secure heap.--secure-heap: Enable the secure heap.--snapshot-blob: Create a snapshot blob of the heap.--test-only: Enable test-only features.--test-reporter-destination: Specify the destination for test reporter output.--test-reporter: Specify the test reporter to use.--test-shard: Specify the test shard to use.--throw-deprecation: Throw an error on deprecation warnings.--title: Specify the title of the process.--tls-cipher-list: Specify the TLS cipher list to use.--tls-keylog: Enable TLS key logging.--tls-max-v1.2: Specify the maximum TLS version 1.2 allowed.--tls-max-v1.3: Specify the maximum TLS version 1.3 allowed.--tls-min-v1.0: Specify the minimum TLS version 1.0 allowed.--tls-min-v1.1: Specify the minimum TLS version 1.1 allowed.--tls-min-v1.2: Specify the minimum TLS version 1.2 allowed.--tls-min-v1.3: Specify the minimum TLS version 1.3 allowed.--trace-atomics-wait: Enable tracing of atomic wait operations.--trace-deprecation: Enable tracing of deprecation warnings.--trace-event-categories: Specify the trace event categories to enable.--trace-event-file-pattern: Specify the file pattern to use for trace event output.--trace-events-enabled: Enable tracing of events.--trace-exit: Enable tracing of process exit.--trace-sigint: Enable tracing of SIGINT signals.--trace-sync-io: Enable tracing of synchronous I/O operations.--trace-tls: Enable tracing of TLS operations.--trace-uncaught: Enable tracing of uncaught exceptions.--trace-warnings: Enable tracing of warnings.--track-heap-objects: Track heap objects.--unhandled-rejections: Enable handling of unhandled Promise rejections.--use-bundled-ca: Use the bundled CA certificates.--use-largepages: Enable the use of large pages for memory allocation.--use-openssl-ca: Use the OpenSSL CA certificates.--v8-pool-size: Specify the size of the V8 thread pool.--watch-path: Specify a path to watch for changes.--watch-preserve-output: Preserve the output of watched files.--watch: Enable watching for changes to files.--zero-fill-buffers: Zero-fill buffers when they are allocated.
V8 Options
The following V8 options can be set using NODE_OPTIONS:
--abort-on-uncaught-exception: Abort the process on uncaught exceptions.--disallow-code-generation-from-strings: Disallow code generation from strings.--enable-etw-stack-walking: Enable ETW stack walking.--huge-max-old-generation-size: Set the maximum size of the old generation heap to 1GB.--interpreted-frames-native-stack: Include interpreted frames in native stack traces.--jitless: Disable JIT compilation.--max-old-space-size: Set the maximum size of the old generation heap.--max-semi-space-size: Set the maximum size of the semi-space.--perf-basic-prof-only-functions: Only profile functions in the basic profile.--perf-basic-prof: Enable basic profiling.--perf-prof-unwinding-info: Enable unwinding information in the profile.--perf-prof: Enable profiling.--stack-trace-limit: Set the maximum call stack size for stack traces.
Real-World Applications
NODE_OPTIONS can be used in a variety of real-world applications, such as:
Debugging: You can use
NODE_OPTIONSto enable debugging options, such as--inspectand--debug-port, which allow you to remotely debug your code.Profiling: You can use
NODE_OPTIONSto enable profiling options, such as--profand--perf-prof, which allow you to measure the performance of your code.Security: You can use
NODE_OPTIONSto enable security options, such as--tls-min-v1.2and--force-context-aware, which can help to protect your code from security vulnerabilities.Testing: You can use
NODE_OPTIONSto enable testing options, such as--test-onlyand--test-reporter, which can help you to write and run tests for your code.Customization: You can use
NODE_OPTIONSto customize the behavior of Node.js, such as by enabling experimental features or disabling warnings.
NODE_PATH is an environment variable that tells Node.js where to look for modules. By default, Node.js will only look for modules in the node_modules directory of the current working directory. However, you can use NODE_PATH to add additional directories to the search path.
This can be useful if you have modules installed in a global location, or if you want to share modules between multiple projects.
To set the NODE_PATH environment variable, you can use the following command:
On Windows, you would use the following command:
Once you have set the NODE_PATH environment variable, you can use the require() function to import modules from the directories that you have specified. For example, the following code will import the my-module module from the /path/to/my-module directory:
Potential applications in real world:
Sharing modules between multiple projects: You can use
NODE_PATHto share modules between multiple projects by adding thenode_modulesdirectory of each project to theNODE_PATHenvironment variable. This can be useful if you have a library of reusable modules that you want to use in multiple projects.Installing modules globally: You can use
NODE_PATHto install modules globally by adding thenode_modulesdirectory of the global installation to theNODE_PATHenvironment variable. This will allow you to use the modules from the global installation in any project.Customizing the module search path: You can use
NODE_PATHto customize the module search path to meet your specific needs. For example, you could add thesrcdirectory of your project to theNODE_PATHenvironment variable to make it easier to import modules from thesrcdirectory.
What is PENDING_DEPRECATION?
PENDING_DEPRECATION is a special environmental variable that allows you to see warnings about changes in Node.js that have the potential to break your code. Any code using features that may later become deprecated (no longer supported or recommended) will trigger a warning with this variable set.
How to Turn It On:
To turn on PENDING_DEPRECATION warnings, set the NODE_PENDING_DEPRECATION environment variable to 1 with your preferred method.
For example:
"Node.js Version 16.13.0" will be printed, along with a warning that the --version option in the command line interface is deprecated.
Why Use PENDING_DEPRECATION?
PENDING_DEPRECATION warnings help you stay ahead of code changes. By catching potential issues early, you have time to make adjustments and avoid problems later on. This is especially useful for large applications or projects that rely on Node.js.
Example Application:
Consider a Node.js application that uses the Buffer object heavily for data manipulation. In future versions of Node.js, the Buffer object may be deprecated in favor of alternative APIs. By enabling PENDING_DEPRECATION warnings, you can be alerted to this potential change and start planning for the migration to avoid any disruptions.
Simplified Explanation:
NODE_PENDING_PIPE_INSTANCES=instances controls the number of pending connections that a pipe server can handle before it starts rejecting new connections.
Detailed Explanation:
When a client tries to connect to a pipe server, the server needs to create a new "instance" of the pipe to handle the connection. If the server has reached its limit of pending instances, it will refuse the new connection until some of the existing instances are closed.
Code Snippet:
Real-World Example:
Imagine a pipe server that's used to communicate with multiple clients. If the server is configured with a low number of pending instances, it may struggle to handle a sudden influx of connections and start dropping clients. By increasing the number of pending instances, the server can handle more simultaneous connections without rejecting them.
Potential Applications:
High-traffic server applications: To ensure that all clients can connect seamlessly, even during peak usage.
Messaging systems: To handle multiple client subscriptions and deliver messages efficiently.
Data transfer applications: To transfer large files over pipes without interruption.
What is NODE_PRESERVE_SYMLINKS=1?
Imagine you have a folder named "projects" with two subfolders linked together like this:
When you try to import a module from "project-a," the module loader normally treats it as a separate module from "project-b." However, if you set NODE_PRESERVE_SYMLINKS=1, the module loader will recognize the symlink and load the module from "project-b" instead.
Why use NODE_PRESERVE_SYMLINKS=1?
Sharing code between projects: You can use symlinks to share common code between multiple projects without having to copy and paste the code into each project.
Testing code in different environments: You can create symlinks to different versions of a module to test how your code behaves in different scenarios.
Example:
When you run node another-project/app.js, it will load module-a from example-project/modules/module-a.js because of the symlink. This allows you to share the same code between both projects.
Real-world applications:
Creating reusable components: You can create a library of common components and then symlink them into different projects, ensuring that all projects use the latest version of the components.
Testing different versions of code: You can create symlinks to different branches or commits of a module to test how your code behaves in different scenarios, such as testing a new feature or fixing a bug.
Developing multiple versions of an application: You can use symlinks to create different versions of an application for different environments, such as a development version and a production version.
Simplified Explanation:
Sometimes when your Node.js program runs, it might encounter issues that need to be reported. By default, these issues are printed to your terminal (the command window).
NODE_REDIRECT_WARNINGS Environment Variable:
The NODE_REDIRECT_WARNINGS environment variable allows you to change where these issues are reported. Instead of printing them to the terminal, you can direct them to a file.
How to Use NODE_REDIRECT_WARNINGS:
Create a file where you want the warnings to be saved. For example,
warnings.txt.Set the
NODE_REDIRECT_WARNINGS=warnings.txtenvironment variable.
Example:
Advantages of NODE_REDIRECT_WARNINGS:
Keeps terminal clean: Avoids cluttering the terminal with warning messages.
Easy to review: Warnings are saved in a file, making it easier to track and review them later.
Automates logging: If you're using automated tools for error reporting, it simplifies the process by directing warnings to a central location.
Real-World Applications:
Production Environment: In production environments, it's crucial to log warnings to a file for easier monitoring and troubleshooting.
Automated Testing: During automated testing, warnings can be captured in a file for detailed analysis.
Debugging: Developers can use the file to trace the source of potential issues and find solutions.
NODE_REPL_EXTERNAL_MODULE=file
In Node.js, the REPL (read-eval-print-loop) is an interactive environment where you can enter JavaScript code and see the results.
By default, Node.js uses its own built-in REPL. However, you can customize the REPL by loading an external Node.js module instead. This allows you to add additional features or change the behavior of the REPL.
To use an external REPL module, set the NODE_REPL_EXTERNAL_MODULE environment variable to the path of the module. For example:
This will load the my-repl-module.js module and use it as the REPL instead of the built-in one.
Real-world applications:
Adding custom commands to the REPL
Changing the REPL's default behavior, such as the output formatting
Integrating the REPL with other tools or environments
Example:
Here's an example of a simple external REPL module that adds a custom command:
To use this module, set the NODE_REPL_EXTERNAL_MODULE environment variable and start Node.js:
Now, you can use the mycmd command in the REPL:
Simplified Explanation:
NODE_REPL_HISTORY is an environment variable that lets you save and use a history of commands entered in Node.js's REPL (Read-Evaluate-Print-Loop).
What is REPL?
REPL is a command-line interface where you can enter Node.js code to be executed immediately. It's like a playground to try out code snippets and interact with Node.js.
What does NODE_REPL_HISTORY do?
By default, REPL does not remember the commands you enter. NODE_REPL_HISTORY lets you store these commands in a file so that you can use them later.
How to use it:
To enable persistent REPL history, set NODE_REPL_HISTORY to the path of a file where you want to save the history, like:
To disable it, set the variable to an empty string:
Example:
Let's say you have a my_repl_history.txt file in your current directory. To start REPL with persistent history:
Now, any commands you enter in REPL will be saved to my_repl_history.txt.
Applications:
Save and reuse common commands: Store commands you use frequently to avoid typing them again.
Share REPL sessions: If you're collaborating with others, you can share your REPL history file to give them access to your commands.
Debug and troubleshoot: By reviewing your history, you can track down errors and identify issues.
Simplified Explanation:
NODE_SKIP_PLATFORM_CHECK is an environment variable you can set to skip the check that Node.js does to make sure it's running on a platform it supports.
How Does It Work?
When you start Node.js, it checks if your operating system and processor type are supported. If they're not, Node.js will give you an error message and refuse to run.
Setting NODE_SKIP_PLATFORM_CHECK to '1' tells Node.js to ignore this check and run anyway.
Why Would You Use It?
You might want to use this if you're running Node.js on an unsupported platform for testing or development purposes. However, keep in mind that Node.js might not work correctly on unsupported platforms, and any problems you encounter won't be fixed by Node.js developers.
Code Snippet:
Real-World Applications:
Testing: You can use this to test your Node.js code on platforms that Node.js doesn't officially support, such as embedded systems or old operating systems.
Development: If you're developing a Node.js application that you intend to run on an unsupported platform, you can use this to test it locally before deploying it to the target platform.
Cautions:
Only use this if you're aware of the risks and understand that Node.js might not work correctly on the unsupported platform.
Do not use this in production environments, as it could lead to unexpected errors and unpredictable behavior.
Simplified Explanation:
When you run Node.js tests, you can use the NODE_TEST_CONTEXT environment variable to control how test results are reported.
Test Reporter Options:
Test reporters display test results in a specific format. By default, Node.js uses its own test reporter, which shows results in a plain text format.
TAP Format:
TAP (Test Anything Protocol) is a standard format for reporting test results. It's popular because it's easy to parse and can be used with various tools.
Overriding Reporter Options:
If you set NODE_TEST_CONTEXT to 'child', it overrides the default test reporter options. This means that test results will be sent to standard output in the TAP format.
Example:
To run your Node.js tests in TAP format, open your terminal and type:
Real-World Applications:
Using TAP format can be useful if you want to:
Analyze test results using external tools
Integrate your test results into other systems
Use consistent reporting across multiple test frameworks
NODE_TLS_REJECT_UNAUTHORIZED=value
This environment variable controls whether or not certificate validation is performed for TLS connections.
TLS (Transport Layer Security) is a security protocol that is used to encrypt communication between two systems over a network. TLS provides two important functions:
Confidentiality: It ensures that data sent between the two systems cannot be intercepted and read by unauthorized parties.
Authentication: It verifies that the system you are connecting to is actually the system you think it is.
Certificate validation is a process of verifying the authenticity of the certificate presented by the server during a TLS connection. This ensures that the server is a legitimate entity and not an imposter.
By default, Node.js will perform certificate validation for all TLS connections. However, you can disable certificate validation by setting the NODE_TLS_REJECT_UNAUTHORIZED environment variable to '0'.
Why would you want to disable certificate validation?
There are several reasons why you might want to disable certificate validation:
You are testing or debugging a TLS connection.
You are using a self-signed certificate.
You are connecting to a server that does not support TLS certificate validation.
It is important to note that disabling certificate validation makes TLS connections insecure. This is because it allows imposters to impersonate legitimate servers and intercept your data.
How to disable certificate validation
To disable certificate validation, you can set the NODE_TLS_REJECT_UNAUTHORIZED environment variable to '0' before making any TLS connections.
Potential applications
Disabling certificate validation can be useful in the following situations:
Testing or debugging TLS connections.
Using a self-signed certificate.
Connecting to a server that does not support TLS certificate validation.
Real-world example
The following example shows how to use the NODE_TLS_REJECT_UNAUTHORIZED environment variable to disable certificate validation when connecting to a self-signed certificate.
NODE_V8_COVERAGE
Explanation: NODE_V8_COVERAGE is an environment variable that allows you to track how your JavaScript code is being used. It generates coverage information that shows which parts of your code were executed and which parts were not.
Benefits:
Helps identify unused code, which can be removed to improve performance.
Aids in debugging by showing which parts of your code are actually being used.
Provides insights into code coverage for testing purposes.
How to use: Set the NODE_V8_COVERAGE environment variable to a directory path where you want the coverage data to be saved.
Example:
V8 JavaScript Code Coverage
Explanation: V8 JavaScript Code Coverage is a feature of Node.js that collects data on which lines of JavaScript code are executed.
Benefits:
Helps optimize code by identifying unused lines and blocks.
Facilitates testing by ensuring that all branches of code are covered.
How to use: NODE_V8_COVERAGE is used to enable V8 Code Coverage.
Source Map
Explanation: A Source Map is a file that maps the original source code to the generated code (e.g., JavaScript code generated from TypeScript).
Benefits:
Allows debugging of the original source code, even when it has been transformed or minified.
Facilitates easy navigation between the source and generated code.
How to use: NODE_V8_COVERAGE also generates Source Map files that map the coverage data to the original source code. This allows you to visualize which lines in the original source code were executed.
Real World Applications:
Code Optimization: By analyzing coverage data, developers can identify and remove unused code to streamline performance. For example, removing unused imports or condition blocks can reduce the size of the codebase and improve execution time.
Testing: Coverage reports help ensure that tests cover all branches of a codebase. This ensures thorough testing and reduces the risk of missing potential bugs or issues.
Error Tracking: Coverage data can be used to diagnose errors and identify the specific lines of code that caused the error. This allows developers to pinpoint the source of the problem more quickly.
Coverage: Measuring Code Execution
In software development, coverage refers to the extent to which your code is being executed during testing. Measuring coverage is important because it helps you identify areas of your code that aren't being tested, potentially leading to bugs or errors.
Node.js's CLI module provides a way to generate coverage reports for your JavaScript code. These reports show you which lines of code were executed during testing and which were not.
Coverage Report Structure
Coverage output is an array of ScriptCoverage objects, each of which represents a specific JavaScript file. Each ScriptCoverage object contains:
scriptId: A unique identifier for the script.url: The URL or path to the script file.functions: An array of coverage information for individual functions within the script.
Understanding Functions Coverage
Each function coverage object includes the following information:
functionName: The name of the function.lineNumber: The line number where the function starts.hits: The number of times the function was called during testing.
Real-World Applications
Coverage reports are useful for:
Identifying untested code: Highlight areas of your code that haven't been executed, allowing you to focus testing efforts on those areas.
Optimizing test suites: Help you identify redundant or unnecessary tests by showing which lines of code are already being covered by existing tests.
Debugging: Assist in finding errors or unintended behavior by revealing which lines of code were executed leading up to a failure.
Example Code:
To generate a coverage report using Node.js's CLI module, you can use the following code:
This example uses the istanbul library to generate and load coverage data, which can then be printed or used for further analysis.
Source map cache
A source map cache is an object that stores the raw source map data, the parsed Source Map v3 information, and the line lengths of the source file. The source map cache is appended to the top-level key source-map-cache on the JSON coverage object.
Source map data
The raw source map data is a string that contains the source map information. The source map information is a JSON object that contains the following information:
The version of the source map specification
The list of source files that the source map applies to
The list of names that the source map applies to
The mappings between the original source code and the generated code
Parsed Source Map v3 information
The parsed Source Map v3 information is a JavaScript object that contains the following information:
The version of the source map specification
The list of source files that the source map applies to
The list of names that the source map applies to
The mappings between the original source code and the generated code
The source root
Line lengths
The line lengths are an array that contains the length of each line in the source file.
Real-world example
The following code shows how to use the source map cache to get the line lengths of a source file:
Potential applications
The source map cache can be used to improve the performance of coverage reporting tools. By caching the source map data, the tools can avoid having to parse the source map every time they need to generate a report.
OPENSSL_CONF Environment Variable
Purpose:
Lets you specify a configuration file for OpenSSL, which is the underlying library used by Node.js to handle encryption and security.
How it works:
When you set OPENSSL_CONF to a file path, OpenSSL will load that file and apply its settings. This allows you to customize various OpenSSL features.
Example:
Potential Applications:
Enabling stricter security settings
Setting up a custom certification authority (CA)
Enabling FIPS-compliant cryptography (for government or finance applications)
Simplified Explanation:
Think of it as an instruction book that tells OpenSSL how to handle security. By loading a configuration file, you can tell OpenSSL to follow specific rules or settings.
Real-World Example:
In a banking application, you could use OPENSSL_CONF to load a configuration file that enforces strong encryption algorithms and disables weak cipher suites. This helps protect sensitive financial data.
--openssl-config Command-Line Option
Purpose:
Similar to OPENSSL_CONF, but sets the OpenSSL configuration file directly from the command line.
Example:
When to Use:
If you want to specify the OpenSSL configuration file when running a Node.js script from the command line, use --openssl-config.
Real-World Example:
You're working on a project that requires FIPS-compliant cryptography. You can use --openssl-config to load the necessary configuration file, ensuring that your code follows FIPS standards.
SSL_CERT_DIR=dir
SSL_CERT_DIR=dirTopic:
Configuring OpenSSL's trusted certificate directory
Explanation:
When using
--use-openssl-cawith node.js, you can specify a custom directory where OpenSSL should look for trusted certificates.This directory overrides OpenSSL's default location for trusted certificates.
Note that this environment variable will be inherited by any child processes.
Code Snippet:
Real-World Application:
You can use this environment variable to change the directory where OpenSSL looks for trusted certificates.
This can be useful if you have a specific set of certificates that you want to trust, or if you want to use a different directory than OpenSSL's default.
SSL_CERT_FILE=file
This environment variable tells Node.js to use the specified file as the source of trusted certificates for OpenSSL. This is useful if you want to use a custom set of trusted certificates, rather than the ones that come with OpenSSL by default.
Usage
To use this environment variable, simply set it to the path of the file containing the trusted certificates. For example:
Potential Applications
This environment variable can be useful in a variety of situations, such as:
Customizing the set of trusted certificates: You can use this environment variable to add or remove trusted certificates from the default set. This can be useful if you need to trust certificates from a specific issuer, or if you want to remove certificates that you no longer trust.
Using a custom CA: You can use this environment variable to use a custom certificate authority (CA) to sign your certificates. This can be useful if you want to create your own PKI infrastructure.
Real-World Example
Here is an example of how you might use this environment variable in a real-world application:
This example will cause Node.js to use the trusted certificates from the specified file when making SSL connections.
What is the TZ environment variable?
The TZ environment variable is a way to tell your computer what time zone you're in. This is important because it affects how your computer interprets dates and times. For example, if you're in the Eastern Time Zone, your computer will interpret a date of "2023-03-08" as March 8th, 2023 at 12:00 AM Eastern Standard Time. If you were in the Pacific Time Zone, your computer would interpret the same date as March 7th, 2023 at 9:00 PM Pacific Standard Time.
How do I set the TZ environment variable?
You can set the TZ environment variable in a few different ways.
In a terminal window:
In a Node.js script:
What are some common TZ values?
Some common TZ values include:
'Etc/UTC': Coordinated Universal Time (UTC)'Europe/Paris': Central European Time (CET)'America/New_York': Eastern Time (ET)
How does Node.js use the TZ environment variable?
Node.js uses the TZ environment variable to set the default time zone for all date and time operations. This means that all dates and times created by Node.js will be interpreted according to the TZ environment variable.
Real-world examples
The TZ environment variable can be used in a variety of real-world applications, such as:
Displaying dates and times in the correct time zone: For example, a website could use the
TZenvironment variable to display dates and times in the local time zone of the user.Scheduling tasks: A scheduling application could use the
TZenvironment variable to schedule tasks to run at a specific time in a specific time zone.Converting dates and times between time zones: A date and time conversion tool could use the
TZenvironment variable to convert dates and times between different time zones.
Simplified Explanation of UV_THREADPOOL_SIZE
Imagine Node.js is a big team of workers who are trying to complete tasks like reading files, encrypting data, and looking up domain names. These tasks can sometimes take a while, and Node.js has a small team of assistants (called a threadpool) to help out.
However, if the team of assistants is too small, and one of the assistants gets stuck on a long task, it can slow down all the other assistants and the tasks they're working on.
How UV_THREADPOOL_SIZE Helps
UV_THREADPOOL_SIZE lets us increase the number of assistants in the threadpool. This means that if one task takes a while, it won't affect the other tasks as much.
Code Example
Real-World Applications
Web servers: If a web server is handling a lot of requests, increasing the threadpool size can help it handle them faster.
File processing: If a program is reading or writing a lot of files, increasing the threadpool size can speed up the process.
Data encryption: Encrypting large amounts of data can take a long time. Increasing the threadpool size can make encryption faster.
Useful V8 Options
V8 is the JavaScript engine that powers Node.js. It provides a set of command-line options that can be used to control the behavior of V8.
Why use V8 options?
V8 options can be used to:
Improve performance
Debug JavaScript code
Enable experimental features
How to use V8 options?
V8 options can be passed to Node.js using the --v8-options flag. For example:
This will run the main.js script with the --track-gc V8 option enabled.
Common V8 options
Here are some of the most commonly used V8 options:
--track-gc: Enables garbage collection tracking. This can be useful for debugging memory leaks.
Real-world example:
--harmony: Enables experimental JavaScript features. These features may not be stable and could change in future versions of V8.
Real-world example:
--trace-hydrogen: Enables hydrogen code tracing. This can be useful for debugging performance issues.
Real-world example:
--max-old-space-size=n: Sets the maximum size of the old generation heap space. This can be used to limit memory usage.
Real-world example:
--log-code: Logs JavaScript code as it is compiled. This can be useful for debugging compilation issues.
Real-world example:
Note:
V8 options are not part of the Node.js API and may change at any time. Use them at your own risk.
Simplified Explanation of --max-old-space-size Flag
What is V8?
V8 is the JavaScript engine used by Node.js. It's responsible for executing JavaScript code in a fast and efficient manner.
What is the Old Memory Section?
V8 divides the memory it uses into two main sections: young and old. The young section stores newly created objects, while the old section stores objects that have survived multiple garbage collection cycles.
What does --max-old-space-size do?
This flag sets the maximum size (in megabytes) that the old memory section can reach.
Why is this important?
When the old memory section becomes too large, V8 spends more time on garbage collection, trying to free up unused memory. This can slow down your Node.js application.
How to use --max-old-space-size
If you have 2GB of memory on your machine, you can set this flag to 1536 (1.5GB) to leave some memory for other uses and prevent swapping (moving data from RAM to hard disk).
Example
In this example, we're setting the max old space size to 1536MB for the Node.js application running the "index.js" script.
Potential Applications
Memory-intensive applications: Applications that create a lot of objects and hold onto them for long periods of time.
Applications running on machines with limited memory: Setting this flag appropriately can prevent crashes and improve performance.
Fine-tuning Node.js performance: By adjusting this flag, you can balance memory usage with application performance.
--max-semi-space-size=SIZE (in megabytes)
--max-semi-space-size=SIZE (in megabytes)This option sets the maximum size of the semi-space in V8's garbage collector. The semi-space is a region of memory used by the garbage collector to store live objects.
Increasing the max size of a semi-space may improve throughput for Node.js at the cost of more memory consumption.
The default value is 16 MiB for 64-bit systems and 8 MiB for 32-bit systems.
To get the best configuration for your application, you should try different max-semi-space-size values when running benchmarks for your application.
For example, on a 64-bit system: