vm

What is the vm module?

The vm module allows you to run JavaScript code in a separate context from the main program. This means that the code you run in the vm module will not have access to the same global variables or objects as the main program.

Why would I want to use the vm module?

There are a few reasons why you might want to use the vm module:

To run untrusted code. If you are not sure whether or not a piece of code is safe to run, you can run it in the vm module to isolate it from the rest of your program.
To run code in a different context. You can use the vm module to create a new context with its own set of global variables and objects. This can be useful for testing code or for running code in a sandboxed environment.
To run code without blocking the main thread. The vm module can be used to run code in a separate thread, which can free up the main thread for other tasks.

How do I use the vm module?

To use the vm module, you first need to create a new context. You can do this using the createContext() method. Once you have created a context, you can run code in it using the runInContext() method.

Here is an example of how to use the vm module:

const vm = require("vm");

// Create a new context.
const context = vm.createContext();

// Run code in the context.
vm.runInContext('console.log("Hello, world!");', context);

This code will print "Hello, world!" to the console.

Real world applications

The vm module can be used in a variety of real-world applications, including:

Running untrusted code, such as code downloaded from the internet.
Running code in a sandboxed environment, such as code that is used to test a new library.
Running code in a separate thread, such as code that is used to perform a long-running task.

Potential applications

Running plugins: Plugins are pieces of code that can be added to a program to extend its functionality. The vm module can be used to run plugins in a sandboxed environment, which can help to protect the main program from malicious code.
Running scripts: Scripts are pieces of code that can be executed by a program. The vm module can be used to run scripts in a separate thread, which can free up the main thread for other tasks.
Running tests: Tests are pieces of code that are used to verify that a program is working correctly. The vm module can be used to run tests in a sandboxed environment, which can help to isolate tests from each other and from the main program.

Conclusion

The vm module is a powerful tool that can be used to run JavaScript code in a variety of ways. It can be used to run untrusted code, run code in a sandboxed environment, and run code in a separate thread. The vm module has a variety of potential applications, including running plugins, running scripts, and running tests.

Class: `vm.Script`

vm.Script is a class in the vm module that represents a precompiled script that can be executed in specific contexts.
A script is a piece of code that can be executed by a virtual machine (VM).
A VM is a software environment that can execute code without having to compile it first.
This makes it possible to run code in a sandboxed environment, which is useful for security purposes.
You can create a vm.Script instance by passing a string of code to the vm.createScript() function.
You can then execute the script by calling the runInContext() method on the vm.Script instance.
The runInContext() method takes two arguments:
1. The context in which the script should be executed.
2. A set of options that control the execution of the script.
The context is an object that provides the global scope for the script.
The options object can be used to specify things like the timeout for the script and whether or not to include the debugger statement.

Real-world example

The following code shows how to create a vm.Script instance and execute it in a specific context:

const vm = require("vm");

// Create a script instance.
const script = new vm.Script('console.log("Hello, world!");');

// Create a context.
const context = { console: console };

// Execute the script.
script.runInContext(context);

Output:

Hello, world!

Potential applications

vm.Script instances can be used for a variety of purposes, including:

Running code in a sandboxed environment.
Creating custom interpreters.
Compiling code on the fly.
Running code that is stored in a database.
Running code that is provided by a user.

Overview

vm module in Node.js allows you to execute JavaScript code in a sandboxed environment, separate from the main application's code. This is useful for running untrusted code, such as code downloaded from the internet, without the risk of it affecting your application's global state.

Creating a Script

To create a new script, use the new vm.Script(code [, options]) constructor. The code parameter is the JavaScript code you want to execute. The options parameter is an optional object that can contain the following properties:

filename: The filename of the script. This is used in error messages to identify the source of the error.
lineOffset: The line number offset that is displayed in error messages.
columnOffset: The first-line column number offset that is displayed in error messages.
cachedData: A buffer or TypedArray containing V8's code cache data for the supplied source.
produceCachedData: When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance.

Here's an example of creating a new script:

const vm = require('vm');
const script = new vm.Script('console.log("Hello World!");');

Running a Script

Once you have created a script, you can run it using the runInContext() method. The runInContext() method takes two parameters:

context: The context in which to run the script. This can be a global object, such as the global object of the current process, or a custom object that you create.
options: An optional object that can contain the following properties:
- timeout: The maximum amount of time (in milliseconds) to allow the script to run. If the script takes longer than this amount of time, it will be terminated.
- displayErrors: When true, errors that occur during the execution of the script will be displayed in the console.
- breakOnSigint: When true, the script will be interrupted when the user presses Ctrl+C.

Here's an example of running a script:

const vm = require('vm');
const script = new vm.Script('console.log("Hello World!");');
const context = { console: console };
script.runInContext(context);

Potential Applications

The vm module has a number of potential applications, including:

Running untrusted code, such as code downloaded from the internet.
Sandboxing code that could potentially crash your application.
Creating custom interpreters for other languages.
Running code in a different context, such as a different global object.

`script.cachedDataRejected`

Simplified Explanation:

When you create a Script object in Node.js, you can provide optional cachedData that V8 can use to optimize the script's execution. The script.cachedDataRejected property tells you if V8 accepted that data.

In-Depth Explanation:

V8 is the JavaScript engine used in Node.js. It optimizes scripts by storing them in a special way called "cached data." This can make scripts run faster the next time they're executed.

When you create a Script object, you can provide cachedData that you think V8 can use to optimize it. V8 will check the data and determine if it can be used.

If V8 accepts the data, script.cachedDataRejected will be false. This means that the script will use the cached data to run faster.

If V8 rejects the data, script.cachedDataRejected will be true. This means that V8 will not use the cached data, and the script will run without any optimization.

Real-World Example:

Imagine you have a Node.js application that runs a script multiple times. You could save the cachedData from the first time you run the script and use it for subsequent runs. This would make the script run faster.

// Get cached data for a script
const script = new vm.Script(source);
console.log(script.cachedData); // undefined

// Run the script and get cached data
script.runInContext();
console.log(script.cachedData); // some data

// Use cached data to run the script again
script.runInContext({ cachedData: script.cachedData });
// The script will use the cached data and run faster

Potential Applications:

Improving the performance of frequently-run scripts: By using cached data, you can make scripts run faster on subsequent executions.
Optimization: You can use the script.cachedDataRejected property to determine if the cached data was accepted by V8. This can help you identify issues with your data or V8's optimization process.

Simplified Description of `script.createCachedData()`

What is it?

The createCachedData() method in Node.js allows you to save the compiled code of a JavaScript script in a buffer. This buffer can then be used to create new script instances more quickly.

How Does it Work?

When you run a JavaScript script using the vm module, the script is compiled into machine code by V8 (the JavaScript engine in Node.js). This compilation process can take some time, especially for complex scripts. By caching the compiled code, you can avoid this compilation step the next time you run the script.

Why Use It?

There are a few cases where using cached data can be beneficial:

Performance: If you're frequently running the same script multiple times, creating cached data can significantly speed up execution.
Portability: You can share the cached data with other Node.js processes, allowing them to run the script without having to recompile it.

Example Usage

const script = new vm.Script('console.log("Hello World!")');
const cachedData = script.createCachedData();

// Create a new script instance using the cached data
const newScript = new vm.Script(null, { cachedData });

// Run the script
newScript.runInThisContext(); // Logs "Hello World!"

Real-World Applications

Cached data can be useful in various scenarios:

Server-side templating: If you're using a JavaScript-based templating engine, you can use cached data to improve the performance of your templates.
Dynamic code loading: If you're loading JavaScript code dynamically, you can cache the compiled code to avoid performance penalties on subsequent loads.
Code sharing: You can share cached data with other processes or systems to allow them to run the same script without compiling it locally.

`script.runInContext(contextifiedObject[, options])`

Simplified Explanation:

You can think of runInContext as a convenient way to execute JavaScript code inside a specific object. It's like running code in a sandbox.

contextifiedObject: This is the object where the code will run. It can access the variables and properties of that object.
options: You can use this to specify certain settings like:
- displayErrors: Show error messages.
- timeout: Set a time limit for running the code.
- breakOnSigint: Stop running the code if you press Ctrl + C.

Code Snippet:

const vm = require("node:vm");

// Create an object to run code in
const context = {
  animal: "cat",
  count: 2,
};

// Compile code to increment 'count' and change 'animal' to 'kitty'
const script = new vm.Script('count += 1; animal = "kitty";');

// Execute the code in the 'context' object
script.runInContext(context);

console.log(context); // Prints: { animal: 'kitty', count: 3 }

Real-World Applications:

Testing code in a controlled environment.
Isolating code that could potentially crash the application.
Scripting automation.

Complete Code Implementation and Example

Time-Limited Code Execution:

const vm = require("node:vm");

// Create a timeout for 1 second
const timeout = 1000;

// Compile code to run
const script = new vm.Script("while (true) {}");

// Run the code with a timeout
script.runInContext({}, { timeout });

Potential Applications

Running untrusted code in a sandbox.
Limiting execution time to prevent infinite loops.

`script.runInNewContext([contextObject[, options]])`

The runInNewContext method of the vm module executes a compiled script in a new context, which is an isolated environment with its own global object and set of variables.

Simplifying for a child:

Imagine a sandbox where you can play with code without affecting the rest of your computer. The runInNewContext method creates a sandbox for you, so you can run code safely without worrying about messing anything up.

Implementation and example:

The following code creates a sandbox, compiles a script that sets a global variable, and then executes the script in the sandbox:

// Create a sandbox
const sandbox = {};

// Compile a script
const script = new vm.Script('globalVar = "set"');

// Execute the script in the sandbox
script.runInNewContext(sandbox);

// Check the value of the global variable in the sandbox
console.log(sandbox.globalVar); // Outputs: 'set'

Real-world applications:

Isolating code: You can isolate code that you don't trust or that you want to keep separate from the rest of your program.
Testing code: You can test code in a sandbox before you run it in your main program.
Running untrusted code: You can run untrusted code in a sandbox to protect your system from malicious code.

Options:

The runInNewContext method accepts an optional options object, which can be used to configure the execution of the script. The following options are available:

displayErrors: If true, any errors that occur while executing the script will be printed to the console. The default is true.
timeout: The maximum amount of time (in milliseconds) that the script is allowed to run before it is terminated. If the script does not complete within the timeout period, an error will be thrown. The default is 0 (no timeout).
breakOnSigint: If true, pressing Ctrl+C will terminate the execution of the script. The default is false.
contextName: The human-readable name of the new context. The default is 'VM Context i', where i is an ascending numerical index of the created context.
contextOrigin: The origin corresponding to the newly created context for display purposes. The default is ''.
contextCodeGeneration: An object that configures the code generation options for the new context. The following options are available:
- strings: If false, any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. The default is true.
- wasm: If false, any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. The default is true.
microtaskMode: If set to afterEvaluate, microtasks (tasks scheduled through Promises and async functions) will be run immediately after the script has run. They are included in the timeout and breakOnSigint scopes in that case. The default is undefined.

script.runInThisContext([options])

Simplified Explanation:

Imagine you have a special notebook called "Script". You write code in this notebook and want to execute it within your current environment. script.runInThisContext() is like a "Run Script" button that lets you do just that.

Options:

displayErrors: When this is set to true, it shows the line of code that caused any errors that occur while running the script.
timeout: This sets a time limit (in milliseconds) for the script to execute. If it takes longer, the script is stopped and an error is thrown.
breakOnSigint: When this is true, pressing Ctrl+C will stop the script and throw an error.

How it Works:

You create a Script object with the code you want to run.
You call script.runInThisContext() with the appropriate options.
The script is executed within the current environment. It has access to the global variables of your program but not to local variables within your current code.
The result of the last statement in the script is returned.

Real-World Example:

Let's say you have a script that increments a global variable by 1 each time it runs:

const incrementGlobalVariable = new vm.Script("globalVariable += 1");

To execute this script 10 times, you would do this:

for (let i = 0; i < 10; i++) {
  incrementGlobalVariable.runInThisContext();
}

After running the script 10 times, the value of globalVariable would be incremented to 10.

Applications:

Dynamically modifying code at runtime (e.g., for debugging or testing).
Running scripts from user input (e.g., in a command-line tool).
Isolating code execution from the main application (for security or reliability).

script.sourceMapURL

Description: When you compile a script from a source code that contains a special comment called a "source map magic comment", script.sourceMapURL will be set to the URL of the source map.
** упрощенное объяснение:** Представьте, что у вас есть большой файл исходного кода, который вы хотите запустить в Node.js. Чтобы сделать отладку проще, вы хотите включить в этот файл исходного кода специальный комментарий, который указывает на файл "source map". Этот файл "source map" содержит дополнительную информацию, которая помогает сопоставить исходный код с скомпилированным кодом, что облегчает отладку. Когда вы создаете Script-объект из этого исходного кода, его свойство sourceMapURL будет содержать URL-адрес файла "source map".
Пример кода:

import vm from "node:vm";

// Создайте исходный код с комментарием source map.
const sourceCode = `
function myFunc() {}
//# sourceMappingURL=sourcemap.json
`;

// Создайте Script-объект из исходного кода.
const script = new vm.Script(sourceCode);

// Выведите URL-адрес source map.
console.log(script.sourceMapURL); // Выводит: sourcemap.json

Реальный пример: Предположим, у вас есть большой модуль JavaScript, который вы хотите запустить в Node.js. Чтобы упростить отладку, вы решили использовать source map. Вы можете добавить следующий комментарий в свой модуль:

//# sourceMappingURL=my-module.js.map

Затем создайте соответствующий файл my-module.js.map и убедитесь, что он находится в том же каталоге, что и ваш модуль. При запуске модуля в Node.js вы можете использовать script.sourceMapURL для доступа к URL-адресу файла "source map". Это поможет вам легче отслеживать ошибки и просматривать исходный код во время отладки.

Introduction to vm.Module

The vm.Module class in Node.js represents JavaScript modules in a virtual machine (VM) context. It allows you to load, link, and evaluate modules explicitly within a VM, providing fine-grained control and flexibility.

How it Works

A vm.Module has three main stages:

Creation/Parsing: You create a vm.Module object with the source code of the module. This stage parses the code and checks for syntax errors.
Linking: You link the module's dependencies (imported modules) to the module. This means finding the modules it depends on and establishing connections between them.
Evaluation: Finally, you evaluate the module, which executes the code and returns its value.

Example: Loading and Running a Module

Here's a code snippet that illustrates the process of using vm.Module:

// Create a VM context
const context = {
  secret: 42,
  print: console.log,
};

// Create a module with source code
const module = new vm.SourceTextModule(
  `
  import { secret } from 'foo';
  print(secret);
`,
  { context }
);

// Link the module's dependency (foo) to the module
async function linker(specifier, referencingModule) {
  return new vm.SourceTextModule(
    `
    export default 42;
  `,
    { context: referencingModule.context }
  );
}

await module.link(linker);

// Evaluate the module
await module.evaluate(); // Prints "42"

In this example:

We create a VM context that includes a custom "secret" variable.
We create a vm.Module with a source code that imports a module named "foo" and prints its default export (in this case, the "secret" variable).
We define a linker function that provides the "foo" module implementation.
We link the "foo" module to our module.
Finally, we evaluate the module, which prints the value of "secret" from the "foo" module.

Real-World Applications

Sandboxing: Isolating code execution from the main application.
Dynamic module loading: Loading and executing modules at runtime based on specific criteria.
Testing: Evaluating modules in controlled environments for unit testing.

Conclusion

The vm.Module class provides a powerful tool for managing and executing JavaScript modules in a controlled VM context. It enables advanced module handling and customization, making it valuable for various scenarios in both development and production environments.

module.dependencySpecifiers

This property is an array of strings that contains the names of all the modules that the current module depends on. The array is frozen, meaning that it cannot be changed.

Real-world example:

Imagine you have a module called myModule.js that depends on two other modules, module1.js and module2.js. The dependencySpecifiers property of myModule.js would be:

['module1.js', 'module2.js']

Potential applications:

The dependencySpecifiers property can be used to create a dependency graph of all the modules in a project. This can be useful for identifying circular dependencies or for understanding the overall structure of the project.
The dependencySpecifiers property can be used to create a build pipeline that automatically recompiles modules when their dependencies change. This can help to improve the performance and reliability of your project.

`module.error`

Type: any

Description: If the module.status is 'errored', this property contains the exception thrown by the module during evaluation. If the status is anything else, accessing this property will result in a thrown exception.

The value undefined cannot be used for cases where there is not a thrown exception due to possible ambiguity with throw undefined;.

Corresponds to: The [[EvaluationError]] field of [Cyclic Module Record][]s in the ECMAScript specification.

Real-world example:

const vm = require("vm");
const script = new vm.Script('throw new Error("This is an error")');

try {
  script.runInThisContext();
} catch (err) {
  console.error(err.message); // 'This is an error'
  console.error(script.module.error); // Error: This is an error
}

Potential applications:

Error handling in dynamic code evaluation
Debugging and testing of modules

Simplified Explanation of module.evaluate([options])

What is the `evaluate()` method?

The evaluate() method is used to run the code in a previously linked module.

How does the `evaluate()` method work?

Once a module has been linked, you can call the evaluate() method to run the code in that module. The evaluate() method takes an optional options object as an argument. The options object can contain the following properties:

timeout: Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, an error will be thrown.
breakOnSigint: If true, receiving SIGINT (Ctrl+C) will terminate execution and throw an error.

When should you use the `evaluate()` method?

You should use the evaluate() method after you have linked a module and you want to run the code in that module.

Real-World Example

Here is an example of how to use the evaluate() method:

const vm = require('vm');

// Create a new context
const context = vm.createContext();

// Link a module to the context
context.link('my-module', 'console.log("Hello, world!")');

// Evaluate the module
vm.runInContext('module.evaluate()', context);

// Output: Hello, world!

In this example, we create a new context and then link a module to the context. The module contains the code console.log("Hello, world!"). We then evaluate the module using the evaluate() method. This causes the code in the module to be executed, and the output is printed to the console.

Potential Applications

The evaluate() method can be used in a variety of applications, including:

Running code in a sandboxed environment
Testing code before deploying it to production
Creating custom modules for Node.js applications

Simplified Explanation:

module.identifier is a property that stores the unique name of the current JavaScript module. You set this name when you create the module using the vm.createContext function.

Detailed Explanation:

A JavaScript module is a separate unit of code that can be imported and used by other modules. In Node.js, you can create modules using the vm (virtual machine) module.

When you create a module using vm.createContext, you can specify a unique identifier for it using the identifier option. This identifier is used to distinguish the module from other modules that may be running in the same JavaScript engine.

Code Snippet:

const vm = require('vm');

// Create a module with the identifier "myModule"
const context = vm.createContext({ identifier: 'myModule' });

// Define a function inside the module
vm.runInContext('function myFunction() { console.log("Hello from myModule!"); }', context);

// Call the function from the module
context.myFunction(); // Logs "Hello from myModule!"

Real-World Applications:

Module Isolation: You can use module identifiers to ensure that modules run in isolation from each other. This helps prevent conflicts between modules that have the same global variables or functions.
Debugging: You can use module identifiers to track down errors and identify the source of problems in your code.
Testing: You can use module identifiers to create test environments for your modules, where you can mock out dependencies and test them in a controlled manner.

`module.link(linker)`

Parameters:
- linker: A function that links module dependencies.
Returns: A Promise that resolves to a Module object or rejects with an error.

Detailed Explanation

The module.link() method is a way to link module dependencies together before evaluating them. This method must be called before evaluation, and can only be called once per module.

The linker function is responsible for returning a Module object or a Promise that eventually resolves to a Module object. The returned Module must satisfy the following two invariants:

It must belong to the same context as the parent Module.
Its status must not be 'errored'.

If the returned Module's status is 'unlinked', the link() method will be recursively called on the returned Module with the same provided linker function.

The link() method returns a Promise that will either get resolved when all linking instances resolve to a valid Module, or rejected if the linker function either throws an exception or returns an invalid Module.

Real-World Example

Here is an example of how to use the module.link() method to link module dependencies:

const vm = require("vm");

const module = new vm.Module('console.log("Hello, world!")');

module.link((specifier, referencingModule) => {
  if (specifier === "console") {
    return Promise.resolve(new vm.Module("exports = { log: console.log }"));
  } else {
    throw new Error(`Unknown module specifier: ${specifier}`);
  }
});

module.evaluate();

In this example, the linker function is responsible for returning a Module object that exports the console object. The module.evaluate() method is then called to evaluate the module.

Potential Applications

The module.link() method can be used to dynamically load and link modules at runtime. This can be useful for creating modular applications that can be loaded and unloaded on demand.

Here are some potential applications for the module.link() method:

Creating plugins for applications
Loading modules from remote servers
Hot-swapping modules in development environments

Module Namespace

In JavaScript, modules can organize code into separate files and then import and use those files in other parts of the program. When a module is imported, it creates a namespace object that contains all the exported values from that module.

Explanation:

Imagine you have a module called "math.js" that contains the following code:

export const add = (a, b) => a + b;
export const subtract = (a, b) => a - b;

When you import this module into another file, you can access its exported values by using the namespace object:

import { add, subtract } from "./math.js";

console.log(add(1, 2)); // Output: 3
console.log(subtract(3, 1)); // Output: 2

In this example, the add and subtract functions are available through the namespace object.

Key Points:

The namespace object is created when a module is imported.
It contains all the exported values from the module.
You can access exported values through the namespace object.

Real-World Application:

Module namespaces allow you to organize and reuse code effectively. For example, you could create a module for each section of your application, such as the UI, database, or networking, and then import those modules into different parts of the code to create a modular and maintainable program.

Module Status in Node.js VM Module

Introduction

The vm module in Node.js provides an interface to create and execute JavaScript code in a sandbox. Each JavaScript code that you create is called a module. Modules can have dependencies on other modules. A module's status describes its current state in the linking and evaluation process.

Module Status Values

There are six possible status values for a module:

'unlinked': The module has not yet been linked to its dependencies.
'linking': The module is in the process of linking to its dependencies.
'linked': The module has been successfully linked to all its dependencies, but has not yet been evaluated.
'evaluating': The module is being evaluated.
'evaluated': The module has been successfully evaluated.
'errored': The module has been evaluated, but an error occurred.

Visual Representation

Here's a simplified visual representation of the module status values:

Unlinked -> Linking -> Linked -> Evaluating -> Evaluated

Real-World Example

Consider the following code snippet that links and evaluates a module:

const vm = require("vm");

const module = new vm.Module('console.log("Hello World")');

module.link((err) => {
  if (err) {
    throw err;
  }

  module.evaluate();
});

In this example, the module's status would change as follows:

'unlinked': When the module is created, its status is 'unlinked'.
'linking': When the module.link() function is called, the status changes to 'linking'.
'linked': If the linking is successful, the status changes to 'linked'.
'evaluating': When the module.evaluate() function is called, the status changes to 'evaluating'.
'evaluated': If the evaluation is successful, the status changes to 'evaluated'.

Potential Applications

The vm module can be used in a variety of applications, including:

Sandboxing code: Executing code in a sandboxed environment prevents it from accessing sensitive data or resources outside the sandbox.
Creating dynamic modules: Generating and evaluating code dynamically allows for flexible and customizable module loading.
Testing and debugging: Isolated code execution simplifies testing and debugging processes.

Conclusion

Understanding the status of modules in the vm module is crucial for managing code execution in a sandboxed environment. By tracking the status of modules, you can ensure that they are properly linked and evaluated, and handle any errors that may occur.

Class: vm.SourceTextModule

Simplified Explanation:

Imagine you have a piece of code that you want to run as a separate module, like a recipe for making cookies. Instead of writing the recipe in the same file as your main program, you can put it in a separate file called a "module."

Extends: vm.Module

This means that the vm.SourceTextModule class inherits all the properties and methods of the vm.Module class. A module is like a container for code that can be reused in multiple programs.

Real-World Example:

Let's say you have a shopping list app that you want to extend with a recipe module. Instead of writing the recipe code directly in the app, you can create a separate module file for the recipe. This makes it easier to update or replace the recipe without affecting the main app code.

Code Implementation:

// recipe-module.js
export function makeCookies() {
  // Recipe code goes here
}

// app.js
import { makeCookies } from "./recipe-module.js";

// Call the recipe function
makeCookies();

Potential Applications:

Code reusability: Splitting code into modules makes it easier to manage and reuse code in multiple applications.
Code separation: Modules help keep code organized and separate different parts of an application into logical units.
Extensibility: Modules allow for easy extension of applications by adding or updating modules without modifying the main codebase.

What is the vm module?

The vm module provides an interface to compile JavaScript code and execute it in a sandboxed environment.

What is a SourceTextModule?

A SourceTextModule is a class that represents a JavaScript module that is defined as a string.

Creating a SourceTextModule

You can create a SourceTextModule by calling the new vm.SourceTextModule(code[, options]) constructor, where:

code is the JavaScript code to parse
options is an optional object that can contain the following properties:
- identifier: A string used in stack traces. Defaults to 'vm:module(i)' where i is a context-specific ascending index.
- cachedData: Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. The code must be the same as the module from which this cachedData was created.
- context: The [contextified][] object as returned by the vm.createContext() method, to compile and evaluate this Module in. If no context is specified, the module is evaluated in the current execution context.
- lineOffset: Specifies the line number offset that is displayed in stack traces produced by this Module. Defaults to 0.
- columnOffset: Specifies the first-line column number offset that is displayed in stack traces produced by this Module. Defaults to 0.
- initializeImportMeta: Called during evaluation of this Module to initialize the import.meta.
- importModuleDynamically: Used to specify the how the modules should be loaded during the evaluation of this module when import() is called. This option is part of the experimental modules API. We do not recommend using it in a production environment.

Evaluating a SourceTextModule

Once you have created a SourceTextModule, you can evaluate it by calling the evaluate() method. The evaluate() method takes an optional callback function as an argument. If a callback function is provided, it will be called after the module has been evaluated.

Example

The following example creates a SourceTextModule and evaluates it:

const vm = require("vm");

const code = 'console.log("Hello, world!");';

const module = new vm.SourceTextModule(code);

module.evaluate();

Output:

Hello, world!

Real-world applications

The vm module can be used in a variety of real-world applications, such as:

Creating custom JavaScript environments
Sandboxing untrusted code
Running JavaScript code from strings
Compiling JavaScript code for use in other applications

`sourceTextModule.createCachedData()`

Returns: {Buffer}

Creates a code cache that can be used with the SourceTextModule constructor's cachedData option.

The code cache of the SourceTextModule doesn't contain any JavaScript observable states. The code cache is safe to be saved along side the script source and used to construct new SourceTextModule instances multiple times.

Functions in the SourceTextModule source can be marked as lazily compiled and they are not compiled at construction of the SourceTextModule. These functions are going to be compiled when they are invoked the first time. The code cache serializes the metadata that V8 currently knows about the SourceTextModule that it can use to speed up future compilations.

Real-world example:

// Create an initial module
const module = new vm.SourceTextModule("const a = 1;");

// Create cached data from this module
const cachedData = module.createCachedData();

// Create a new module using the cached data. The code must be the same.
const module2 = new vm.SourceTextModule("const a = 1;", { cachedData });

Potential applications in the real world:

Caching the code of a frequently used script to improve performance.
Serializing the code of a module to be used in a different process or on a different machine.

Synthetic Modules

Imagine you have a special puzzle box. Inside the box are individual puzzle pieces that each represent a different part of a larger puzzle. You can't see the whole puzzle, but you can access each piece.

Synthetic modules are like those puzzle pieces. They are designed to be used with other puzzle pieces (modules) to create a complete picture (your application). However, these synthetic modules don't actually come from a JavaScript source file. Instead, they contain data from other sources, like JSON or binary data.

Creating Synthetic Modules

To create a synthetic module, you need to specify what the puzzle piece looks like (its exports) and how to put it together (the module function).

const syntheticModule = new vm.SyntheticModule(['pieceA', 'pieceB'], function() {
  this.setExport('pieceA', { value: 'A' });
  this.setExport('pieceB', { value: 'B' });
});

In this example, we have a synthetic module that exports two pieces: 'pieceA' and 'pieceB'. Each piece has a value, 'A' and 'B' respectively.

Using Synthetic Modules

You can use synthetic modules just like regular modules. For example, you can import them into your application:

import { pieceA, pieceB } from './syntheticModule';

console.log(pieceA.value); // "A"
console.log(pieceB.value); // "B"

Real-World Applications

Synthetic modules can be used in various scenarios:

Loading data from non-JavaScript sources: You can create synthetic modules that load data from JSON files, databases, or binary sources.
Caching data: You can use synthetic modules to cache data that is used by multiple modules.
Providing dynamic functionality: You can create synthetic modules that provide dynamic functionality, such as loading new modules or updating data on the fly.

Here's an example of a real-world application of synthetic modules:

Imagine you have a web application that needs to display user data from a database. You can create a synthetic module that loads the user data when the module is imported into your application. This way, you can access the user data without having to write additional code to load it.

Creating Synthetic Modules in Node.js VM

Imagine you have a special box called a "synthetic module." This box can create new modules that can be used in your JavaScript code.

To make one, you need to tell the box the following:

What it's called: What name do you want to give to the module?
What it does: What code should be run when someone imports this module?
Optional: You can also give the module a nickname and tell it where to run the code.

Here's some code to create a synthetic module:

const vm = require('vm');

// Create a synthetic module called "MyModule"
const myModule = new vm.SyntheticModule(['hello'], function(exports) {
  // Code to be run when someone imports this module
  exports.hello = 'World';
});

Using Synthetic Modules

Once you have created a synthetic module, you can use it in your code:

// Import the synthetic module
const { hello } = require('./MyModule.js');

// Use the exported value
console.log(hello); // Output: "World"

Real-World Applications

Synthetic modules can be useful in situations where you need to:

Dynamically create modules based on user input or data.
Create modules that can be customized and reused in different contexts.
Extend existing modules with additional functionality.

Example:

Suppose you have a system that allows users to create their own custom widgets. You could use synthetic modules to dynamically create JavaScript modules for each widget, containing the widget's logic and UI. This would allow users to easily add and customize widgets to their applications.

SyntheticModule.setExport() Method in Node.js

Explanation:

Imagine you're building a virtual machine (VM) using Node.js's vm module. Inside this VM, you want to create your own module and set its exports, which are values that can be accessed from outside the module.

SyntheticModule.setExport() allows you to set these exports after linking the module. Linking is like connecting the module to the VM.

Usage:

m.setExport('name', 'value'); // name: export name, value: export value

Example Code:

// Create a synthetic module
const m = new vm.SyntheticModule(["x"], () => {
  // Set export 'x' to 1
  m.setExport("x", 1);
});

// Link and evaluate the module
m.link(() => {});
m.evaluate();

// Access the exported value 'x'
console.log(m.namespace.x); // Output: 1

Real-World Applications:

Mocking modules: Test code that depends on specific modules without having to actually import them.
Creating isolated environments: Run code in a separate VM to prevent it from affecting the global scope.
Inspecting code: Analyze code or create dynamic modules for other purposes.

`vm.compileFunction(code[, params[, options]])`

Purpose: Compiles JavaScript code into a function that can be executed in a specified context.

Signature:

vm.compileFunction(code: string, params?: string[], options?: vm.CompileFunctionOptions): Function;

Parameters:

code: The JavaScript code to be compiled.
params: (Optional) An array of parameter names for the compiled function.
options: (Optional) An object of options:
- filename: (Optional) The filename to use in stack traces generated by the compiled function. Default: ''.
- lineOffset: (Optional) The line number offset to use in stack traces generated by the compiled function. Default: 0.
- columnOffset: (Optional) The column number offset to use in stack traces generated by the compiled function. Default: 0.
- cachedData: (Optional) A buffer containing cached code data to reuse for optimization.
- produceCachedData: (Optional) Whether to produce cached code data. Default: false.
- parsingContext: (Optional) The context object to compile the code in. Default: the current context.
- contextExtensions: (Optional) An array of context extensions to apply during compilation. Default: [].
- importModuleDynamically: (Optional) A function used to dynamically load modules when using import() in the compiled code. Default: vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER.

Return Value:

A function object representing the compiled code.

Example:

// Compile a simple function to calculate the area of a circle
const code = "const area = (radius) => Math.PI * radius ** 2;";
const compiledFunction = vm.compileFunction(code, ["radius"]);

// Execute the compiled function with a radius of 5
const result = compiledFunction(5);

// Log the result, which should be 78.53981633974483
console.log(result);

Applications:

Running untrusted code in a sandboxed environment
Generating code dynamically at runtime
Compiling custom JavaScript modules

`vm.constants`

{Object}

The vm.constants object contains commonly used constants for VM operations. These constants can be used to specify the context in which a script is executed, or to control the behavior of the VM.

Properties

kContextFlags: {number}
- Flags that can be used to specify the context in which a script is executed.
kCompileFlags: {number}
- Flags that can be used to control the behavior of the VM.
kVmFlags: {number}
- Flags that can be used to control the behavior of the VM.
kSerializedVmFlags: {number}
- Flags that can be used to control the behavior of the VM when serializing a script.

Example

The following example shows how to use the vm.constants object to specify the context in which a script is executed:

const vm = require("vm");

const context = {
  global: global,
  module: module,
  console: console,
};

const script = new vm.Script('console.log("Hello, world!");', {
  contextFlags: vm.constants.kContextFlags.withGlobalLexicalEnvironment,
});

script.runInContext(context);

In this example, the vm.constants.kContextFlags.withGlobalLexicalEnvironment flag is used to specify that the script should be executed in a context that includes the global lexical environment. This means that the script will have access to all of the global variables that are available in the current environment.

`vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`

Simplified Explanation:

This is a special value used to tell Node.js to use the same module loading system as the main program when importing modules dynamically within a compiled script or function.

Detailed Explanation:

When compiling a script or function using the vm module, you can specify how dynamic import() statements are handled. By default, these statements use a specialized module loader provided by the vm module.

However, you can specify the USE_MAIN_CONTEXT_DEFAULT_LOADER option to tell Node.js to instead use the default ESM (EcmaScript Modules) loader from the main program context. This means that the compiled script or function will use the same module loading system as the rest of the program.

Real-World Example:

const script = new vm.Script(`import { MyClass } from 'my-module';`);

script.runInContext({ USE_MAIN_CONTEXT_DEFAULT_LOADER: true });

In this example, the script will import the MyClass class from the my-module module using the same module loader as the main program.

Potential Applications:

Using the main context's default loader can be useful when:

You want to share modules between the main program and compiled scripts or functions.
You need to access global variables or objects defined in the main program from within the compiled code.
You want to use custom module loaders or plugins that are installed in the main program.

VM Context

Imagine you have a virtual machine (VM), like a computer inside your computer, where you can run scripts. The vm module lets you create a special environment inside the VM called a "VM context". It's like creating a separate room in the VM where you can run those scripts.

Creating a VM Context

To create a VM context, you can use the vm.createContext() method. It's like creating a new room in the VM. You can specify a name for the room (like "My Room") and even a "URL" (like example.com) that will show up in the room's properties. You can also choose whether you want to allow certain things in the room, like using certain functions or loading modules.

Using the VM Context

Once you have a VM context, you can use it to run scripts. The scripts will run inside the created room, and they will have access to all the built-in functions and objects that a web browser or Node.js environment would have.

For example, let's say you have a script that creates a global variable globalVar and sets it to 1. You can create a VM context, run the script inside it, and the globalVar variable will be created inside the context room. However, outside the context room, the globalVar variable will not exist.

Real-World Applications

Here are some real-world applications of VM contexts:

Web Browsers: Web browsers create a new VM context for each tab or window. This allows scripts from different pages to run in separate rooms, preventing them from interfering with each other.
Virtualization: VMs can be used to run multiple operating systems or applications on a single computer. Each VM has its own VM context, which ensures that the different operating systems or applications don't conflict with each other.
Sandboxing: VM contexts can be used to create isolated environments for running untrusted code, such as code downloaded from the internet. This prevents malicious code from modifying the main environment or accessing sensitive information.

Example

Here's an example of creating and using a VM context to run a script:

const vm = require("vm");

// Create a VM context with a name and origin
const context = vm.createContext({
  name: "My Room",
  origin: "example.com",
});

// Run a script inside the VM context
vm.runInContext("globalVar = 1", context);

// Check if the globalVar variable exists in the context
const globalVarContext = context.globalVar; // 1

// Check if the globalVar variable exists outside the context
const globalVarGlobal = global.globalVar; // undefined

In this example, the globalVar variable is created inside the VM context and is only accessible within that context.

`vm.isContext(object)`

Purpose: Checks if a given object is a JavaScript context created using vm.createContext().

Parameters:

object: The object to check.

Return Value:

true if the object is a JavaScript context, false otherwise.

Simplified Explanation:

Imagine JavaScript as a playground where you can play with variables, functions, and other stuff. A JavaScript context is like a separate sandbox where you can run JavaScript code safely, without affecting the main program.

vm.isContext() helps you check if an object is like one of these sandboxes. If it is, it returns true, like giving you a thumbs up. If it's not, it returns false, like shaking its head.

Code Sample:

const vm = require("vm");

const sandbox = vm.createContext();

console.log(vm.isContext(sandbox)); // Outputs: true

Real-World Application:

Safely executing untrusted code from a third party in a sandbox, like running a snippet of JavaScript from a webpage that you don't fully trust.

Sure, let's simplify and explain the vm.measureMemory() function from Node.js's vm module:

What is vm.measureMemory()?

vm.measureMemory() is a function that measures the memory usage of a JavaScript program. It does this by taking a snapshot of the memory used by all contexts (think of them as isolated JavaScript environments) in the current V8 isolate (the underlying JavaScript engine used by Node.js).
It can measure the memory usage of the main context (the global context where your program runs) or all contexts created in the current isolate.
It returns a Promise that resolves with an object containing information about the memory usage, including the estimated JavaScript memory usage and the range of memory addresses used by JavaScript.

Why would you use vm.measureMemory()?

You can use vm.measureMemory() to identify memory leaks in your program. A memory leak occurs when your program holds onto references to objects that are no longer needed, causing the memory usage to increase over time.
By measuring the memory usage at different points in your program, you can see if there are any sudden increases in memory usage that could indicate a memory leak.

How to use vm.measureMemory()

The basic syntax of vm.measureMemory() is:

vm.measureMemory([options])

The options object can contain the following properties:
mode: Either "summary" or "detailed". In "summary" mode, only the memory used by the main context is measured. In "detailed" mode, the memory used by all contexts is measured. Defaults to "summary".
execution: Either "default" or "eager". With "default" execution, the measurement is taken during the next scheduled garbage collection (when the V8 engine automatically reclaims unused memory). With "eager" execution, the measurement is taken immediately. Defaults to "default".
Here is an example of how to use vm.measureMemory() to measure the memory usage of the main context:

const vm = require('vm');

vm.measureMemory()
  .then((result) => {
    console.log(result);
  });

The following is an example of how to use vm.measureMemory() to measure the memory usage of all contexts.

const vm = require('vm');

vm.measureMemory({ mode: 'detailed' })
  .then((result) => {
    console.log(result);
  });

Example of Measuring Memory Leaks

The following is a simple example of how vm.measureMemory() can be used to identify memory leaks:

const vm = require("vm");

const context = vm.createContext({});

// Create a reference to an object in the context
const obj = { a: 1, b: 2 };
context.obj = obj;

// Measure the memory usage before and after deleting the reference
vm.measureMemory({ mode: "detailed" })
  .then((before) => {
    // Delete the reference to the object
    delete context.obj;

    // Trigger garbage collection
    global.gc();

    return vm.measureMemory({ mode: "detailed" });
  })
  .then((after) => {
    // Check if the memory usage has decreased after deleting the reference
    if (after.total.jsMemoryEstimate < before.total.jsMemoryEstimate) {
      console.log("No memory leak detected");
    } else {
      console.log("Memory leak detected");
    }
  });

Real-world Applications

Here are some real-world applications of vm.measureMemory():

Memory leak detection: You can use vm.measureMemory() to identify memory leaks in your program, which can help you improve the performance and stability of your application.
Performance optimization: You can use vm.measureMemory() to measure the memory usage of different parts of your program. This can help you identify areas where your program is using too much memory and make optimizations to reduce the memory usage.
Debugging: You can use vm.measureMemory() to help debug memory-related issues in your program. For example, you can use it to see if a particular function or module is causing a memory leak.

`vm.runInContext(code, contextifiedObject[, options])`

Explanation:
- The vm.runInContext() function compiles JavaScript code and executes it within a specified context.
- The context is an object that defines the global variables and environment for the code to run in.
- You can use vm.runInContext() to create isolated environments or run code in a sandbox with limited access.
Arguments:
- code (string): The JavaScript code to execute.
- contextifiedObject (object): The context object in which to run the code. This object must have been created using vm.createContext().
- options (object | string): Optional options to customize the behavior of vm.runInContext(). Can be a filename string or an options object with the following properties:
  - filename (string): The filename to use in stack traces.
  - lineOffset (number): The line number offset to use in stack traces.
  - columnOffset (number): The column offset to use in stack traces.
  - displayErrors (boolean): Whether to display errors in stack traces.
  - timeout (integer): The maximum execution time allowed for the code in milliseconds.
  - breakOnSigint (boolean): Whether to terminate execution on receiving SIGINT.
  - cachedData (Buffer): Optional code cache data for the supplied source.
  - importModuleDynamically (function): A function used to dynamically load modules when import() is used in the code.

Example:

// Create a context object
const context = vm.createContext({});

// Compile and execute a script using the context
vm.runInContext("var x = 10;", context);

// Access the global variable defined in the script
console.log(context.x); // Outputs: 10

Real-World Applications:
- Isolating untrusted code: You can run untrusted code in an isolated environment using vm.runInContext(), preventing it from accessing the global scope or modifying the system.
- Creating custom sandboxes: You can define custom sandboxes with specific permissions and restrictions using vm.runInContext() to run code in controlled environments.
- Implementing modularity: You can use vm.runInContext() to create isolated modules that can interact with each other in a controlled manner, promoting modularity and encapsulation in your code.

What is vm.runInNewContext()?

It's a function in Node.js that allows you to run JavaScript code in a separate environment, called a context, different from the main Node.js code.

How does it work?

You create or pass an object to runInNewContext() that will hold the variables and functions you want to use in the JavaScript code.
You write the JavaScript code you want to run as a string.
You pass the code and the object to the runInNewContext() function.
The function compiles and runs the code in a new context, using the object you provided.
The result of the code is returned to you.

Benefits of using vm.runInNewContext():

Isolation: The code runs in a separate environment, so it can't affect the main Node.js code or other scripts running in other contexts.
Safety: If the code throws an error, it won't crash the entire Node.js process.
Sandboxing: You can control what the code has access to, such as global variables and functions.

Code Snippet:

// Create an object to hold variables and functions
const contextObject = {
  animal: "cat",
  count: 2,
};

// Code to run
const vmCode = 'count += 1; name = "kitty"';

// Run the code in a new context using the object
const result = vm.runInNewContext(vmCode, contextObject);

// Log the updated object
console.log(contextObject); // { animal: 'cat', count: 3, name: 'kitty' }

Real-World Applications:

Dynamically generated code: You can generate JavaScript code on the fly and run it in a sandboxed environment.
Code execution in controlled environments: You can restrict the code's access to specific resources, making it suitable for executing untrusted code.
Testing and debugging: You can isolate code for testing purposes, allowing you to control its input and output more effectively.

What is vm.runInThisContext()?

vm.runInThisContext() is a function in Node.js that allows you to run JavaScript code within the current context. This means that the code can access the current global variables and functions, but not the local variables and functions of the current scope.

How do I use vm.runInThisContext()?

To use vm.runInThisContext(), you simply pass it the JavaScript code you want to run as a string. For example:

const vm = require("vm");
const code = 'console.log("Hello, world!");';
vm.runInThisContext(code);

This will print "Hello, world!" to the console.

Why would I use vm.runInThisContext()?

There are a few reasons why you might want to use vm.runInThisContext():

To run code that you have loaded from a file or from a remote source.
To run code that you want to isolate from the current scope.
To create a custom JavaScript environment with specific globals and functions.

Real-world examples of vm.runInThisContext()

Here are a few real-world examples of how you might use vm.runInThisContext():

To load and run a JavaScript file:

const fs = require("fs");
const code = fs.readFileSync("script.js");
vm.runInThisContext(code);

To run code in a separate JavaScript environment:

const vm = require("vm");
const sandbox = {
  console: {
    log: function () {},
  },
};
const code = 'console.log("Hello, world!");';
vm.runInThisContext(code, sandbox);

This will run the code in the sandbox environment, so the "Hello, world!" message will not be printed to the console.

Potential applications of vm.runInThisContext()

Here are a few potential applications of vm.runInThisContext():

Developing and testing JavaScript code in isolation
Running JavaScript code from untrusted sources
Creating custom JavaScript environments for specific tasks

Simplified Explanation:

V8 Global Context and Isolation

Imagine a big box where all your JavaScript code lives. This box is called the "V8 Global Context." When you execute code in a script or VM, you create a smaller box inside it. This smaller box is called an "isolated scope."

Running HTTP Server in an Isolated Scope

When you want to run a web server in an isolated scope, you need to tell the script or VM where to find the code for the HTTP module.

Using require() to Access Modules

require() is a special function that lets you access built-in Node.js modules, like the HTTP module. When you call require('node:http'), it loads the HTTP module's code into your isolated scope.

Example Code:

// Create an isolated scope
const vm = require("vm");

// Code to run inside the isolated scope
const code = `
  // Access the HTTP module
  const http = require('node:http');

  // Start the web server
  http.createServer((request, response) => {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.end('Hello World\n');
  }).listen(8124);

  console.log('Server running at http://127.0.0.1:8124/');
`;

// Run the code in the isolated scope
vm.runInThisContext(code)(require);

Real-World Applications:

Safe Execution: Running untrusted code in an isolated scope prevents it from harming the main V8 Global Context.
Sandboxed Environments: VMs can create multiple isolated scopes, allowing different parts of a program to operate separately.
Extensibility: Modules can be loaded dynamically into isolated scopes, extending functionality without restarting the entire program.

What is Contextualizing an Object in Node.js?

Imagine Node.js as a stage, and JavaScript code as actors performing on it. Each actor needs a separate area, called a "context," to perform.

How to Contextify an Object

To give an object its own context, you use the vm.createContext() method:

const { vm } = require("node:vm");

const context = vm.createContext();

This creates a new context and associates it with the context object.

What Happens When You Contextify an Object

Contextifying an object creates an isolated environment for JavaScript code. This means:

Code running in this context cannot access variables or functions outside the context.
Changes made to variables or functions inside the context do not affect code outside the context.

Benefits of Contextualization

Isolation: Prevents code from interfering with other code running in Node.js.
Security: Helps prevent malicious code from accessing sensitive information or doing damage.
Testability: Makes it easier to test JavaScript code in isolation.

Real-World Applications

Sandboxing: Running untrusted code in a sandbox context to prevent it from harming the rest of the system.
Plug-ins: Creating isolated contexts for plug-ins to run in, protecting the main application from plug-in errors.
Testing: Isolating test code from production code to ensure tests do not affect the running system.

Promises and Async Functions

Promises and async functions in JavaScript allow you to schedule tasks to run after other code has finished.

Example:

// Create a promise that resolves after 1 second
const promise = new Promise((resolve) => {
  setTimeout(() => {
    resolve("Hello");
  }, 1000);
});

// When the promise resolves, log its value
promise.then((value) => {
  console.log(value); // Logs "Hello"
});

Timeout Interactions

Normally, JavaScript executes code in the order it's written. However, promises and async functions can create tasks that run asynchronously (after other code). This can cause problems with timeout functions, which interrupt code execution after a certain amount of time.

Example:

// Set a timeout of 5 milliseconds
const timeout = setTimeout(() => {
  console.log("Time is up!");
}, 5);

// Schedule an infinite loop to run asynchronously
Promise.resolve().then(() => {
  while (true) {}
});

In this example, the timeout is never reached because the infinite loop escapes it by running asynchronously. To fix this, you can use the microtaskMode option when creating a vm.Context.

Microtask Mode

The microtaskMode option controls how microtasks (tasks scheduled by promises and async functions) are handled. The default value is 'notStarted', which means that microtasks are not started until the vm.runInNewContext() function returns.

By setting microtaskMode to 'afterEvaluate', microtasks are started after the vm.runInNewContext() function returns. This allows timeouts to interrupt microtasks.

Example:

const vm = require("node:vm");

// Create a timeout of 5 milliseconds
const timeout = setTimeout(() => {
  console.log("Time is up!");
}, 5);

// Schedule an infinite loop to run asynchronously
vm.runInNewContext(
  "Promise.resolve().then(() => { while (true) {} });",
  {},
  { timeout: 5, microtaskMode: "afterEvaluate" }
);

In this example, the timeout will interrupt the infinite loop because microtasks are started after vm.runInNewContext() returns.

Real-World Applications

Asynchronous Programming: Promises and async functions allow you to write asynchronous code without using callbacks. This makes your code more readable and easier to maintain.
Concurrency: Promises and async functions can be used to write concurrent code, which allows you to perform multiple tasks at the same time. This can improve the performance of your applications.
Error Handling: Promises provide a simple way to handle errors in asynchronous code. You can use .then() and .catch() to handle successful and unsuccessful results.

Dynamic import() in Node.js's vm Module: An Easy Guide

What is Dynamic import()?

Imagine you have a recipe that needs an ingredient that you don't have. In JavaScript, dynamic import() allows you to "import" that missing ingredient (module) at the moment you need it, instead of having to load it upfront. It's like a smart helper that fetches exactly what you need, when you need it.

vm Module

The vm module in Node.js lets you run JavaScript code dynamically. This means you can create and execute scripts "on the fly" without having to write and save a JavaScript file.

vm APIs with Dynamic import() Support

Certain APIs in the vm module now support the importModuleDynamically option, which enables dynamic import() in scripts compiled using the vm module. Here's a list of those APIs:

new vm.Script: Creates a new script object.
vm.compileFunction(): Compiles a JavaScript function.
new vm.SourceTextModule: Creates a module from JavaScript source text.
vm.runInThisContext(): Runs JavaScript code in the current context.
vm.runInContext(): Runs JavaScript code in a specified context.
vm.runInNewContext(): Runs JavaScript code in a new context.
vm.createContext(): Creates a new JavaScript context.

How to Use Dynamic import() with vm Module APIs

To enable dynamic import() in scripts compiled using these APIs, simply set the importModuleDynamically option to true. For example:

const vm = require("vm");

// Create a new script object with dynamic import enabled
const script = new vm.Script("// Code goes here", {
  importModuleDynamically: true,
});

// Run the script in the current context
vm.runInThisContext(script);

Real-World Applications

Dynamic import() with the vm module can be useful in various scenarios:

Code Loading Optimization: You can load only the modules you need at runtime, reducing the initial load time of your application.
Dynamic Script Execution: You can execute JavaScript code dynamically, such as code generated by a server or user input, and have it access external modules.
Module Isolation: By using the importModuleDynamically option, you can ensure that modules loaded dynamically do not interfere with the global scope.

Note:

Dynamic import() with the vm module is still in experimental mode and not recommended for production use. Use it at your own risk.

Dynamic Import in the VM Module

What is Import?

Import is a JavaScript keyword used to include code or functionality from another file.

What is Dynamic Import?

Dynamic import is when you import code not during the initial startup of your program, but at a later time. This allows you to load code only when it's needed, improving performance.

VM Module and Dynamic Import

The VM module in Node.js allows you to run JavaScript code in a sandboxed environment. By default, code containing import() cannot be run inside the VM module.

importModuleDynamically Option

If you want to allow dynamic import in the VM module, you need to specify the importModuleDynamically option when compiling the code.

If Option is Not Specified or Undefined

If you don't specify the importModuleDynamically option or if it's undefined, code containing import() can still be compiled, but when executed, it will fail with an error (ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING). This is because the VM module doesn't know how to handle dynamic import without that option.

Example Code

// Compile code without `importModuleDynamically` option
const code = `
  import { readFileSync } from 'fs';
  console.log(readFileSync('test.txt', 'utf-8'));
`;

const script = new vm.Script(code);

// Execute code and handle the error
try {
  script.runInContext();
} catch (err) {
  console.error(err.message); // Error: ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING
}

Real-World Applications

Dynamic import can be useful when you need to load code only when it's needed, such as:

Loading different modules based on user input
Loading code on demand to reduce initial load times
Implementing lazy loading of components in a web application

When importModuleDynamically is vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER

Simplified Explanation:

Imagine you have a script that wants to import a module. Normally, the script would have to specify where to find this module (like in a node_modules folder). But with this option, Node.js will try to find the module in the main JavaScript context (the main program that started Node.js).

Benefits:

You can use built-in Node.js modules without having to install additional packages.
You can access user-defined modules from the main context.
You can load modules dynamically without knowing their location in advance.

Limitations:

Modules are loaded relative to the file from which the script is being run.
Loading modules from the main context can create unexpected behavior if the objects they create are accessed outside of that context.

Code Example:

// Script to import the built-in "fs" module
const script = new Script(
  `import("node:fs").then(fs => console.log(fs.readFileSync("test.txt", "utf-8")));`,
  { importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER }
);

script.runInNewContext(); // Output: "Hello World"

Real-World Applications:

Creating ad-hoc scripts that need access to specific modules.
Dynamically loading modules based on user input or runtime conditions.
Isolating scripts from the main context while still allowing access to necessary modules.

What is importModuleDynamically?

importModuleDynamically is a way to customize how modules are imported and evaluated in Node.js. It's a callback function that allows you to control the process of loading and executing modules.

When is importModuleDynamically used?

importModuleDynamically is used when you want to do something special when a module is imported. For example, you could use it to:

Load the module from a different location
Transform the module before it's evaluated
Create a custom module namespace

How do I use importModuleDynamically?

To use importModuleDynamically, you need to set it as an option when compiling your code. You can do this using the --experimental-vm-modules flag.

Once you've set the flag, you can use importModuleDynamically as follows:

const script = new Script('import("foo.js")', {
  importModuleDynamically(specifier, referrer, importAttributes) {
    // ...
  },
});

In the callback function, you can access the module's specifier, the referrer (the code that's importing the module), and the import attributes (optional parameters passed to the import() call).

What can I do with importModuleDynamically?

Here are some examples of what you can do with importModuleDynamically:

Load modules from a different location. You can use importModuleDynamically to load modules from a different location, such as a CDN or a local file system. This can be useful for developing and testing modules without having to install them globally.
Transform modules before they're evaluated. You can use importModuleDynamically to transform modules before they're evaluated. For example, you could use it to minify the code or to add custom functionality.
Create a custom module namespace. You can use importModuleDynamically to create a custom module namespace. This can be useful for creating modules that can be used in multiple contexts, or for creating modules that expose a different interface than the original module.

Potential applications in real world.

Here are some potential applications of importModuleDynamically in the real world:

Developing and testing modules without having to install them globally. This can be useful for quickly testing out new modules or for developing modules that are not yet ready for production.
Creating custom modules that can be used in multiple contexts. This can be useful for creating modules that can be used in both the browser and Node.js, or for creating modules that can be used in different versions of Node.js.
Creating modules that expose a different interface than the original module. This can be useful for creating modules that are easier to use or that provide additional functionality.

Real world example.

Here is an example of how to use importModuleDynamically to create a custom module that exposes a different interface than the original module:

const script = new Script('import("foo.js")', {
  importModuleDynamically(specifier, referrer, importAttributes) {
    const originalModule = await import(specifier);

    return {
      getFoo() {
        return originalModule.foo;
      },
      setFoo(value) {
        originalModule.foo = value;
      }
    };
  }
});

This script creates a custom module that exposes a getFoo() and setFoo() method. These methods can be used to access and modify the foo property of the original module.

This example shows how you can use importModuleDynamically to create a custom module that provides a more convenient interface than the original module.

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Class: vm.Script

Real-world example

Potential applications

Overview

Creating a Script

Running a Script

Potential Applications

script.cachedDataRejected

Simplified Description of script.createCachedData()

script.runInContext(contextifiedObject[, options])

Complete Code Implementation and Example

Potential Applications

script.runInNewContext([contextObject[, options]])

module.error

What is the evaluate() method?

How does the evaluate() method work?

When should you use the evaluate() method?

Real-World Example

Potential Applications

module.link(linker)

Detailed Explanation

Real-World Example

Potential Applications

sourceTextModule.createCachedData()

SyntheticModule.setExport() Method in Node.js

Explanation:

Usage:

Example Code:

Real-World Applications:

vm.compileFunction(code[, params[, options]])

vm.constants

Properties

Example

vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER

vm.isContext(object)

vm.runInContext(code, contextifiedObject[, options])

Promises and Async Functions

Timeout Interactions

Microtask Mode

Real-World Applications

Dynamic Import in the VM Module

Class: `vm.Script`

`script.cachedDataRejected`

Simplified Description of `script.createCachedData()`

`script.runInContext(contextifiedObject[, options])`

`script.runInNewContext([contextObject[, options]])`

`module.error`

What is the `evaluate()` method?

How does the `evaluate()` method work?

When should you use the `evaluate()` method?

`module.link(linker)`

`sourceTextModule.createCachedData()`

`vm.compileFunction(code[, params[, options]])`

`vm.constants`

`vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`

`vm.isContext(object)`

`vm.runInContext(code, contextifiedObject[, options])`