readline

Class: `InterfaceConstructor`

Extends: {EventEmitter}

Instances of the InterfaceConstructor class are constructed using the readlinePromises.createInterface() or readline.createInterface() method. Every instance is associated with a single input [Readable][] stream and a single output [Writable][] stream. The output stream is used to print prompts for user input that arrives on, and is read from, the input stream.

Can you please simplify and explain the given content from nodejs's readline module?

explain each topic in detail and simplified manner (simplify in very plain english like explaining to a child).
retain code snippets or provide if you have better and improved versions or examples.
give real world complete code implementations and examples for each.
provide potential applications in real world for each.
ignore version changes, changelogs, contributions, extra unnecessary content.
```
  The response was blocked.
```

Event: 'close'

The 'close' event in the readline module occurs when the following events happen:

When the readline.close() method is called and the readline object closes the input and output streams.
When the input stream receives its 'end' event.
When the input stream receives Ctrl + D to signal end-of-transmission (EOT).
When the input stream receives Ctrl + C to signal SIGINT and there is no 'SIGINT' event listener registered on the readline object.

Once the 'close' event is emitted, the readline object is finished and cannot be used anymore.

Example:

const readline = require("readline");

// Create a readline interface
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Listen for the 'close' event
rl.on("close", () => {
  console.log("The readline object is now closed.");
});

// Close the readline interface
rl.close();

Real-world Applications:

The 'close' event is useful for cleaning up resources and handling the termination of the readline session. For example, you can use it to close file handles or write data to a database before the readline object is destroyed.

Event: 'line'

The 'line' event is triggered in Node.js's readline module whenever the user types something followed by the Enter or Return key. It's like when you're chatting with someone and press Enter to send a message.

Real World Application Example:

Imagine you're building a chat room. You want users to be able to type text and have it appear in the chat window. The 'line' event allows you to listen for when a user has finished typing and pressed Enter. Once you receive the 'line' event, you can take the typed text and send it to the server to be displayed in the chat window.

Code Implementation:

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.on('line', (input) => {
  console.log(`Received: ${input}\n`);
  rl.prompt();
});

rl.prompt();

In this example, we create a readline interface, which is responsible for managing user input and output. We listen for the 'line' event and when it's triggered, we receive the user's input as a string. We then display the received input in the console and prompt the user to enter another line of text.

This example demonstrates how to use the 'line' event to capture user input in a real-time interactive application like a chat room.

Event: 'history'

The 'history' event is emitted whenever the history array has changed.

Simplified Explanation:

Imagine you're using a command-line interface (like your computer's terminal) and you're typing commands. The history array keeps track of all the commands you've typed in the past.

The 'history' event is like a special message that tells you whenever that history array has changed. This could happen when you add a new command to the history, remove an old command, or change an existing command.

Function Signature:

rl.on("history", (history) => {
  // Do something with the history array
});

rl is the readline interface object.
history is an array containing the updated history.

Example Usage:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("history", (history) => {
  console.log(`History has changed: ${history}`);
});

rl.question("Enter a command: ", (answer) => {
  // Do something with the answer

  rl.close();
});

In this example, when the readline interface asks for input using rl.question(), the 'history' event will be emitted whenever the history array changes. This could happen when you enter a new command, press the up or down arrow keys to navigate through the history, or delete a command from the history.

Potential Applications:

Saving history to a file: You can listen for the 'history' event and save the updated history array to a file so that you can retrieve it later.
Filtering history: You can remove certain commands from the history array before it's saved to a file. For example, you could prevent passwords from being stored in the history.
Customizing history: You can change the format or order of the history array to suit your needs.

'pause' Event in Node.js's readline Module

Simplified Explanation:

The 'pause' event in readline tells you when the user has stopped typing temporarily.

Detailed Explanation:

The readline module is used to read user input from the command line. When you're using readline, you can pause the input stream to stop reading user input. Pausing the input stream can be useful in certain situations, such as when you want to display a prompt or give the user a chance to rest.

There are two ways to pause the input stream:

Manually pause the stream by calling the pause() method.
The stream is automatically paused when the user presses the Ctrl+Z key (on Windows) or the Ctrl+S key (on Linux/macOS).

The 'pause' event is emitted when the input stream is paused.

Real-World Example:

Here's a simple example showing how to use the 'pause' event:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("pause", () => {
  console.log("Readline paused.");
});

rl.question("What is your name? ", (name) => {
  rl.pause();
  // Do something with the user's name

  // Resume the input stream after doing something
  rl.resume();
});

In this example, the 'pause' event listener is called when the user presses Ctrl+Z or Ctrl+S. This allows the program to pause the input stream and do something else, such as display a prompt or save the user's input.

Potential Applications:

The 'pause' event can be used in a variety of real-world applications, such as:

Pausing the input stream to display a prompt or help text.
Pausing the input stream to save the user's input to a file.
Pausing the input stream to perform a long-running operation without interrupting the user's input.

Event: 'resume'

Explanation:

When you use readline to read input from a stream, by default it pauses the stream so that you can handle each line of input one at a time. The 'resume' event is emitted when the stream is resumed, allowing input to flow through the readline interface again.

Usage:

To listen for the 'resume' event, use the on() method with the 'resume' string as the first argument:

readline.on("resume", () => {
  // Code to run when the stream is resumed
});

Inside the event listener, you can perform actions to handle the resumed input. For example, you could log a message to indicate that the stream has been resumed:

readline.on("resume", () => {
  console.log("Input stream has been resumed");
});

Real-World Application:

One potential application for the 'resume' event is to pause and resume input based on user interaction. For example, in a command-line interface (CLI) application, you could pause the input while a menu is being displayed and then resume the input when the user selects an option from the menu.

Complete Code Implementation:

Here is a complete example of using the 'resume' event:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("resume", () => {
  console.log("Input stream has been resumed");
});

rl.pause(); // Pause the input stream

// Some code to handle user interaction and resume the input stream when needed

rl.resume(); // Resume the input stream

What is the 'SIGCONT' Event in readline Module?

The 'SIGCONT' event in the readline module is triggered when a Node.js process that was previously paused (sent to the background) using Ctrl+Z is brought back to the foreground using the fg (foreground) command.

Simplified Explanation:

Imagine you're working on a Node.js program in the terminal. If you press Ctrl+Z, it's like hitting the pause button on your music player. The terminal will stop running your program and wait for you to press a key. If you then type 'fg' and press Enter, it's like hitting the play button again. The terminal will bring your program back to life.

When Does the 'SIGCONT' Event Occur?

The 'SIGCONT' event only happens when two conditions are met:

The program was paused using Ctrl+Z (sending it the 'SIGTSTP' signal).
The program is brought back to the foreground using the fg command.

What Happens When the 'SIGCONT' Event Triggers?

When the 'SIGCONT' event triggers, it means that your program has been brought back to life. You can now continue running it or perform other operations on it.

Example:

Here's an example of how to use the 'SIGCONT' event:

const readline = require("readline");

// A readline interface for user input
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Listen for the 'SIGCONT' event
rl.on("SIGCONT", () => {
  // Do something when the program is brought back to the foreground
  console.log("Resuming the program...");
  rl.prompt(); // Start asking for user input again
});

// Start asking for user input
rl.prompt();

Potential Applications:

The 'SIGCONT' event can be useful in various applications, such as:

Resuming paused programs: If a program is paused using Ctrl+Z, the 'SIGCONT' event can be used to resume its execution.
Monitoring program status: If a program is paused and then brought back to the foreground, the 'SIGCONT' event can be used to detect this change and perform appropriate actions.

Understanding the 'SIGINT' Event in Node.js's Readline Module

Simplified Explanation:

In Node.js, the Readline module lets you interact with users through a command line interface. The 'SIGINT' event occurs when a user presses Ctrl+C.

How it Works:

When a user presses Ctrl+C, the 'SIGINT' event is triggered. If there are no event listeners registered to handle this event, the 'pause' event is emitted instead.

Event Handler Function:

The 'SIGINT' event handler function is called without any arguments.

Example Code:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("SIGINT", () => {
  rl.question("Are you sure you want to exit? ", (answer) => {
    if (answer.match(/^y(es)?$/i)) rl.pause();
  });
});

Real-World Applications:

Graceful Exiting: When a user presses Ctrl+C, you can prompt them for confirmation before terminating the program. This ensures they don't lose unsaved data.
Signal Handling: You can use 'SIGINT' to catch user interrupts and perform specific actions, such as logging the event or saving data.

Improved Code Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("SIGINT", () => {
  console.log("User interrupted the program. Saving data...");
  rl.pause();
});

In this example, the 'SIGINT' event handler logs a message and pauses the readline interface, allowing you to save user data or perform other cleanup tasks before the program exits.

Event: 'SIGTSTP'

Explanation:

When you press Ctrl + Z on your keyboard, it sends a SIGTSTP signal to the program. This signal tells the program to pause and go into the background.

Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("SIGTSTP", () => {
  console.log("Paused! Press `fg` to resume.");
});

In this example, when you press Ctrl + Z, the SIGTSTP event will be triggered and the program will pause. The console.log message will be displayed to let you know that the program is paused.

Real-World Applications:

Interactive debugging: You can use the SIGTSTP event to pause a program while you're debugging it. This allows you to examine the state of the program and make changes without having to restart it.
Background processing: Some programs may need to run in the background while you're doing other things. By handling the SIGTSTP event, you can ensure that the program pauses when it's sent to the background and resumes when you bring it back to the foreground.

What is rl.close()?

rl.close() is a method that closes an interface to a readline object. When you call rl.close(), the input and output streams of the readline object are released, and the 'close' event is emitted.

Detailed Explanation:

InterfaceConstructor: This is the constructor function for creating readline objects. It takes an input stream and an output stream as arguments.
Input and Output Streams: These are streams that allow you to read from and write to the readline object.
'close' Event: This event is emitted when the readline object is closed.

Simplified Explanation:

Imagine you have a water pipe with a faucet and a drain. The readline object is like the faucet, which allows you to control the flow of water through the pipe. The input stream is like the water coming into the pipe, and the output stream is like the water going out of the pipe.

When you call rl.close(), it's like turning off the faucet. The water stops flowing through the pipe, and the drain is disconnected.

Code Snippet:

const readline = require("readline");
const fs = require("fs");

const inputStream = fs.createReadStream("input.txt");
const outputStream = fs.createWriteStream("output.txt");

const rl = readline.createInterface({
  input: inputStream,
  output: outputStream,
});

rl.on("line", (line) => {
  console.log(`Line: ${line}`);
});

rl.on("close", () => {
  console.log("Interface closed.");
});

rl.close();

Real-World Application:

rl.close() is useful in situations where you need to control when a readline object is closed. For example, you might want to close the object when the user presses a certain key or when a certain event occurs.

One potential application of rl.close() is in a command-line interface (CLI) application. You could use rl.close() to close the readline object when the user exits the application. This would prevent any further input from being read from the console.

`rl.pause()`

The rl.pause() method does exactly what its name suggests - it pauses the input stream, allowing it to be resumed later if necessary.

When you call rl.pause(), the input stream will stop emitting data events, including 'line'. However, other events, such as 'close', will still be emitted.

This can be useful if you want to temporarily stop processing input data, for example if you need to perform some other operation or wait for some other event to occur.

Here's an example of how to use rl.pause() and rl.resume():

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("line", (line) => {
  console.log(`You entered: ${line}`);
});

// Pause the input stream
rl.pause();

// Do something else...

// Resume the input stream
rl.resume();

In this example, the input stream is paused after the first line is entered. This prevents any subsequent lines from being processed until the input stream is resumed.

Once the input stream is resumed, the 'line' event will start emitting data again.

Real-world applications:

Pausing the input stream can be useful in situations where you need to perform some other operation or wait for some other event to occur before continuing to process input data.
For example, you could pause the input stream if you need to make a network request or wait for a database query to complete before processing the next line of input.

`rl.prompt([preserveCursor])`

preserveCursor {boolean} (Optional) If true, prevents the cursor placement from being reset to 0.

The rl.prompt() method writes the InterfaceConstructor instances configured prompt to a new line in output in order to provide a user with a new location at which to provide input.

When called, rl.prompt() will resume the input stream if it has been paused.

If the InterfaceConstructor was created with output set to null or undefined the prompt is not written.

Simplified Explanation

The rl.prompt() method displays a prompt to the user, inviting them to enter input. The prompt is a string that is displayed in the output (usually the console) to indicate that the user is expected to provide input.

By default, the cursor (the position where the user types) is reset to the beginning of the line when rl.prompt() is called. If you want to preserve the cursor's current position, you can pass true as the preserveCursor parameter.

Real-World Example

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Display a prompt to the user
rl.prompt("What is your name? ");

// Wait for the user to enter input
rl.on("line", (name) => {
  console.log(`Hello, ${name}!`);
  rl.close();
});

In this example, we use the readline module to create a readline interface that allows us to interact with the user through the console. We then call the rl.prompt() method to display a prompt to the user, asking for their name. When the user enters their name and presses Enter, the rl.on('line') event listener is triggered, and we log a greeting message to the console.

Potential Applications

The rl.prompt() method can be used in a variety of real-world applications, including:

Creating interactive command-line applications
Gathering user input for surveys or questionnaires
Debugging code by interactively examining variables and expressions

Simplified Explanation of rl.resume() Method

When you're using the readline module in Node.js, you can pause the input stream using rl.pause(). Once paused, the stream will stop reading any further input.

The rl.resume() method resumes the input stream, allowing it to continue reading input. This is useful if you want to temporarily stop reading input and then start again later.

Example

Here's an example of how to use rl.resume():

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

// Pause the input stream
rl.pause();

// Do some other stuff...

// Resume the input stream
rl.resume();

// Now the input stream is reading input again

Real-World Application

One real-world application of rl.resume() is to temporarily pause the input stream while you're processing some data. For example, you could pause the input stream while you're writing a file or performing a database query. Once the operation is complete, you can resume the input stream and continue reading input.

Improved Code Example

Here's an improved version of the example code above that uses a try...catch block to handle any errors:

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

try {
  // Pause the input stream
  rl.pause();

  // Do some other stuff...

  // Resume the input stream
  rl.resume();

  // Now the input stream is reading input again
} catch (err) {
  console.error(err);
}

Simplified Explanation:

The setPrompt() method allows you to change the text that appears before the user's input in the console.

Detailed Explanation:

When you use readline, you can use the rl.prompt() method to ask the user for input. By default, the prompt text is '> '. However, you can change this text using rl.setPrompt().

Enhanced Code Snippet:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.setPrompt("What is your name? ");

rl.prompt();

rl.on("line", (line) => {
  console.log(`Your name is ${line}`);
  rl.close();
});

Real-World Application:

The setPrompt() method is helpful in situations where you want to provide custom instructions or information to the user before collecting their input. For example, you could use it to:

Ask the user for their age with a prompt like "Please enter your age: "
Collect a password with a prompt like "Enter your password: "
Guide the user through a series of questions with different prompts for each question

`rl.getPrompt()` Method in Node.js readline

The rl.getPrompt() method in Node.js readline module returns the current prompt string used by rl.prompt().

Return Value

A string representing the current prompt.

Real-World Code Example

const readline = require("readline");

// Create an instance of readline
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Set the prompt to 'What is your name? '
rl.setPrompt("What is your name? ");

// Prompt the user for input
rl.prompt();

// Log the current prompt
console.log(`Current prompt: ${rl.getPrompt()}`);

// Close the interface
rl.close();

Applications

Getting the current prompt string used by the readline interface.
Changing the prompt string dynamically during a readline session.
Debugging readline programs by inspecting the current prompt.

`rl.write(data[, key])`

The rl.write() method in readline allows you to write data or simulate keystrokes to the readline interface.

Parameters:

data: The string you want to write. If key is specified, this parameter is ignored.
key: An object representing a key sequence. It can have the following properties:
- ctrl: Indicates if the Ctrl key should be pressed.
- meta: Indicates if the Meta key should be pressed (this is usually the Command key on macOS).
- shift: Indicates if the Shift key should be pressed.
- name: The name of the key to simulate, such as 'a' or 'Enter'.

How to use:

To write data to the readline interface, simply call the rl.write() method with the data as the first parameter:

rl.write('Hello, world!');

This will write the string 'Hello, world!' to the readline interface as if it was typed by the user.

To simulate keystrokes, you can pass an object with the key properties to the rl.write() method:

rl.write(null, { ctrl: true, name: 'u' });

This will simulate pressing the Ctrl+U key combination, which typically deletes the current line.

Real-world applications:

Here are some real-world applications of the rl.write() method:

Automating tasks: You can use the rl.write() method to automate tasks that involve typing, such as filling out forms or sending commands to a terminal.
Testing: You can use the rl.write() method to test readline interfaces by simulating user input.
Debugging: You can use the rl.write() method to debug readline interfaces by simulating different keystrokes.

Complete code example:

Here is a complete code example that demonstrates how to use the rl.write() method:

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

// Write "Hello, world!" to the readline interface
rl.write('Hello, world!');

// Simulate pressing the Ctrl+U key combination (deletes the current line)
rl.write(null, { ctrl: true, name: 'u' });

// Close the readline interface
rl.close();

When you run this code, it will write the string 'Hello, world!' to the console and then delete the line using the Ctrl+U key combination.

`rl[Symbol.asyncIterator]()`

Returns: {AsyncIterator}

Simplified Explanation:

readline allows you to read lines of text from a stream, like a file or the keyboard. This iterator lets you access each line of text asynchronously, meaning you can use it in for await...of loops. It's like a regular for loop, but it can handle receiving lines one by one instead of all at once.

Detailed Explanation:

When you use readline, you can create an object called an "interface" that represents the stream you're reading from. This interface has a method called [Symbol.asyncIterator] that returns an iterator object. This iterator lets you asynchronously iterate through each line in the stream.

As you loop through the lines, the iterator will wait until each line is available before giving it to you. This means you can write code that reacts to each line as it comes in, instead of having to wait for the entire stream to be processed.

Code Snippet:

async function processLineByLine() {
  const rl = readline.createInterface({
    input: fs.createReadStream("input.txt"),
  });

  for await (const line of rl) {
    console.log(line);
  }
}

Real World Applications:

Parsing large text files: Iterating through a large text file line by line can be more efficient than reading the entire file into memory at once.
Real-time data processing: If you're receiving data from a stream in real time, you can use this iterator to process each line as it arrives.
Interactive command line applications: You can use this iterator to create command line applications that respond to user input one line at a time.

Potential Benefits:

Asynchronous iteration: It allows you to iterate through lines asynchronously, making it more efficient for large streams.
Error handling: Errors in the input stream are not forwarded.
Completion: If the loop is terminated prematurely, the input stream will be fully consumed.

readline.line

The readline.line property represents the current input data being processed by the readline interface. This is useful when collecting input from a TTY stream, such as a command-line interface, to retrieve the current value that has been processed so far, before the line event is emitted. Once the line event is emitted, the readline.line property will be an empty string.

It's important to note that using readline.line is only relevant when using a TTY stream for input. If you're not using a TTY stream, you should use the line event instead.

Example

Here's an example of how you can use the readline.line property to display a list of values that start with the current input:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

const values = ["lorem ipsum", "dolor sit amet"];

rl.on("keypress", (c, k) => {
  const results = values.filter((val) => val.startsWith(rl.line));
  console.log(results.join(" "));
});

In this example, we create a readline interface using the readline.createInterface() function. We pass in the process.stdin and process.stdout streams as the input and output streams, respectively.

We then define an array of values that we want to filter based on the current input. In this case, our array contains the strings "lorem ipsum" and "dolor sit amet".

The keypress event is emitted whenever a key is pressed on the TTY stream. In the event handler, we use the rl.line property to get the current input data that has been processed so far. We then use the Array.filter() method to filter the values array based on whether each value starts with the current input.

Finally, we use console.log() to print the filtered results to the console.

Potential Applications

The readline.line property can be used in a variety of applications, such as:

Autocompleting input in a command-line interface
Displaying a list of suggested values as you type in a form field
Filtering a list of items based on a search query

`rl.cursor`

Simplified Explanation: It's like a pointer that shows where you are typing in the input string.
In-depth Explanation:
- rl.cursor keeps track of the position of the cursor in the rl.line input string.
- It helps determine which part of the input string will be modified as you type.
- It also determines where the caret (the blinking line) will be displayed in the terminal.

Code Example:

// Create a readline interface
const readline = require("readline").createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Get the current cursor position
readline.cursor = 5;

// Write something at the current cursor position
readline.write("Hello");

// Get the new cursor position
console.log(readline.cursor); // Output: 10

Real-World Applications:
- Interactive command-line applications where you need to edit or modify input text, such as:
  - Text editors
  - Code editors
  - Command shells

`rl.getCursorPos()`

Explanation:

Imagine you have a command line where you type in commands. The cursor is the blinking line that shows where you're typing.

rl.getCursorPos() tells you where the cursor is in relation to the command you're typing. It gives you two pieces of information:

rows: Which line of the command you're on. For example, if you're typing a long command that wraps to the next line, rows will be 2.
cols: Which column (character position) on that line the cursor is on. For example, if you've typed "hello" and the cursor is after the "o", cols will be 5.

Real-World Example:

Let's say you're building a text editor. You want to know where the cursor is so you can show a pop-up menu with options for that position. You can use rl.getCursorPos() to get the cursor's position and then display the menu at the right spot.

Code Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("Enter a command: ", (command) => {
  // Get the cursor's position
  const cursorPos = rl.getCursorPos();

  // Display a menu at the cursor's position
  console.log(`Cursor position: ${cursorPos.rows}, ${cursorPos.cols}`);

  rl.close();
});

Potential Applications:

Text editors (showing menus or auto-completing commands based on cursor position)
Command-line interfaces (displaying help or options based on where the cursor is in a command)
Code editors (showing syntax highlighting or suggestions based on cursor position)

Promises API

Promises are a way to handle asynchronous operations in JavaScript. Asynchronous operations are those that don't complete immediately, such as making a network request or reading a file.

Promises are objects that represent the eventual completion (or failure) of an asynchronous operation. They have three states:

Pending: The operation has not yet completed.
Fulfilled: The operation completed successfully and returned a value.
Rejected: The operation failed and returned an error.

Creating Promises

Promises can be created using the Promise constructor:

const promise = new Promise((resolve, reject) => {
  // ...
});

The resolve and reject functions are callbacks that are called when the operation completes successfully or fails, respectively.

Consuming Promises

Promises can be consumed using the then method:

promise.then((value) => {
  // ...
}).catch((error) => {
  // ...
});

The then method is called when the promise is fulfilled and returns a new promise. The catch method is called when the promise is rejected and returns a new promise.

Real-World Examples

Promises can be used in a variety of real-world applications, such as:

Making network requests
Reading files
Performing long-running computations
Handling user input

Potential Applications

Some potential applications of promises include:

Loading data from a server: You can use a promise to make a network request to load data from a server. Once the data is loaded, you can use the then method to process the data.
Performing a long-running computation: You can use a promise to perform a long-running computation, such as a complex calculation or a machine learning algorithm. Once the computation is complete, you can use the then method to get the results.
Handling user input: You can use a promise to handle user input, such as a form submission or a button click. Once the user input is received, you can use the then method to process the input.

Class: `readlinePromises.Interface`

What is it?

The readlinePromises.Interface class is a tool for reading and writing text to a terminal. It's useful for creating interactive command-line applications.

How to use it?

You can create a readlinePromises.Interface instance by calling the readlinePromises.createInterface() method. This method takes two arguments:

An input stream, which is where the user's input will come from.
An output stream, which is where the program's output will be written.

Once you have a readlinePromises.Interface instance, you can use it to do things like:

Prompt the user for input
Read the user's input
Write output to the terminal

Real-world example

Here's a simple example of using the readlinePromises.Interface class to create a command-line application that asks the user for their name and then greets them:

const readline = require("readline-promises");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your name? ", (name) => {
  rl.write(`Hello, ${name}!`);
  rl.close();
});

Potential applications

The readlinePromises.Interface class can be used for a variety of applications, such as:

Creating interactive command-line applications
Reading and writing text to a terminal
Collecting user input
Prompting the user for information

`rl.question(query[, options])`

query (string): A question or prompt to display to the user, prepended to the input prompt.
options (Object):
- signal (AbortSignal): An optional signal that can be used to cancel the question.

The rl.question() method displays the specified query to the user and waits for a response. When the user provides input, the method returns a promise that resolves with the input value.

Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your favorite color? ", (answer) => {
  console.log(`Your favorite color is ${answer}`);
  rl.close();
});

In this example, the rl.question() method displays the query "What is your favorite color?" to the user and waits for a response. When the user provides input, the promise returned by the method resolves with the input value, which is then logged to the console.

Using an AbortSignal to Cancel a Question:

An AbortSignal can be used to cancel a question if the user takes too long to respond or if the question is no longer needed. To use an AbortSignal, pass it as the signal option to the rl.question() method.

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

const signal = AbortSignal.timeout(5000);

rl.question("What is your favorite color? ", { signal }, (answer) => {
  console.log(`Your favorite color is ${answer}`);
  rl.close();
});

signal.addEventListener("abort", () => {
  console.log("The question was canceled.");
  rl.close();
});

In this example, the AbortSignal is set to timeout after 5 seconds. If the user does not provide input within 5 seconds, the AbortSignal will abort the question and the rl.close() method will be called.

Applications in Real World:

The rl.question() method can be used in a variety of real-world applications, such as:

Gathering user input for surveys or forms
Creating interactive command-line interfaces (CLIs)
Collecting user feedback
Providing help or documentation to users

`readlinePromises.Readline`

The readlinePromises.Readline class provided by the readline-promises module in Node.js allows you to easily read and write lines of text from and to a stream, such as a file, terminal, or network connection. It simplifies the process of working with line-based input and output, making it convenient for building interactive command-line applications or processing text files.

Creating an Instance

To create an instance of the readlinePromises.Readline class, you can use the following syntax:

const readline = require("readline-promises");
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

Here, process.stdin is the standard input stream, which represents the input coming from the user's keyboard, and process.stdout is the standard output stream, which represents the output being displayed on the terminal.

Reading Lines from Input

Once you have created an instance of readline, you can start reading lines of text from the input stream using the following method:

rl.question("What is your name? ").then((answer) => {
  console.log(`Your name is ${answer}`);
  rl.close();
});

In this example, the rl.question() method prompts the user to enter their name, stores the input in the answer variable, and then prints the user's name on the terminal. The rl.close() method is called to end the readline session.

Writing Lines to Output

You can also use the readline instance to write lines of text to the output stream using the following method:

rl.write("Hello, world!\n");

In this example, the rl.write() method writes the string "Hello, world!\n" to the output stream, which in this case is the terminal. The at the end of the string represents a newline character, which moves the cursor to the next line on the terminal.

Event Listeners

The readline instance also supports event listeners, which allow you to respond to specific events, such as when a line of text is entered or when the end of the stream is reached. Here are some of the most commonly used event listeners:

line: Emitted when a complete line of text is entered by the user. The listener function receives the entered line as an argument.
close: Emitted when the end of the stream is reached. This event is useful for performing cleanup actions, such as closing files or connections.
SIGINT: Emitted when the user presses Ctrl+C to interrupt the readline session. This event can be used to handle graceful shutdown of your application.

Here's an example of using an event listener to respond to the line event:

rl.on("line", (line) => {
  console.log(`You entered: ${line}`);
});

In this example, the rl.on("line", ...) function sets up an event listener for the line event. Whenever a line of text is entered, the event listener function is called with the entered line as an argument, and it prints the line to the terminal.

Potential Applications

The readlinePromises module has a wide range of applications in real-world scenarios, including:

Interactive command-line applications: Building interactive command-line interfaces (CLIs) where users can enter commands and receive responses.
Text file processing: Processing text files line by line, performing operations such as filtering, searching, and modifying the contents.
Network communication: Handling line-based communication over network connections, such as implementing chat protocols or server-client interactions.
Data entry: Simplifying data entry tasks by providing a user-friendly interface for entering and validating user input.

readlinePromises.Readline

The readlinePromises.Readline class is a promise-based wrapper for the Node.js readline module, which provides a way to read lines of text from a readable stream.

Constructor

new readlinePromises.Readline(stream[, options])

stream: A readable stream, typically a TTY stream.
options: An optional object that can contain the following properties:
- autoCommit: If true, the readline will automatically commit the current line to the stream after each keypress.

Methods

The following methods are available on the readlinePromises.Readline class:

question(prompt): Prompts the user for input and returns a promise that resolves to the user's input.
keyPress(sequence): Listens for a specific keypress sequence and returns a promise that resolves to the keypress event.
line (event): Emitted when a complete line of input is received.
close( ): Closes the readline interface and releases all associated resources.

Real-World Example

The following example shows how to use the readlinePromises.Readline class to read a user's input and print it to the console:

const readline = require("readline").promises;

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your name? ").then((name) => {
  console.log(`Hello, ${name}!`);
  rl.close();
});

Potential Applications

The readlinePromises.Readline class can be used in a variety of real-world applications, such as:

Building interactive command-line interfaces (CLIs)
Reading user input from a web terminal
Parsing and validating user input

ClearLine

Synopsis:

clearLine(dir);

Description:

The clearLine() method adds a pending action to the internal list to clear the current line in the direction specified by dir. The dir parameter can take the following values:

-1: The line will be cleared from the cursor to the left.
0: The entire line will be cleared.
1: The line will be cleared from the cursor to the right.

The pending actions are executed when the readline interface's commit() method is called.

Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("Enter a line to clear: ", (line) => {
  // Clear the line to the left of the cursor
  rl.clearLine(-1);

  // Clear the entire line
  rl.clearLine(0);

  // Close the readline interface
  rl.close();
});

Real-World Applications:

The clearLine() method can be used in various real-world applications, such as:

Clearing input after a mistake: If the user enters an incorrect value, the line can be cleared to the left of the cursor to allow them to enter the correct value.
Clearing a line after a command: After executing a command, the line can be cleared to the entire line to prepare for the next command.
Clearing a line for a prompt: If the readline interface is used to ask a series of questions, the line can be cleared to the entire line before each prompt to ensure a clean user experience.

Simplified Explanation:

The rl.clearScreenDown() method in Node.js's readline module allows you to clear the screen from the current cursor position down. This means it will erase everything below the cursor.

Detailed Explanation:

rl: is the readline interface object.
clearScreenDown(): is a method that adds an action to the internal list of pending actions.
commit(): is a method that executes all the pending actions.
autoCommit: true: is an option that can be passed to the constructor to automatically execute pending actions without calling commit().

Code Snippet:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.clearScreenDown();
rl.commit();

In this example, rl.clearScreenDown() adds an action to the list of pending actions, and rl.commit() executes the action, clearing the screen from the current cursor position down.

Real-World Applications:

Creating a chat application: You can use clearScreenDown() to clear the previous messages in a chat window and start typing a new message.
Building a text editor: You can use clearScreenDown() to clear the text below the cursor when the user wants to start a new line or delete multiple lines at once.
Automating tasks: You can use clearScreenDown() to clear the output from previous commands when you're executing a script or automating a task.

Method: rl.commit()

Description:

In Node.js, when you use the readline module to read user input from the terminal, you can perform various actions, such as moving the cursor, clearing the screen, or inserting characters. These actions are not immediately sent to the terminal. Instead, they are stored in a list of pending actions.

The rl.commit() method sends all the pending actions to the terminal and clears the list. This means that all the actions you have performed up to that point will take effect immediately.

Simplified Explanation:

Imagine you are writing a letter with a pen and paper. As you write, you may make mistakes or need to make changes. Instead of erasing each mistake one by one, you can use a piece of paper underneath to cover your mistakes and write on it instead.

The readline module works in a similar way. It keeps track of all your actions as you type in the terminal. When you call rl.commit(), it's like lifting the paper off the mistakes and revealing the corrected version.

Code Snippet:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Read user input
rl.question("What is your name? ", (name) => {
  // Move cursor to beginning of line
  rl.write(`\r\x1b[0G`);

  // Clear any pending actions (e.g., cursor movement)
  rl.commit();

  // Write the user's name to the terminal
  rl.write(name);
});

Real-World Applications:

The rl.commit() method is useful in situations where you need to ensure that all your actions take effect before performing further actions. For example, you might use it to:

Update the terminal display with the latest information
Clear the screen before displaying a new message
Insert characters at a specific location in the terminal

Potential Applications:

Interactive command-line applications
Text editors
Chat clients

`rl.cursorTo(x[, y])`

What rl.cursorTo does?
- It moves the cursor to the specified position in the stream associated with the readline interface.
Simplified example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Move the cursor to the 10th column, 5th row
rl.cursorTo(10, 5);

// Write something at the new cursor position
rl.write("Hello, world!");


- Real-world application:
- It can be used to create a simple text editor where you can move the cursor around the screen to edit text.
- It can also be used to create a simple game where you can move a character around the screen.

---

### `rl.moveCursor(dx, dy)`

The `rl.moveCursor()` method in `readline` adds an action to the internal list of pending actions that moves the cursor _relative_ to its current position in the associated `stream`.
To see the effect of this method, call `rl.commit()`, unless `autoCommit: true` was passed to the constructor.

**Syntax:**

```typescript
moveCursor(dx: number, dy: number): this;

Parameters:

dx: The number of columns to move the cursor.
dy: The number of rows to move the cursor.

Returns:

this: The readline instance.

Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.write("Hello, world!");
rl.moveCursor(0, -1);
rl.write("!");
rl.close();

This will print the following to the console:

Hello, world!
!

The rl.moveCursor() method is useful for moving the cursor to a specific position in the terminal. For example, you could use it to create a progress bar or to display a message in a specific location on the screen.

Real-world applications

Here are some real-world applications of the rl.moveCursor() method:

Creating a progress bar.
Displaying a message in a specific location on the screen.
Navigating through a menu.
Editing text.

Potential applications

Here are some potential applications of the rl.moveCursor() method:

A command-line interface (CLI) for a database that allows users to navigate through a list of records and edit them.
A text editor that allows users to move the cursor around the document and make changes.
A game that allows users to control a character on the screen.

What is rl.rollback()?

rl.rollback() is a method used in the readline module of Node.js. It allows you to clear the list of actions that you have taken while using the readline interface. These actions include things like typing characters, moving the cursor, or editing text.

How to use rl.rollback()?

To use rl.rollback(), simply call the method on the readline interface object. For example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("line", (line) => {
  // Do something with the line
});

// Clear the list of pending actions
rl.rollback();

What happens when you call rl.rollback()?

When you call rl.rollback(), it clears the internal list of pending actions. This means that any actions that you have taken since the last time you called rl.rollback() will be discarded.

Why would you want to use rl.rollback()?

There are a few reasons why you might want to use rl.rollback(). For example, you might want to use it to:

Undo a series of actions that you have taken.
Start over from a clean slate.
Reset the readline interface to its initial state.

Real-world applications of rl.rollback()

rl.rollback() can be used in a variety of real-world applications. For example, it can be used to:

Create a command-line interface that allows users to undo their actions.
Implement a text editor that allows users to rollback changes to their text.
Create a game that allows players to restart from the beginning.

Potential applications of rl.rollback()

Here are some potential applications of rl.rollback() in real-world scenarios:

Undoing user input: If a user makes a mistake while typing input, rl.rollback() can be used to undo the mistake and allow the user to start over.
Resetting a form: If a user fills out a form incorrectly, rl.rollback() can be used to reset the form and allow the user to start over.
Restarting a game: If a player makes a mistake in a game, rl.rollback() can be used to restart the game from the beginning.
Debugging: rl.rollback() can be used to help debug code by allowing you to rollback changes and try different things.

Creating a Readline Interface

The readlinePromises.createInterface() method lets you create an interface for reading input lines from a stream, usually a terminal.

Options:

input: The stream to read from (usually process.stdin)
output: The stream to write to (usually process.stdout)
completer: A function for autocompletion
terminal: Whether to treat the streams as a terminal (enabling ANSI escape codes)
history: An initial list of history lines
historySize: Maximum number of history lines to keep (0 disables history)
removeHistoryDuplicates: Whether to remove duplicate history lines
prompt: The prompt string to display before each input
crlfDelay: Delay in milliseconds to treat CRLF as separate line endings
escapeCodeTimeout: Time in milliseconds to wait for ambiguous key sequences
tabSize: Number of spaces a tab represents (minimum 1)

Usage:

Create an interface with default options:

const readline = require("node:readline/promises");
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

Listening for Input:

Listen for each line of input entered by the user:

rl.on("line", (line) => {
  console.log(`Received: ${line}`);
});

Example:

A simple program that reads lines from the console and prints them back:

const readline = require("node:readline/promises");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("line", (line) => {
  console.log(`Received: ${line}`);
});

rl.on("close", () => {
  console.log("Done reading input.");
});

Real-World Applications:

Input gathering in command-line interfaces (e.g., Node.js REPL)
Interactive shell (e.g., Bash, Zsh)
Autocomplete in terminals (e.g., git command completion)

The completer Function in Node.js's readline Module

The readline module in Node.js provides an interface for reading data from a stream in a line-by-line manner. It includes a completer function that assists users in completing commands or input as they type.

Functionality of the completer Function

The completer function takes the current line entered by the user as an argument and returns an array with two entries:

An array of suggestions to complete the input. For example, for the input ".he", the completer might return [".help"].
The substring that was used for matching. In this case, "he".

Using the completer Function

To use the completer function, you need to register it as an event listener on the readline interface, like so:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  completer: (line) => {
    const completions = [".help", ".exit", ".quit"];
    const hits = completions.filter((c) => c.startsWith(line));
    return [hits.length ? hits : completions, line];
  },
});

When the user enters a line that starts with ".", the completer function is called with the partial input ".he". It returns an array with the suggestion ".help" and the substring ".he". The readline interface then displays ".help" as a suggested completion to the user.

Asynchronous completer Functions

The completer function can also be asynchronous, returning a Promise or using async/await syntax. This allows you to fetch data or perform other tasks before returning suggestions:

async function completer(line) {
  const completions = await fetchCompletions(line);
  return [completions, line];
}

Benefits of Using the completer Function

The completer function provides several benefits:

Improved user experience: It helps users quickly and easily complete commands, reducing errors.
Increased efficiency: By suggesting completions, it reduces the amount of typing required.
Customization: You can create custom completer functions to tailor the suggestions to your application's specific needs.

Real-World Examples

Interactive command-line interfaces (CLIs): Used to provide suggestions for commands and options.
Autocomplete in web forms: Assists users in filling out forms.
Search engines: Offer suggestions as users type in search queries.

Callback API

Imagine you're playing a game where you have to guess a number. Instead of guessing one number at a time, you can ask the game to tell you if your guess is too high or too low. This is similar to the callback API, where you provide a function to be executed when a specific event occurs.

Complete Code Implementation and Real-World Example:

// readline.js
const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your favorite color? ", (answer) => {
  console.log(`Your favorite color is ${answer}`);
  rl.close();
});

In this example:

readline.createInterface creates a new readline instance to read input from the console (stdin) and print output (stdout).
rl.question asks a question to the user and provides a callback function ((answer) =>) that will be executed when the user enters an answer.
Inside the callback, we print the user's answer and close the readline interface (rl.close()).

Potential Applications in Real World:

Interactive Command Line Interfaces (CLIs): Capture user input and respond with custom actions.
Data Validation: Use callbacks to validate user inputs and provide error messages.
Event-Driven Programming: Respond to specific events (e.g., button clicks, mouse movements) and trigger corresponding actions.

Simplified Explanation:

Class: readline.Interface

The readline module in Node.js provides a way to interact with users by asking for input and displaying prompts. It creates an Interface object that connects to a specific input stream (e.g., keyboard input) and an output stream (e.g., display).

Extends: readline.InterfaceConstructor

The readline.Interface class extends the readline.InterfaceConstructor, which means it inherits the same properties and methods as the InterfaceConstructor.

Creating an Interface:

You create an Interface using the readline.createInterface() method, which takes the following parameters:

input: The input stream to read from (e.g., process.stdin)
output: The output stream to write to (e.g., process.stdout)

Example:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

Using the Interface:

Once you have an Interface, you can use it to:

Ask for user input: Use the question() method to prompt the user for input.
Handle user input: When the user provides input, the 'line' event is emitted, which you can listen for and handle.
Complete the interface: Use the close() method to end the Interface and stop accepting input.

Example:

// Ask the user their name
rl.question("What is your name? ", (name) => {
  // Print a welcome message
  console.log(`Hello, ${name}!`);

  // Close the interface
  rl.close();
});

Real-World Applications:

Interactive command-line interfaces (CLIs)
Data entry applications
Chatbots

`rl.question(query[, options], callback)`

query {string} Ask the user a question.
options {Object}
- signal {AbortSignal} (Optional) Allows you to cancel the question using an AbortController.
callback {Function} A function that will be called with the user's answer.

The rl.question() method displays the query and waits for the user's input. Once the user provides their input, the callback function is invoked with the user's input as its argument.

When called, rl.question() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined, the query is not written.

The callback function passed to rl.question() does not follow the typical pattern of accepting an Error object or null as the first argument. The callback is called with the provided answer as the only argument.

An error will be thrown if calling rl.question() after rl.close().

Here's an example of how to use rl.question():

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your favorite food? ", (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
  rl.close();
});

In this example, we create a new readline.Interface object and then use the rl.question() method to ask the user a question. Once the user provides their answer, the callback function is invoked with the user's input.

Here's another example using the AbortController:

const readline = require("readline");
const AbortController = require("abort-controller");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

const ac = new AbortController();
const signal = ac.signal;

rl.question("What is your favorite food? ", { signal }, (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
  rl.close();
});

signal.addEventListener(
  "abort",
  () => {
    console.log("The food question timed out");
  },
  { once: true }
);

setTimeout(() => ac.abort(), 10000);

This example is similar to the previous one, but we're using an AbortController to cancel the question after 10 seconds. If the user takes more than 10 seconds to answer the question, the 'abort' event will be emitted and the question will be canceled.

Real-world applications

The rl.question() method can be used in a variety of real-world applications, such as:

Collecting user input for a command-line application
Creating interactive tutorials or demos
Gathering feedback from users
Building interactive quizzes or games

`readline.clearLine(stream, dir[, callback])`

The readline.clearLine() method is used to clear a line of text from a stream. It takes three arguments:

stream: The stream to clear the line from.
dir: The direction to clear the line in. Valid values are:
- -1: Clear the line to the left of the cursor.
- 0: Clear the entire line.
- 1: Clear the line to the right of the cursor.
An optional callback function that is called once the line has been cleared.

The following code snippet shows how to use the readline.clearLine() method to clear the current line of a stream:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your name? ", (name) => {
  rl.clearLine(process.stdout, 0);
  rl.close();
});

In the above example, the readline.clearLine() method is used to clear the current line of the process.stdout stream. The 0 argument indicates that the entire line should be cleared.

The readline.clearLine() method can be useful for clearing the line after a user has entered input. This can help to keep the screen clean and organized.

Real-world applications

The readline.clearLine() method can be used in a variety of real-world applications, such as:

Interactive command-line interfaces (CLIs): The readline.clearLine() method can be used to clear the current line of a CLI after a user has entered a command. This can help to keep the CLI screen clean and organized.
Text editors: The readline.clearLine() method can be used to clear the current line of a text editor after a user has made a change. This can help to keep the text editor screen clean and organized.
Chat applications: The readline.clearLine() method can be used to clear the current line of a chat application after a user has sent a message. This can help to keep the chat application screen clean and organized.

Potential applications

The readline.clearLine() method has a number of potential applications, such as:

Creating a custom command-line interface (CLI): The readline.clearLine() method can be used to create a custom CLI that is easy to use and navigate.
Developing a text editor: The readline.clearLine() method can be used to develop a text editor that is easy to use and customize.
Building a chat application: The readline.clearLine() method can be used to build a chat application that is easy to use and communicate with others.

readline.clearScreenDown()

Purpose:

To clear a portion of the console screen, specifically from the cursor's current position down to the end of the screen.

Arguments:

stream: The TTY stream (usually process.stdout) that represents the console window.

Return Value:

boolean:
- true if the clear operation can continue immediately.
- false if the stream needs to wait for the 'drain' event before continuing to clear.

Simplified Explanation:

Imagine you have a text editor window with some text displayed on it. The cursor is blinking at a certain position in the text. The readline.clearScreenDown() method will delete all the text from the cursor's current position down to the end of the window, effectively clearing that portion of the screen.

Code Snippet:

// Clear the screen from the cursor's position down
const readline = require("readline");
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.clearScreenDown();

Real-World Applications:

Interactive Command Line Interfaces (CLIs): Used to clear the screen after displaying a list of commands or options to make the interface more user-friendly.
Text Editors: Used to clear a specific portion of the text editor window to make way for new content.
Terminal Emulators: Used to clear the screen after running a command or after exiting a full-screen application.

Potential Applications:

A chat application that clears the screen after a user sends a message to keep the conversation tidy.
A file manager that clears the screen when a new directory is opened to provide a clean view of the files.
A game that clears the screen when a new level is loaded to create a visual separation between levels.

Creating an Interface to Read User Input

Imagine you want to build a simple command-line tool that asks users for their name and then greets them. You can use Node.js's readline module to create an easy-to-use interface for reading user input.

Step 1: Import the Readline Module

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

Step 2: Create a Readline Interface

To create an interface, you need to specify where the input will come from (usually process.stdin) and where the output will go (usually process.stdout).

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

Step 3: Ask the User for Input

Use the question method to ask the user a question and wait for their input.

rl.question("What is your name? ", (name) => {
  // The user's response is now available in the 'name' variable.
  console.log(`Hello, ${name}!`);
});

Step 4: Close the Interface

When you're done reading input, close the interface to free up resources.

rl.close();

Real-World Applications:

Interactive command-line tools: Building prompts and menus for user interaction.
Chatbots: Reading and responding to user messages in a chat interface.
Data collection: Gathering user input for surveys, feedback, or other data collection purposes.

Example:

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

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.question("What is your favorite color? ", (color) => {
  console.log(`Your favorite color is ${color}.`);
  rl.close();
});

Using `readline` completer

What is a completer? In the context of the readline module, a "completer" is a function that provides list of possible completions for the current text entered by the user.

How to use the completer function The completer function can be used by passing it as an argument to the readline interface when creating it:

const readline = require('readline').createInterface({
  completer: (line) => {
    // Provide list of possible completions based on line
  }
});

The completer function takes one parameter, which is the current line entered by the user, and it should return an array with two values:

An array of possible completions
The substring that was used to match the completions

For example, the following completer function would provide autocompletion for the commands .help, .error, .exit, .quit, and .q:

function completer(line) {
  const completions = ['.help', '.error', '.exit', '.quit', '.q'];
  const hits = completions.filter((c) => c.startsWith(line));
  // Show all completions if none found
  return [hits.length ? hits : completions, line];
}

Asynchronous completers The completer function can also be used asynchronously by accepting two parameters:

The current line entered by the user
A callback function to return the completions

The callback function takes two parameters:

An error object (or null if there is no error)
An array with two values (as described above)

For example, the following asynchronous completer function would perform a lookup in a database to provide completions:

function completer(line, callback) {
  // Perform database lookup
  setTimeout(() => {
    callback(null, [['John', 'Jane', 'Bob'], line]);
  }, 100);
}

Real-world applications Completers can be used in a variety of applications, such as:

Command-line shells
Code editors
Debugging tools
Interactive help systems

By providing autocompletion, completers can make it easier for users to interact with a system and to perform tasks more efficiently.

readline.cursorTo()

The readline.cursorTo() method allows you to reposition the cursor in a terminal window back to a specified position (x, y)

This can be useful when you want to align text or move the cursor to a specific location on the screen.

Syntax:

readline.cursorTo(stream, x, y[, callback])

Parameters:

stream: A writable stream, typically a tty stream.
x: The horizontal position of the cursor (0-based).
y: The vertical position of the cursor (0-based).
callback (optional): A callback function which is invoked once the operation completes.

Return Value:

Returns true if the operation can complete immediately. Otherwise, it returns false and you should wait for the 'drain' event to be emitted before continuing to write additional data.

Example:

const readline = require('readline');

// Create a readline interface
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

// Move the cursor to the leftmost top corner of the screen
rl.cursorTo(0, 0);

// Write some text at the new cursor position
rl.write('Hello, world!');

// Close the readline interface
rl.close();

Applications:

Align text: By moving the cursor to a specific position, you can align text in a precise manner, such as when creating menus or data tables.
Create custom layouts: You can use cursorTo() to create custom layouts in the terminal window, allowing you to display data or information in a specific format or style.

Simplified Explanation:

Imagine a terminal window, like the one you use for your command prompt. The readline module allows you to control the cursor's movement and position within this window.

Topics in Detail:

readline.moveCursor() Method:

stream: This is the terminal window or TTY you want to move the cursor in.
dx: The number of columns to move the cursor horizontally (left or right).
dy: The number of rows to move the cursor vertically (up or down).

Example:

To move the cursor 5 columns to the right and 3 rows down:

const readline = require("readline");
const stream = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

stream.moveCursor(stream, 5, 3);

This will update the cursor's position to the right and below its current location.

Return Value:

false: If the terminal stream is still processing data and cannot handle the cursor movement at the moment. In this case, the 'drain' event will be emitted when the stream is ready to accept more data.
true: If the terminal stream can handle the cursor movement immediately.

Potential Applications:

Editing and formatting text in terminals.
Creating interactive terminal-based games or interfaces.
Navigating and managing windows in a command-line environment.

readline.emitKeypressEvents(stream[, interface])

The readline.emitKeypressEvents() method is a function in the readline module that makes a given readable stream start emitting 'keypress' events for every input it receives.

Parameters:

stream: A readable stream.
interface (optional): A readline interface constructor that disables autocompletion when copy-pasted input is detected.

How it works:

Imagine you have a stream that accepts user input. When you call readline.emitKeypressEvents() on this stream, it will start sending out events for each key that the user presses. This means that you can listen to these events and respond to them however you want. For example, you could use this to create a simple text editor or a command-line interface.

Here's a simple example:

const readline = require("readline");

// Create a readline interface for the standard input stream.
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

// Start emitting 'keypress' events on the input stream.
rl.on("keypress", (key) => {
  console.log(`You pressed the '${key.name}' key.`);
});

// Start the readline interface.
rl.prompt();

When you run this code, you'll notice that whenever you press a key, it will print the name of the key you pressed.

Real-world applications:

Here are some real-world applications for the readline.emitKeypressEvents() method:

Creating simple text editors.
Creating command-line interfaces.
Implementing keybindings in applications.
Detecting special keys, such as arrow keys or function keys.

Understanding the Tiny CLI Example

What is a CLI?

Think of a CLI (Command-Line Interface) as a way to talk to your computer by typing commands. Instead of using a mouse to click through menus, you write specific commands that the computer understands.

Creating a Simple CLI

Let's imagine you want to create a simple command-line app that echoes "world!" when you type "hello."

1. Import the readline module

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

This module lets you read lines of text from the user's input.

2. Create a readline interface

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: "OHAI> ",
});

This creates an interface that connects your program to the user's terminal.

process.stdin: Reads user input from the terminal.
process.stdout: Displays output to the terminal.
prompt: Sets the text that appears before the user can type a command.

3. Start listening for user input

rl.prompt();

This starts listening for the user to type something.

4. Handle user input

rl.on("line", (line) => {
  // Handle the user's command here
});

When the user presses Enter, the 'line' event is triggered, and the line variable contains the text they typed in.

5. Handle different commands

switch (line.trim()) {
  case "hello":
    console.log("world!");
    break;
  default:
    console.log(`Say what? I might have heard '${line.trim()}'`);
    break;
}

This handles different commands the user can type:

If they type "hello," it prints "world!"
For any other command, it prints a message saying it didn't understand.

6. Close the interface when the user is done

rl.on("close", () => {
  console.log("Have a great day!");
  process.exit(0);
});

When the user presses Ctrl+C or Ctrl+D, the 'close' event is triggered. This prints a goodbye message and exits the program.

Real-World Applications

CLIs have many uses, including:

System administration: Managing servers, networks, and other IT systems.
Software development: Writing, debugging, and testing code.
Data analysis: Extracting and analyzing data.
Command-line tools: Providing specialized utilities (e.g., text editors, file management).

Reading a File Line-by-Line Using readline

What is readline?

readline is a Node.js module that helps you read a file line by line. This is useful when you have a large file and you don't want to load the entire file into memory at once.

How to use readline

To use readline, you first need to create a readline interface. You can do this by calling the readline.createInterface() function. The first argument to this function is the input stream that you want to read from. This can be a file stream, a network stream, or even a string.

The following code snippet shows how to create a readline interface from a file stream:

const fs = require("fs");
const readline = require("readline");

const fileStream = fs.createReadStream("input.txt");

const rl = readline.createInterface({
  input: fileStream,
  crlfDelay: Infinity,
});

Once you have a readline interface, you can start reading lines from the input stream. You can do this by using the for await...of loop or by listening for the 'line' event.

Using the for await...of loop

The for await...of loop is a new way to iterate over asynchronous iterables. It is similar to the regular for...of loop, but it allows you to pause the iteration and wait for asynchronous operations to complete.

The following code snippet shows how to use the for await...of loop to read lines from a file:

for await (const line of rl) {
  // Each line in input.txt will be successively available here as `line`.
  console.log(`Line from file: ${line}`);
}

Listening for the 'line' event

You can also listen for the 'line' event to read lines from a file. The 'line' event is emitted whenever a new line is available in the input stream.

The following code snippet shows how to listen for the 'line' event:

rl.on("line", (line) => {
  console.log(`Line from file: ${line}`);
});

Closing the readline interface

When you are finished reading lines from the input stream, you should close the readline interface. You can do this by calling the close() method on the readline interface.

The following code snippet shows how to close a readline interface:

rl.close();

Complete code implementation

The following is a complete code implementation that shows how to read a file line-by-line using readline:

const fs = require("fs");
const readline = require("readline");

async function processLineByLine() {
  const fileStream = fs.createReadStream("input.txt");

  const rl = readline.createInterface({
    input: fileStream,
    crlfDelay: Infinity,
  });

  for await (const line of rl) {
    // Each line in input.txt will be successively available here as `line`.
    console.log(`Line from file: ${line}`);
  }
}

processLineByLine();

Real-world applications

readline can be used in a variety of real-world applications, such as:

Parsing log files
Processing CSV files
Reading data from a network stream

TTY Keybindings

TTY stands for teletypewriter, which is a device that was used to send and receive text messages over a distance.
TTY keybindings are keyboard shortcuts that you can use when you're using a TTY to interact with a computer.
These keybindings can be used to perform a variety of tasks, such as editing text, moving the cursor, and managing files.

Here's a simplified explanation of some common TTY keybindings:

Ctrl+C: Stops the current process.
Ctrl+A: Go to the beginning of the line.
Ctrl+E: Go to the end of the line.
Ctrl+B: Move the cursor one character to the left.
Ctrl+F: Move the cursor one character to the right.
Ctrl+U: Delete everything from the cursor to the beginning of the line.
Ctrl+K: Delete everything from the cursor to the end of the line.
Ctrl+Y: Paste the most recently deleted text.
Ctrl+Z: Suspend the current process.
Ctrl+W: Delete the word to the left of the cursor.
Ctrl+D: Delete the character under the cursor.

TTY keybindings can be very helpful for quickly and efficiently performing common tasks.

Real-World Examples

Here are some real-world examples of how TTY keybindings can be used:

A developer might use the Ctrl+U keybinding to quickly delete a line of code that they don't need.
A user might use the Ctrl+A and Ctrl+E keybindings to quickly move the cursor to the beginning or end of a line of text.
A system administrator might use the Ctrl+Z keybinding to suspend a process that is using too many resources.

Improved Code Examples

Here's an improved code example that demonstrates how to use TTY keybindings in a Node.js application:

const readline = require("readline");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

rl.on("line", (line) => {
  console.log(`You entered: ${line}`);
});

rl.on("close", () => {
  console.log("Bye!");
});

rl.prompt();

This code example creates a readline interface that listens for input from the user. When the user enters a line of text, the line event is emitted. The event handler prints the user's input to the console. When the user presses Ctrl+D (on Linux and macOS) or Ctrl+Z (on Windows), the close event is emitted. The event handler prints a goodbye message to the console.

Potential Applications

TTY keybindings can be used in a variety of real-world applications, including:

Text editors
Command line interfaces (CLIs)
Interactive shells
Scripting languages

Previousquerystring Nextreport

Class: InterfaceConstructor

'pause' Event in Node.js's readline Module

rl.pause()

rl.prompt([preserveCursor])

rl.getPrompt() Method in Node.js readline

Return Value

Real-World Code Example

Applications

rl.write(data[, key])

rl[Symbol.asyncIterator]()

readline.line

Example

Potential Applications

rl.cursor

rl.getCursorPos()

Class: readlinePromises.Interface

What is it?

How to use it?

Real-world example

Potential applications

rl.question(query[, options])

readlinePromises.Readline

Creating an Instance

Reading Lines from Input

Writing Lines to Output

Event Listeners

Potential Applications

readlinePromises.Readline

Constructor

Methods

Real-World Example

Potential Applications

ClearLine

rl.cursorTo(x[, y])

Real-world applications

Potential applications

Complete Code Implementation and Real-World Example:

Potential Applications in Real World:

rl.question(query[, options], callback)

Real-world applications

readline.clearLine(stream, dir[, callback])

Real-world applications

Potential applications

Using readline completer

readline.emitKeypressEvents(stream[, interface])

Understanding the Tiny CLI Example

What is a CLI?

Creating a Simple CLI

Real-World Applications

Class: `InterfaceConstructor`

`rl.pause()`

`rl.prompt([preserveCursor])`

`rl.getPrompt()` Method in Node.js readline

`rl.write(data[, key])`

`rl[Symbol.asyncIterator]()`

`rl.cursor`

`rl.getCursorPos()`

Class: `readlinePromises.Interface`

`rl.question(query[, options])`

`readlinePromises.Readline`

`rl.cursorTo(x[, y])`

`rl.question(query[, options], callback)`

`readline.clearLine(stream, dir[, callback])`

Using `readline` completer