readline

What is readline?

The readline npm package is a core Node.js module that provides an interface for reading data from a Readable stream (like process.stdin) one line at a time. It is particularly useful for creating command-line interfaces and reading large files line by line.

What are readline's main functionalities?

Reading Input from stdin

This code sets up a readline interface to read user input from the standard input (stdin). It asks the user 'What is your name?' and prints a greeting with the user's input.

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});
rl.question('What is your name? ', (answer) => {
  console.log(`Hello, ${answer}!`);
  rl.close();
});

Reading a File Line by Line

This example demonstrates how to use readline to read a file line by line. It creates a readline interface with a file stream as input and logs each line to the console as it is read.

const fs = require('fs');
const readline = require('readline');
const rl = readline.createInterface({
  input: fs.createReadStream('file.txt'),
  output: process.stdout,
  terminal: false
});
rl.on('line', (line) => {
  console.log(line);
});

Other packages similar to readline

readline-sync

readline-sync is an npm package that provides synchronous readline capabilities. Unlike the asynchronous nature of readline, readline-sync blocks the event loop until input is received. This is useful for CLI applications where synchronous operations are preferred.

inquirer

Inquirer.js is a powerful library for building interactive command-line interfaces. It offers more features than readline, such as checkboxes, lists, confirmations, and more. It is more suitable for complex CLI applications that require more than just line-by-line input.

README

readline

Read a file line by line.

Install

Important. In node 10 there is a core module named readline. Please use linebyline instead, it is the same module just renamed:

Npm linebyline

npm install linebyline

Test

npm install .
npm test

What's this?

Simple streaming readline module for NodeJS. Reads a file and buffers new lines emitting a line event for each line.

Usage

Simple

  var readline = require('linebyline'),
      rl = readline('./somefile.txt');
  rl.on('line', function(line, lineCount, byteCount) {
    // do something with the line of text
  })
  .on('error', function(e) {
    // something went wrong
  });

ASCII file decoding

As the underlying fs.createReadStream doesn't care about the specific ASCII encoding of the file, an alternative way to decode the file is by telling the readline library to retain buffer and then decoding it using a converter (e.g. iconv-lite).

  var readline = require('linebyline'),
      rl = readline('./file-in-win1251.txt', {
    retainBuffer: true //tell readline to retain buffer 
  });
  rl.on("line", function (data,linecount){
    var line = iconv.decode(data, 'win1251');
    // do something with the line of converted text
  });

##API

readLine(readingObject[, options])

Params:

• readingObject - file path or stream object
• options can include:
  • maxLineLength - override the default 4K buffer size (lines longer than this will not be read)
  • retainBuffer - avoid converting to String prior to emitting 'line' event; will pass raw buffer with encoded data to the callback

Return:

• EventEmitter

License

BSD © Craig Brookes