How to read a file line by line using node.js ?

The ability to read a file line by line allows us to read large files without entirely storing it to the memory. It is useful in saving resources and improves the efficiency of the application. It allows us to look for the information that is required and once the relevant information is found, we can stop the search process and can prevent unwanted memory usage.

We will achieve the target by using the Readline Module and Line-Reader Module.

Method 1: Using the Readline Module: Readline is a native module of Node.js, it was developed specifically for reading the content line by line from any readable stream. It can be used to read data from the command line.
Since the module is the native module of Node.js, it doesn’t require any installation and can be imported as

const readline = require('readline');

Since readline module works only with Readable streams, so we need to first create a readable stream using the fs module.

const file = readline.createInterface({
    input: fs.createReadStream('source_to_file'),
    output: process.stdout,
    terminal: false
});

Now, listen for the line event on the file object. The event will trigger whenever a new line is read from the stream:

file.on('line', (line) => {
    console.log(line);
});

Example:

// Importing the Required Modules
const fs = require('fs');
const readline = require('readline');

// Creating a readable stream from file
// readline module reads line by line
// but from a readable stream only.
const file = readline.createInterface({
    input: fs.createReadStream('gfg.txt'),
    output: process.stdout,
    terminal: false
});

// Printing the content of file line by
//  line to console by listening on the
// line event which will triggered
// whenever a new line is read from
// the stream
file.on('line', (line) => {
    console.log(line);
});

Output:

Method 2: Using Line-reader Module: The line-reader module is an open-source module for reading file line by line in Node.js. It is not the native module, so you need to install it using npm(Node Package Manager) using the command:

npm install line-reader --save

The line-reader module provides eachLine() method which reads the file line by line. It had got a callback function which got two arguments: the line content and a boolean value that stores, whether the line read, was the last line of the file.

const lineReader = require('line-reader');

lineReader.eachLine('source-to-file', (line, last) => {
    console.log(line);
});

Example:

// Importing required libraries
const lineReader = require('line-reader');

// eachLine() method call on gfg.txt
// It got a callback function
// Printing content of file line by line
// on the console
lineReader.eachLine('gfg.txt', (line, last) => {
    console.log(line);
});

Output: