Read file line by line javascript

How to read a file line by line in Node.js

There are multiple ways to read a file line by line in Node.js. Earlier, we discussed how to read a file line by line in Java.

Let us look at Node.js ways of reading a file line by line.

The simplest way of reading a file line by line in Node.js is by using the native fs module’s fs.readFileSync() method:

const fs = require('fs') try  // read contents of the file const data = fs.readFileSync('file.txt', 'UTF-8') // split the contents by new line const lines = data.split(/\r?\n/) // print all lines lines.forEach(line =>  console.log(line) >) > catch (err)  console.error(err) > 

1. It is blocking, which means that it will stop the program execution until the entire file is loaded into the memory.
2. It will have a severe impact on memory consumption if the file is in gigabytes or more.

The first issue could be resolved using the non-blocking version fs.readFile() , but reading the entire file into memory is not something you want to do in a production environment.

However, if you want to read small files only, it works fine.

Readline is another Node.js native module that was developed specifically for this purpose — reading one line at a time from any readable stream. You can even use this module to read input data from the command line. Here is how you can access it in your code (no installation required):

const readline = require('readline') 

Since the readline module works with readable streams, we have to first create a stream by using the fs module like below:

const rl = readline.createInterface( input: fs.createReadStream('file.txt'), output: process.stdout, terminal: false >) 

Now we can listen for the line event on the rl object that will be triggered whenever a new line is read from the stream:

rl.on('line', line =>  console.log(line) >) 

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) >) 

line-reader is an open-source module for reading a file line by line in Node.js. You can add it to your project by running the following command in your terminal:

The line-reader module provides the eachLine() method that reads each line of the given file. It takes a callback function with two arguments: the line content and a boolean value specifying whether the line read was the last line of the file. Here is an example:

const lineReader = require('line-reader') lineReader.eachLine('file.txt', (line, last) =>  console.log(line) >) 

Another benefit of using this module is to stop the reading when some condition turns true. It can be achieved by returning false from the callback function:

const lineReader = require('line-reader') lineReader.eachLine('file.txt', line =>  console.log(line) // stop if line contains `NEW` if (line.includes('NEW'))  // stop reading and close the file return false > >) 

linebyline is another open-source library that can read a file line by line in Node.js. Let us add it to your project:

This package simply streams the native readline module internally, reads and buffers new lines emitting a line event for each line:

const readline = require('linebyline') // read all lines rl = readline('file.txt') // listen for `line` event rl.on('line', (line, lineCount, byteCount) =>  console.log(line) >).on('error', err =>  console.error(err) >) 

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How to Read File Line by Line in JavaScript

Reading a file through a browser is an essential task for any website that interacts with its users. The file can be accessed without storing it on the internet. JavaScript provides a built-in method, FileReader() which can be used to read a file. Moreover, various NPM modules can also be used to read a file in JavaScript. This post demonstrates various methods for reading a file line by line via JavaScript. The content of this post is as follows:

How to Read File Line by Line in JavaScript?

JavaScript is famous for providing a variety of methods, and properties to facilitate the user. The built-in FileReader() method can read the file content of each line. For instance, the “readline” module is also utilized to access the file and read it line by line. Moreover, users can read the file through websites or local machines.

Example 1: Using FileReader() Method for Reading a File Line by Line in JavaScript

Here, HTML and JavaScript code is practiced showing the usage of the FileReader() method for reading a file line by line using the functionalities of JavaScript.

Example to Read Local text file < / h1 >

In this code, a file selection field is provided by giving the name “readfile” in the tag. After that, a JavaScript file is integrated by providing the source as “test.js”.

 h1> Read Local text file using JavaScript h1> input type="file" name="myfile" id="myfile" /> p>check console for result p> script src="main.js"> script>  

let file = document.getElementById("myfile"); file.addEventListener("change", function ( ) < var reader = new FileReader(); reader.onload = function (progressEvent) < console.log(this.result); >; reader.readAsText(this.files[0]); >);