Read a file one line at a time in node.js?

Since Node.js v0.12 and as of Node.js v4.0.0, there is a stable readline core module. Here's the easiest way to read lines from a file, without any external modules:

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  });  // Note: we use the crlfDelay option to recognize all instances of CR LF  // ('\r\n') in input.txt as a single line break.  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();

Or alternatively:

var lineReader = require('readline').createInterface({  input: require('fs').createReadStream('file.in')});lineReader.on('line', function (line) {  console.log('Line from file:', line);});

The last line is read correctly (as of Node v0.12 or later), even if there is no final \n.

UPDATE: this example has been added to Node's API official documentation.

For such a simple operation there shouldn't be any dependency on third-party modules. Go easy.

var fs = require('fs'),    readline = require('readline');var rd = readline.createInterface({    input: fs.createReadStream('/path/to/file'),    output: process.stdout,    console: false});rd.on('line', function(line) {    console.log(line);});

You don't have to open the file, but instead, you have to create a ReadStream.

fs.createReadStream

Then pass that stream to Lazy