Read a File One Line At a Time in Node.Js

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.

node.js: read a text file into an array. (Each line an item in the array.)

If you can fit the final data into an array then wouldn't you also be able to fit it in a string and split it, as has been suggested?
In any case if you would like to process the file one line at a time you can also try something like this:

var fs = require('fs');

function readLines(input, func) {
  var remaining = '';

  input.on('data', function(data) {
    remaining += data;
    var index = remaining.indexOf('\n');
    while (index > -1) {
      var line = remaining.substring(0, index);
      remaining = remaining.substring(index + 1);
      func(line);
      index = remaining.indexOf('\n');
    }
  });

  input.on('end', function() {
    if (remaining.length > 0) {
      func(remaining);
    }
  });
}

function func(data) {
  console.log('Line: ' + data);
}

var input = fs.createReadStream('lines.txt');
readLines(input, func);

EDIT: (in response to comment by phopkins) I think (at least in newer versions) substring does not copy data but creates a special SlicedString object (from a quick glance at the v8 source code). In any case here is a modification that avoids the mentioned substring (tested on a file several megabytes worth of "All work and no play makes Jack a dull boy"):

function readLines(input, func) {
  var remaining = '';

  input.on('data', function(data) {
    remaining += data;
    var index = remaining.indexOf('\n');
    var last  = 0;
    while (index > -1) {
      var line = remaining.substring(last, index);
      last = index + 1;
      func(line);
      index = remaining.indexOf('\n', last);
    }

    remaining = remaining.substring(last);
  });

  input.on('end', function() {
    if (remaining.length > 0) {
      func(remaining);
    }
  });
}

How to read a file line by line in Javascript and store it in an array

Maybe this will help you.

Async Version:

const fs = require('fs')

const emails = [];
const names = [];

fs.readFile('file.txt', (err, file) => {

  if (err) throw err;

  file.toString().split('\n').forEach(line => {
    const splitedLine = line.split(':');

    emails.push(splitedLine[0]);
    names.push(splitedLine[1]);

  });
});

Sync Version:

const fs = require('fs')

const emails = [];
const names = [];

fs.readFileSync('file.txt').toString().split('\n').forEach(line => {
  const splitedLine = line.split(':');

  emails.push(splitedLine[0]);
  names.push(splitedLine[1]);
})

console.log(emails)
console.log(names)

How to read a file line by line 1 line at a time

I dont really understand why you use readline interface to read a file.

I suggest you to use split, such as

  var splitstream = fs.createReadStream(file).pipe(split())
  splitstream
    .on('data', function (line) {
      //each chunk now is a seperate line!
      splitstream.pause();
      setTimeout(function (){ splitstream.resume() }, 500);
    })

you will definitely get one line at a time.

** edited for the comment.

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