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

Question

Synchronous:

var fs = require('fs');
var array = fs.readFileSync('file.txt').toString().split("\n");
for(i in array) {
    console.log(array[i]);
}

Asynchronous:

var fs = require('fs');
fs.readFile('file.txt', function(err, data) {
    if(err) throw err;
    var array = data.toString().split("\n");
    for(i in array) {
        console.log(array[i]);
    }
});

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);
    }
  });
}

Using the Node.js readline module.

var fs = require('fs');
var readline = require('readline');

var filename = process.argv[2];
readline.createInterface({
    input: fs.createReadStream(filename),
    terminal: false
}).on('line', function(line) {
   console.log('Line: ' + line);
});

js:

var array = fs.readFileSync('file.txt', 'utf8').split('\n');

ts:

var array = fs.readFileSync('file.txt', 'utf8').toString().split('\n');

use readline (documentation). here's an example reading a css file, parsing for icons and writing them to json

var results = [];
  var rl = require('readline').createInterface({
    input: require('fs').createReadStream('./assets/stylesheets/_icons.scss')
  });


  // for every new line, if it matches the regex, add it to an array
  // this is ugly regex :)
  rl.on('line', function (line) {
    var re = /\.icon-icon.*:/;
    var match;
    if ((match = re.exec(line)) !== null) {
      results.push(match[0].replace(".",'').replace(":",''));
    }
  });


  // readline emits a close event when the file is read.
  rl.on('close', function(){
    var outputFilename = './icons.json';
    fs.writeFile(outputFilename, JSON.stringify(results, null, 2), function(err) {
        if(err) {
          console.log(err);
        } else {
          console.log("JSON saved to " + outputFilename);
        }
    });
  });

Essentially this will do the job: .replace(/\r\n/g,'\n').split('\n'). This works on Mac, Linux & Windows.

Code Snippets

Synchronous:

const { readFileSync } = require('fs');

const array = readFileSync('file.txt').toString().replace(/\r\n/g,'\n').split('\n');

for(let i of array) {
    console.log(i);
}

Asynchronous:

With the fs.promises API that provides an alternative set of asynchronous file system methods that return Promise objects rather than using callbacks. (No need to promisify, you can use async-await with this too, available on and after Node.js version 10.0.0)

const { readFile } = require('fs').promises;

readFile('file.txt', function(err, data) {
    if(err) throw err;

    const arr = data.toString().replace(/\r\n/g,'\n').split('\n');

    for(let i of arr) {
        console.log(i);
    }
});

More about \r & \n here: \r\n, \r and \n what is the difference between them?

file.lines with JFile package

Pseudo

var JFile=require('jfile');

var myF=new JFile("./data.txt");
myF.lines // ["first line","second line"] ....

Don't forget before :

npm install jfile --save