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

js:

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

ts:

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

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?

This is a variation on the answer above by @mtomis.

It creates a stream of lines. It emits 'data' and 'end' events, allowing you to handle the end of the stream.

var events = require('events');

var LineStream = function (input) {
    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;
            this.emit('data', line);
            index = remaining.indexOf('\n', last);
        }
        remaining = remaining.substring(last);
    }.bind(this));

    input.on('end', function() {
        if (remaining.length > 0) {
            this.emit('data', remaining);
        }
        this.emit('end');
    }.bind(this));
}

LineStream.prototype = new events.EventEmitter;

Use it as a wrapper:

var lineInput = new LineStream(input);

lineInput.on('data', function (line) {
    // handle line
});

lineInput.on('end', function() {
    // wrap it up
});

To read a big file into array you can read line by line or chunk by chunk.

line by line refer to my answer here

var fs = require('fs'),
    es = require('event-stream'),

var lines = [];

var s = fs.createReadStream('filepath')
    .pipe(es.split())
    .pipe(es.mapSync(function(line) {
        //pause the readstream
        s.pause();
        lines.push(line);
        s.resume();
    })
    .on('error', function(err) {
        console.log('Error:', err);
    })
    .on('end', function() {
        console.log('Finish reading.');
        console.log(lines);
    })
);

chunk by chunk refer to this article

var offset = 0;
var chunkSize = 2048;
var chunkBuffer = new Buffer(chunkSize);
var fp = fs.openSync('filepath', 'r');
var bytesRead = 0;
while(bytesRead = fs.readSync(fp, chunkBuffer, 0, chunkSize, offset)) {
    offset += bytesRead;
    var str = chunkBuffer.slice(0, bytesRead).toString();
    var arr = str.split('\n');

    if(bytesRead = chunkSize) {
        // the last item of the arr may be not a full line, leave it to the next chunk
        offset -= arr.pop().length;
    }
    lines.push(arr);
}
console.log(lines);

Using Node.js v8 or later has a new feature that converts normal function into an async function.

util.promisify

It's an awesome feature. Here's the example of parsing 10000 numbers from the txt file into an array, counting inversions using merge sort on the numbers.

// read from txt file
const util = require('util');
const fs = require('fs')
fs.readFileAsync = util.promisify(fs.readFile);
let result = []

const parseTxt = async (csvFile) => {
  let fields, obj
  const data = await fs.readFileAsync(csvFile)
  const str = data.toString()
  const lines = str.split('\r\n')
  // const lines = str
  console.log("lines", lines)
  // console.log("str", str)

  lines.map(line => {
    if(!line) {return null}
    result.push(Number(line))
  })
  console.log("result",result)
  return result
}
parseTxt('./count-inversion.txt').then(() => {
  console.log(mergeSort({arr: result, count: 0}))
})

Another answer using an npm package. The nexline package allows one to asynchronously read a file line-by-line:

"use strict";

import fs from 'fs';
import nexline from 'nexline';

const lines = [];
const reader = nexline({
    input: fs.createReadStream(`path/to/file.ext`)
});

while(true) {
    const line = await reader.next();
    if(line === null) break; // line is null if we reach the end
    if(line.length === 0) continue; // Ignore empty lines
    
    // Process the line here - below is just an example
    lines.push(line);
}

This approach will work even if your text file is larger than the maximum allowed string length, thereby avoiding the Error: Cannot create a string longer than 0x1fffffe8 characters error.