change a file using node.js

Here are two options for Sync or Async depending on what you want.

var fs = require('fs');function readWriteAsync() {  fs.readFile('filelist.txt', 'utf-8', function(err, data){    if (err) throw err;    var newValue = data.replace(/^\./gim, 'myString');    fs.writeFile('filelistAsync.txt', newValue, 'utf-8', function (err) {      if (err) throw err;      console.log('filelistAsync complete');    });  });}function readWriteSync() {  var data = fs.readFileSync('filelist.txt', 'utf-8');  var newValue = data.replace(/^\./gim, 'myString');  fs.writeFileSync('filelistSync.txt', newValue, 'utf-8');  console.log('readFileSync complete');}readWriteAsync();readWriteSync();

This regex should work: /^\.(.+)/gm

It performs a global, multi-line match(/gm) for all lines that begin with a period(^\.), captures whatever follows the period((.+)) and replaces the entire string with 'myString' concatenated with the captured value($1).

var fs = require('fs'),fileList = '/filelist.txt';fs.readFile(fileList, function(err, data) {    if(err) throw err;    data = data.toString();    data = data.replace(/^\.(.+)/gm, 'myString$1');    fs.writeFile(fileList, data, function(err) {        err || console.log('Data replaced \n', data);    });});

If you want to update/edit/change lines/files with a really big file, in my experience the best way is:

• Using readline module from Nodejs, it wont cause any memory issues
• For each line you read, you can use fs.appendFileSync to write data in another file

Code example:

const fs = require('fs');const readline = require('readline');const rd = readline.createInterface({    input: fs.createReadStream('./old-data.csv'),    output: process.stdout,    console: false});const regex_pattern = /(.*),.*,.*,.*,(.*),.*,.*,.*,.*/rd.on('line', function(line) {    // if your line matches the pattern, you need to process it    if(regex_pattern .test(line)) {      // do your job here to get your string that you want      // my_string_1 = ''      // my_string_2 = ''      fs.appendFileSync('new-data.csv', `\n${my_string_1 }`, 'utf-8')      fs.appendFileSync('new-data.csv', `\n${my_string_2 }`, 'utf-8')    } else { // otherwise, append to a new file      fs.appendFileSync('new-data.csv', `\n${line}`, 'utf-8')    }});

In my case, when edit a file with more than 1 million rows, it only consumes around 10-12% cpu with very little memory