change a file using node.js

Question

I have a text file that looks like this:

./4F6cFLnAAFc.png
./4Flj9plmKGQ.png
./4fQeHtSdw80.png
./4I9iibPLdBw.png
./4J-dRA3MGc8.png
./4mdEsouIXGM.png
./4-_-wQc3EGE.png
./595Tiga1gIg.png
./60u_ctp6UIw.png
./64bH_27Ehoc.png
./64_ZfIemRGM.png
./66pQLO--Ghk.png
./69rDtSpshAw.png
./69-YUSazuic.png

I'd like to replace the . at the beginning of each line with 'myString' using node.js. So each will end up looking like: myString/4F6cFLnAAFc.png

I am trying to use a regular expression but I suspect it can be done using read and write streams.

var fs = require('fs');

fs.readFile('filelist.txt', 'utf-8', function(err, data){
    if (err) throw err;

    console.log(typeof(data)); // string

    data.replace(/\.\/+/, 'myString');

});

What regex will work or what's the best way to edit this file using node?

Answer 1

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();

Answer 2

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);
    });
});

Answer 3

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