node.js - Replace a line in txt file using JavaScript - Stack Overflow

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I am trying to simply replace a line in a text file using JavaScript.

The idea is:

var oldLine = 'This is the old line';
var newLine = 'This new line replaces the old line';

Now i want to specify a file, find the oldLine and replace it with the newLine and save it.

Anyone who can help me here?

I am trying to simply replace a line in a text file using JavaScript.

The idea is:

var oldLine = 'This is the old line';
var newLine = 'This new line replaces the old line';

Now i want to specify a file, find the oldLine and replace it with the newLine and save it.

Anyone who can help me here?

• javascript
• node.js
• fs
• writefile
• appendfile

Share Improve this question Follow asked Nov 23, 2018 at 12:16 Wibo KuipersWibo Kuipers 8322 gold badges22 silver badges1111 bronze badges 1

• 1 Possible duplicate of Replace a string in a file with nodejs – vahdet Commented Nov 23, 2018 at 12:18

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2 Answers 2

Sorted by: Reset to default 18

Just building on Shyam Tayal's answer, if you want to replace an entire line matching your string, and not just an exact matching string do the following instead:

fs.readFile(someFile, 'utf8', function(err, data) {
  let searchString = 'to replace';
  let re = new RegExp('^.*' + searchString + '.*$', 'gm');
  let formatted = data.replace(re, 'a completely different line!');

  fs.writeFile(someFile, formatted, 'utf8', function(err) {
    if (err) return console.log(err);
  });
});

The 'm' flag will treat the ^ and $ meta characters as the beginning and end of each line, not the beginning or end of the whole string.

So the above code would transform this txt file:

one line
a line to replace by something
third line

into this:

one line
a completely different line!
third line

This should do it

var fs = require('fs')
fs.readFile(someFile, 'utf8', function (err,data) {

  var formatted = data.replace(/This is the old line/g, 'This new line replaces the old line');

 fs.writeFile(someFile, formatted, 'utf8', function (err) {
    if (err) return console.log(err);
 });
});

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