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What's the best way to overwrite a line in a large (2MB+) text file using node.js?

My current method involves

• copying the entire file into a buffer.
• Spliting the buffer into an array by the new line character (\n).
• Overwriting the line by using the buffer index.
• Then overwriting the file with the buffer after join with \n.

What's the best way to overwrite a line in a large (2MB+) text file using node.js?

My current method involves

• copying the entire file into a buffer.
• Spliting the buffer into an array by the new line character (\n).
• Overwriting the line by using the buffer index.
• Then overwriting the file with the buffer after join with \n.

• javascript
• node.js
• filesystems

Share Improve this question Follow asked Jul 27, 2012 at 17:37 Larry BattleLarry Battle 9,18855 gold badges4343 silver badges5555 bronze badges 4

• fs.write() gives you the ability to write to a specific offset/position, do you know that index ahead of time? – Dominic Barnes Commented Jul 27, 2012 at 17:55
• Not really. How can you pute the offset of each line? – Larry Battle Commented Jul 27, 2012 at 18:47
• I figured it was worth a shot, sometimes you have a specified format for a file, which makes calculations like that possible. – Dominic Barnes Commented Jul 27, 2012 at 20:32
• If you use fixed-length lines in your text file the offset approach might work...but at that point I would reconsider the approach. Text files are great if you only need to append them. After that I would consider a database. – Hector Correa Commented Jul 27, 2012 at 20:49

Add a ment  | 

3 Answers 3

Sorted by: Reset to default 4

First, you need to search where the line starts and where it ends. Next you need to use a function for replacing the line. I have the solution for the first part using one of my libraries: Node-BufferedReader.

var lineToReplace = "your_line_to_replace";
var startLineOffset = 0;
var endLineOffset = 0;

new BufferedReader ("your_file", { encoding: "utf8" })
    .on ("error", function (error){
        console.log (error);
    })
    .on ("line", function (line, byteOffset){
        startLineOffset = endLineOffset;
        endLineOffset = byteOffset - 1; //byteOffset is the offset of the NEXT byte. -1 if it's the end of the file, if that's the case, endLineOffset = <the file size>

        if (line === lineToReplace ){
            console.log ("start: " + startLineOffset + ", end: " + endLineOffset +
                    ", length: " + (endLineOffset - startLineOffset));
            this.interrupt (); //interrupts the reading and finishes
        }
    })
    .read ();

Maybe you can try the package replace-in-file

suppose we have a txt file as below

// file.txt
"line1"
"line2"
"line5"
"line6"
"line1"
"line2"
"line5"
"line6"

and we want to replace:

line1 -> line3
line2 -> line4

Then, we can do it like this:

const replace = require('replace-in-file');

const options = {
    files: "./file.txt",
    from: [/line1/g, /line2/g],
    to: ["line3", "line4"]
};

replace(options)
.then(result => {
    console.log("Replacement results: ",result);
})
.catch(error => {
    console.log(error);
});

the result as below:

// file.txt
"line3"
"line4"
"line5"
"line6"
"line3"
"line4"
"line5"
"line6"

More details please refer to its docs: https://www.npmjs./package/replace-in-file

This isn't file size focoused solution, but Overwrites a line in a file using node.js. It may help other people that search engines redirect to this post, like me.

import *  as fs from 'fs'

const filename = process.argv[2]
const lineIndexToUpdate = parseInt(process.argv[3]) - 1
const textUpdate = process.argv[4]

function filterLine(indexToUpdate, dataString) {
  return dataString
      .split('\n')
      .map((val, index) => {
        if (index === indexToUpdate)
          return textUpdate
        else
          return val
       })
      .join('\n')

}

fs.readFile(filename, 'utf8', (err, data) => {
  if (err) throw err

  fs.writeFile(filename, filterLine(lineIndexToUpdate, data), (err, data) => {
    if (err) throw err
    console.log("Line removed")
  })
})

Script use exemple:
node update_line.js file 10 "te voglio benne"

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