Node.js How to delete first line in file

Question

I'm making simple Node.js app and I need to delete first line in file. Please is any way how to do it? I think that It will be possible with fs.write, but how?

Answer 1

Here is streamed version of removing first line from file.
As it uses streams, means you don't need to load whole file in memory, so it is way more efficient and fast, as well can work on very large files without filling memory on your hardware.

var Transform = require('stream').Transform;
var util = require('util');


// Transform sctreamer to remove first line
function RemoveFirstLine(args) {
    if (! (this instanceof RemoveFirstLine)) {
        return new RemoveFirstLine(args);
    }
    Transform.call(this, args);
    this._buff = '';
    this._removed = false;
}
util.inherits(RemoveFirstLine, Transform);

RemoveFirstLine.prototype._transform = function(chunk, encoding, done) {
    if (this._removed) { // if already removed
        this.push(chunk); // just push through buffer
    } else {
        // collect string into buffer
        this._buff += chunk.toString();

        // check if string has newline symbol
        if (this._buff.indexOf('\n') !== -1) {
            // push to stream skipping first line
            this.push(this._buff.slice(this._buff.indexOf('\n') + 2));
            // clear string buffer
            this._buff = null;
            // mark as removed
            this._removed = true;
        }
    }
    done();
};

And use it like so:

var fs = require('fs');

var input = fs.createReadStream('test.txt'); // read file
var output = fs.createWriteStream('test_.txt'); // write file

input // take input
.pipe(RemoveFirstLine()) // pipe through line remover
.pipe(output); // save to file

Another way, which is not recommended.
If your files are not large, and you don't mind loading them into memory, load file, remove line, save file, but it is slower and wont work well on large files.

var fs = require('fs');

var filePath = './test.txt'; // path to file

fs.readFile(filePath, function(err, data) { // read file to memory
    if (!err) {
        data = data.toString(); // stringify buffer
        var position = data.toString().indexOf('\n'); // find position of new line element
        if (position != -1) { // if new line element found
            data = data.substr(position + 1); // subtract string based on first line length

            fs.writeFile(filePath, data, function(err) { // write file
                if (err) { // if error, report
                    console.log (err);
                }
            });
        } else {
            console.log('no lines found');
        }
    } else {
        console.log(err);
    }
});