Node.js read and write file lines

Question

I tried to read a file line by line, and output it to another file, using Node.js.

My problem is the sequence of lines sometimes messed up due to async nature of Nod

Answer 1

I suppose you want to perform some calculations and/or transformations on every line. If not, simple copy is one-liner (take a look at createReadStream documentation)

fs.createReadStream('./input.txt').pipe(fs.createWriteStream('./output.txt'));

Now, you are trying to open file each time you want to write line, and yes, order is unpredictable here. More correct version of your code:

var lines = fs.readFileSync('./input.txt').toString().split('\n')
function writeLineFromArray(lines) {
    var line = arr.shift();
    fs.open("./output.txt", 'a', 0666, function(err, fd) {
        fs.writeSync(fd, line + '\n', null, undefined, function(err, written) {
           writeLineFromArray(lines);
        });
    }); 
}
writeLinesFromArray();

I'd probably use one of 'given input stream, notify me on each line' modules, for example node-lazy or byline:

var fs = require('fs'),
    byline = require('byline');

var stream = byline(fs.createReadStream('sample.txt'));
stream.on('line', function(line) { 
    // do stuff with line
});
stream.pipe(fs.createWriteStream('./output');