javascript - 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 Node.js.

eg my input file is like: line 1 line 2 line 3

but output file could be like: line 1 line 3 line 2

Below is my code.

var fs  = require("fs");
var index = 1;

fs.readFileSync('./input.txt').toString().split('
').forEach(
function (line) { 
    console.log(line);
        fs.open("./output.txt", 'a', 0666, function(err, fd) {
            fs.writeSync(fd, line.toString() + "
", null, undefined, function(err, written) {
            })});
    }
);

Any thoughts would be appreciated, thanks.

Answer 1

If you're writing a synchronous code, use only the synchronous functions:

var fs  = require("fs");

fs.readFileSync('./input.txt').toString().split('
').forEach(function (line) { 
    console.log(line);
    fs.appendFileSync("./output.txt", line.toString() + "
");
});

For asynchronous approach you could write something like

var fs = require('fs'),
    async = require('async'),
    carrier = require('carrier');

async.parallel({
    input: fs.openFile.bind(null, './input.txt', 'r'),
    output: fs.openFile.bind(null, './output.txt', 'a')
}, function (err, result) {
    if (err) {
        console.log("An error occured: " + err);
        return;
    }

    carrier.carry(result.input)
        .on('line', result.output.write)
        .on('end', function () {
            result.output.end();
            console.log("Done");
        });
});