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

Any thoughts would be appreciated, thanks.

1
23
8/16/2012 11:42:12 AM

Accepted Answer

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

var fs  = require("fs");

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

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");
        });
});
35
3/11/2014 4:09:05 PM

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');

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