For-loop and async callback in node.js?

Question

I'm new to JavaScript and to node.js. I want to loop through a directory and add all file stat (not other directories) to an array. As you see below there is a problem with my code since the callback will probably get called after the for loop has finished so using the "i"-variable in the callback method will not work. But how should the code look so that the below snippet works? Does it have something to do with closures?

Thanks for help!

    fs.readdir(SYNCDIR, function(err1, files) {
        var filesOnly = [];

        if(!err1) {

            for(var i = 0; i < files.length; i++) {

                var imgFilePath = SYNCDIR + '/' + files[i];
                fs.stat(imgFilePath, function(stat){

                    if (stat.isFile()){
                        filesOnly[i] = stat; // This will not be correct since the for-loop has finished
                    }
                });

            }
        }
    });

I think since you posted this node has added some synch functions to the fs namespace. — jcollum
– jcollum, Commented Jul 26, 2013 at 18:21
related (if not even duplicate): JavaScript closure inside loops – simple practical example — Bergi
– Bergi, Commented Jul 23, 2014 at 12:49

Gary Chambers · Accepted Answer · 2011-09-01 10:55:05Z

32

You are right about needing to use a closure. You should wrap the contents of the for loop in a self-invoking function to preserve the value of i for each iteration.

fs.readdir(SYNCDIR, function(err1, files) {
    var filesOnly = [];

    if(!err1) {

        for(var i = 0; i < files.length; i++) {

            (function(i) {
                var imgFilePath = SYNCDIR + '/' + files[i];
                fs.stat(imgFilePath, function(stat){
                    if (stat.isFile()){
                        filesOnly[i] = stat;
                    }
                });
            })(i);

        }
    }
});

answered Sep 1, 2011 at 10:55

Gary Chambers

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Comments

einaros · Accepted Answer · 2011-09-01 10:58:28Z

10

One way is to rewrite the innards of the loop to use a closure:

fs.readdir(SYNCDIR, function(err1, files) {
    var filesOnly = [];
    if(!err1) {
        for(var i = 0; i < files.length; i++) {
            (function(index) {
                var imgFilePath = SYNCDIR + '/' + files[index];
                fs.stat(imgFilePath, function(stat){
                    if (stat.isFile()){
                        filesOnly[index] = stat;
                    }
                });
            })(i);
        }
    }
});

A better looking example, achieving the same, using Array.prototype.forEach:

fs.readdir(SYNCDIR, function(err1, files) {
    var filesOnly = [];
    if(!err1) {
        files.forEach(function(file, i) {
            var imgFilePath = SYNCDIR + '/' + file;
            fs.stat(imgFilePath, function(stat){
                if (stat.isFile()){
                    filesOnly[i] = stat;
                }
            });
        });
    }
});

answered Sep 1, 2011 at 10:58

einaros

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1 Comment

Alnitak Over a year ago

+1 for the .forEach approach. It's far easier, and avoids creating a new bound scope for each iteration.

Victor Bjelkholm · Accepted Answer · 2014-03-25 15:44:23Z

0

Alternatively use the new threads module ( https://github.com/robtweed/Q-Oper8 ) and then you can do all this stuff much more simply using standard synchronous coding within the threads child processes, since they only deal with one user's request at a time.

Goodbye to async logic and nested callbacks!

edited Mar 25, 2014 at 15:44

Victor Bjelkholm

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answered Sep 1, 2011 at 12:17

Rob Tweed

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