Are closures different in node.js?

Question

I have worked quite a lot with javascript but yesterday, I started using node.js. It's a little script that runs jslint on the files of a folder. For this example, I changed the command to call ls instead of jslint.

var sys = require("sys");
var fs = require("fs");
var cp = require('child_process');

var path = fs.realpathSync("./src/");

fs.readdir(fs.realpathSync("./src/"), function(err, files) {
  for (var i = 0; i < files.length; i++) {
    var filename = files[i];
    var complete = path + filename;

    // Run jslint on each file
    var jslint = cp.exec("ls " + complete, function(error, stdout, stderr) {
      console.log(filename + " : " + stdout);
    });    
  }
});

The output is this :

jskata.nofreeze.js : /home/dan/php/jskata/src/jskata.undo.js

jskata.nofreeze.js : /home/dan/php/jskata/src/jskata.nofreeze.js

jskata.nofreeze.js : /home/dan/php/jskata/src/jskata.timezone.js

Why do the line console.log(filename + " : " + stdout); always prints jskata.nofreeze.js when the filename should obviously match the result of the ls? Are closures and scopes different in node.js than in javascript?

Answer 1

No they aren't any different, this is just the most common pitfall with closures in JavaScript.

See, you're in a loop, you're assigning the local variable filename on every iteration of the loop, therefore you're in fact overriding it's value. Since it's always the same local variable, and the closure works per reference, the value of filename inside the callback gets updates on each iteration of the loop.

'jskata.nofreeze.js' just happens to be the last file in the directory and therefore it's also the last value that gets assigned to filename.

To solve it you need to pass the value of filename per value:

// Run jslint on each file
(function(c, f) {
    cp.exec("cat " + c, function(error, stdout, stderr) {
      console.log(f + " : " + stdout);
    });
})(complete, filename);

This makes the whole thing work, even though it's a bit ugly. You can of course move this out into a named function if you don't want to clutter your inner loop with more anonymous functions.