Dealing with asynchronous function

Question

I have just started programming with JS and Node, and I haven't got used to asynchronous stuff yet. Basically, i have the following code:

for (var i=0, len=sources.length; i<len; i++) {
    processSource(sources[i], function(info) {
        doesOtherStuff(sources[i], info);
    });
}

It doesn't really work, because, as processSource takes a while to finish, the function doesOtherStuff is called with unmatching arguments, like sources[2] and the processed info for sources[0].

What is the correct way of dealing with this? Is there something inherently wrong with the design of these functions. (both processSource and doesOtherStuff are my functions).

"int" hmmmm, that seems wrong. – epascarello Dec 12 '13 at 17:47 — epascarello, Dec 12 '13 at 17:47
Typo, sorry =P. Problem still holds. – Márcio Paiva Dec 12 '13 at 17:50 — Márcio Paiva, Dec 12 '13 at 17:50
you show me a reason to use javascript forEach! – damphat Dec 12 '13 at 19:11 — damphat, Dec 12 '13 at 19:11

score 2 · Accepted Answer · answered Dec 12 '13 at 17:51

2

The problem with the code is the fact that i is not want you expect it to be.

When the loop completes, the function-level variable i has the value of sources.length. So when doesOtherStuff runs that is what the inner function uses.

for (var i=0, len=sources.length; i<len; i++) {
    (function(i) {
        processSource(sources[i], function(info) {
            doesOtherStuff(sources[i], info);
        });
    })(i);
}

answered Dec 12 '13 at 17:51

epascarello

204,599
20
195
236

So this is actually standard, is is it more like a hack to code that was bad-written in the first place? I'm still trying to get my head around event-driven. – Márcio Paiva Dec 12 '13 at 17:57
1

That pattern is commonly called an IIFE (immediately-invoked function expression) in case you're looking for more information about it. – Aaron Dufour Dec 12 '13 at 18:10
Love this, also we can do +function() {...}(i); :) – damphat Dec 12 '13 at 19:05

score 1 · Answer 2 · answered Dec 12 '13 at 19:03

1

javascript style could help you:

sources.forEach(function (e) {
    processSource(e, function(info) {
        doesOtherStuff(e, info);
    });
}

answered Dec 12 '13 at 19:03

damphat

18,246
8
45
59

Max Truxa · Answer 3 · 2013-12-12T18:16:56.177

0

1) Use var instead of int.

2) You have a superfluous ) in your call to processSource.

processSource(sources[i], function(info) /* No 2nd ')' here */ {
    doesOtherStuff(sources[i], info);
});

should work.

edited Dec 12 '13 at 18:16

answered Dec 12 '13 at 17:50

Max Truxa

3,308
25
38

Sorry, those were typos, not actually the issue. Problem is indeed what epascarello has guessed. – Márcio Paiva Dec 12 '13 at 17:56
To avoid typos you should never *type* code into SO but *copy* it from your source. – Max Truxa Dec 12 '13 at 17:57
Sorry... The code was actually a little more complex, so i tried to reduce it to be clearer about the issue. Guess it didn't help – Márcio Paiva Dec 12 '13 at 17:58

score 0 · Answer 4 · answered Dec 12 '13 at 17:53

0

Try using Caolan's async library - this works on Node and in the browser. You can then do something like this:

async.map(sources, function (item, callback) {
    // Do your actions on each item here
    processSource(item, function (info) {
        var result = doOtherStuff(item, info);

        // Send callback so the next item can be processed
        callback(null, result);
    });
}, function (err, results) {
   // Handle the processed results here
});

answered Dec 12 '13 at 17:53

Clarence Leung

2,446
21
24

Wouldn't this be an overkill? Or is this library used in the majority of node projects, (like Boost library to C++) ? – Márcio Paiva Dec 12 '13 at 18:21

Dealing with asynchronous function

4 Answers4