Javascript For loop callback?

Question

Trying to think in Javascript rather than jQuery, so I'm wondering if I'm doing this right.

I want to have a callback when my loop is finished. Is this the proper way?

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

  /* do some stuff */ 

  if ( i === (divs.length - 1)) {  /* call back */  }

}

I should add that I don't mean something like a JSON request callback, just when the loop has finished.

Excuse my ignorance, but I don't understand it. If you want to call it when the loop is finished, why not calling after the loop instead of inside it? — Aleadam
– Aleadam, Commented Apr 9, 2011 at 2:08
If you're going to think in JavaScript, then do it. Do your loop, then call your function. — JAAulde
– JAAulde, Commented Apr 9, 2011 at 2:11
@Aleadam, because the loop might contain some asynchronous activities. E.g. You might be adding something to a database. This is my challenge anyhow... — daCoda
– daCoda, Commented Jan 7, 2019 at 7:24

codelahoma · Accepted Answer · 2012-02-14 06:52:23Z

24

For clarity, you should go with @mu's answer, but if you really must include the callback within the for construct, you can use the comma operator*:

for(var i = 0;
    i < divs.length || function(){ /* call back */ }(), false;
    i++) {

/* do some stuff */ 

}

*As explained in this fascinating article.

edited Feb 14, 2012 at 6:52

answered Apr 9, 2011 at 4:15

codelahoma

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Comments

mu is too short · Accepted Answer · 2011-04-09 02:14:55Z

22

Why not say what you really mean and call the callback after the loop?

function thing_with_callback(divs, callback) {
    for(var i = 0; i < divs.length; i++) {
        /* do some stuff */ 
    }
    callback();
}

answered Apr 9, 2011 at 2:14

mu is too short

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5 Comments

Thomas Eding Over a year ago

because that would just be rediculous. plus what if you wanted to edit i interactively in a debugger?

Matthew Crumley Over a year ago

Technically, that's not equivalent if the array is empty since it would still run. It's easy to work around though by adding an if statement.

mu is too short Over a year ago

@Matthew: The question was "I want to have a callback when my loop is finished." so putting the callback(); outside the loop is the only way and there is no need for a conditional on divs.length.

Matthew Crumley Over a year ago

Yes, that's probably what he meant to do, but the example in the question wouldn't call the callback if divs.length is zero, because the for loop would never run. You're correct that putting the callback after the loop is almost certainly what he wanted though.

mu is too short Over a year ago

@Matthew: Yeah, there's some ambiguity involved so it could go either way. Hard to say what should happen when divs.length == 0 too.

user489419 · Accepted Answer · 2012-03-07 11:03:13Z

6

It seems to me the question is about the execution order of javascript code. And the answers are:

Yes you can put the callback outside because javascript code is executed line by line. In case of asynchonous ajax calls there might be other things to consider.

answered Mar 7, 2012 at 11:03

user489419

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