Nested jQuery $.when

Question

Essentially I am tring to write this:

var async1 = $.when( a1() ).then(function(){ a2() });
var async2 = $.when( a3() ).then(function(){ a4() });

$.when(async1, async2).then(function(){ 
    console.log("complete");
});

But at the moment when a1 and a3 have executed the function considers itself resolved.

I put together the same example in a fiddle: http://jsfiddle.net/Z7fzR/

@ExplosionPills when a1, a2 a3 and a4 are all resolved I would expect the final when to be resolved. If you follow? — Jon Wells
– Jon Wells, Commented Dec 12, 2012 at 15:43
Just guessing... What would happen if you changed it to this? $.when( a1().then(function(){ a2() })); — chrisfrancis27
– chrisfrancis27, Commented Dec 12, 2012 at 15:48
@ChrisFrancis that did work but ExplosionPills has nailed it — Jon Wells
– Jon Wells, Commented Dec 12, 2012 at 15:55

Explosion Pills · Accepted Answer · 2012-12-12 15:52:50Z

11

You never actually return the promise objects created by a2() and a4() from the callback; this effectively returns null, which apparently counts as a completion for $.when purposes:

http://jsfiddle.net/Z7fzR/1/

answered Dec 12, 2012 at 15:52

Explosion Pills

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jbabey · Accepted Answer · 2012-12-12 15:55:09Z

5

You are throwing away the promise objects that a2 and a4 return, essentially passing undefined back to the original when, which causes it to resolve immediately:

If a single argument is passed to jQuery.when and it is not a Deferred, it will be treated as a resolved Deferred and any doneCallbacks attached will be executed immediately.

Add some returns and it works fine.

var async1 = $.when(a1()).then(function(){ return a2(); });
var async2 = $.when(a3()).then(function(){ return a4(); });

$.when(async1, async2).then(function(){
    console.log("complete");
});

http://jsfiddle.net/Z7fzR/2/

answered Dec 12, 2012 at 15:55

jbabey

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