I'm reading about Deferreds and Promises and keep coming across $.when.apply($, someArray)
. I'm a little unclear on what this does exactly, looking for an explanation that one line works exactly (not the entire code snippet). Here's some context:
var data = [1,2,3,4]; // the ids coming back from serviceA
var processItemsDeferred = [];
for(var i = 0; i < data.length; i++){
processItemsDeferred.push(processItem(data[i]));
}
$.when.apply($, processItemsDeferred).then(everythingDone);
function processItem(data) {
var dfd = $.Deferred();
console.log('called processItem');
//in the real world, this would probably make an AJAX call.
setTimeout(function() { dfd.resolve() }, 2000);
return dfd.promise();
}
function everythingDone(){
console.log('processed all items');
}
.apply
is used to call a function with an array of arguments. It takes each element in the array, and uses each as a parameter to the function. .apply
can also change the context (this
) inside a function.
So, let's take $.when
. It's used to say "when all these promises are resolved... do something". It takes an infinite (variable) number of parameters.
In your case, you have an array of promises; you don't know how many parameters you're passing to $.when
. Passing the array itself to $.when
wouldn't work, because it expects its parameters to be promises, not an array.
That's where .apply
comes in. It takes the array, and calls $.when
with each element as a parameter (and makes sure the this
is set to jQuery
/$
), so then it all works :-)