javascript - What does $.when.apply($, someArray) do?

Question

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');
}

Answer 1

.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 :-)