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');

}