Passing Data to Callbacks
A Promise can give its callbacks additional information. For example, these two Ajax snippets are equivalent:
| | // Using a callback directly |
| | $.get(url, successCallback); |
| | // Binding a callback to a Promise |
| | var fetchingData = $.get(url); |
| | fetchingData.done(successCallback); |
When you resolve or reject a Deferred, any arguments you provide are relayed to the corresponding callbacks.
| | var aDreamDeferred = new $.Deferred(); |
| | aDreamDeferred.done(function(subject) { |
| | console.log('I had the most wonderful dream about', subject); |
| | }); |
| | aDreamDeferred.resolve('the JS event model'); |
| <= | I had the most wonderful dream about the JS event model |
There are also special methods for running the callbacks in a particular context (that is, setting this to a particular value): resolveWith and rejectWith. Just pass the context as the first argument, and pass all other arguments in as an array.
| | var slashdotter = { |
| | comment: function(editor){ |
| | console.log('Obviously', editor, 'is the best text editor.'); |
| | } |
| | }; |
| | var grammarDeferred = new $.Deferred(); |
| | grammarDeferred.done(function(verb, object) { |
| | this[verb](object); |
| | }); |
| | grammarDeferred.resolveWith(slashdotter, ['comment', 'Emacs']); |
| <= | Obviously Emacs is the best text editor. |
Having to wrap your arguments in an array is a pain, though. So, here’s a handy tip: instead of using resolveWith/rejectWith, you can just invoke plain resolve/reject in the desired context. That’s because resolve/reject pass their context right to the callbacks they fire. So, in the previous example, we could achieve the same result with the following:
| | grammarDeferred.resolve.call(slashdotter, 'comment', 'Emacs'); |