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Async JavaScript by Trevor Burnham

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Making Promises

We started this chapter by showing how the Ajax methods in jQuery 1.5+ ($.ajax, $.get, and $.post) return Promises. But to really understand Promises, we need to make a few of our own.

Let’s give the user a prompt to hit either Y or N. The first thing we’ll do is create an instance of $.Deferred that represents the user’s decision.

 
var​ promptDeferred = ​new​ $.Deferred();
 
promptDeferred.always(​function​(){ console.log(​'A choice was made:'​); });
 
promptDeferred.done(​function​(){ console.log(​'Starting game...'​); });
 
promptDeferred.fail(​function​(){ console.log(​'No game today.'​); });

(Note: always is available only in jQuery 1.6+.)

You’re probably wondering why I created an instance of Deferred when this section ...

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