Learning jQuery Deferreds

Appendix A. Hints for Selected Challenges

Here you’ll find solutions or hints for almost all the challenges in the book. Where an answer requires a significant amount of code, we make some suggestions but have left the coding to you.

A Replacement for the setTimeout Function

1. This is pretty easy:

   function wait(timeout){
       return $.Deferred(function(deferred){
           setTimeout(deferred.resolve, timeout);
       }).promise();
   }

2. You’d write something like this:

   var promise = wait(500);
   
   promise.done(function(){
       console.log('Timeout fired!');
   });
   
   one(promise);
   two(promise);

   Of course the one and two functions have to be able to accept a promise.

   We described this challenge as “subtly different” because it’s meant to demonstrate that our replacement setTimeout function is more than just a syntactic nicety. It gives you something you didn’t have before: a value that you can pass around to others. Although this is a trivial example, the principle is important.

3. We just need to store the timeout identifier and add a function to the returning promise to clear the timeout and reject the deferred:

   function wait(timeout){
       var deferred = $.Deferred(),
           promise = deferred.promise(),
           timeoutId = setTimeout(deferred.resolve, timeout);
   
       promise.cancel = function(){
           clearTimeout(timeoutId);
           deferred.reject.apply(deferred, arguments);
       };
   
       return promise;
   }

   Note that this isn’t the best coding style. We probably shouldn’t be adding attributes to the promise in this way. How would you fix this shortcoming? ...