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);
  2. You’d write something like this:

    var promise = wait(500);
        console.log('Timeout fired!');

    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(){
            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? ...

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