Callbacks

As you explored in Chapter 10, Accessing External Resources, callbacks are the oldest and simplest means of executing asynchronous functionality in JavaScript. A callback is a specified function to be called once the result of an operation is ready. You saw this with the jQuery $.ajax() and $.getJSON() methods, where a function is called once a successful service call response is available; for example:

$.getJSON('https:/www.somesite.com/someservice',
      function(data) {
      // this function is a callback and is called once
             // the response to the service call is received
      }
    );

Another area where callbacks are heavily used is for event handlers. Events can be considered asynchronous, as they can happen at unpredictable times and in any order. The callbacks to...