Dojo: The Definitive Guide

Chapter 4. AJAX and Server Communication

The common thread of this chapter is server-side communications. Performing asynchronous requests, using the IFRAME transport to submit forms behind the scenes, serializing to and from JavaScript Object Notation (JSON), and using JSONP (JSON with Padding) are a few of the topics that are introduced in this chapter. You'll also learn about Deferred, a class that forms the lynchpin in the toolkit's IO subsystem by providing a uniform interface for handling asynchronous activity.

Quick Overview of AJAX

AJAX^[12] (Asynchronous JavaScript and XML) has stirred up considerable buzz and revitalized web design in a refreshing way. Whereas web pages once had to be completely reloaded via a synchronous request to the server to perform a significant update, JavaScript's XMLHttpRequest object allows them to now behave much like traditional desktop applications. XHR is an abbreviation for the XMLHttpRequest object and generally refers to any operation provided the object.

Web pages may now fetch content from the server via an asynchronous request behind the scenes, as shown in Figure 4-1, and a callback function can process it once it arrives. (The image in Figure 4-1 is based on http://adaptivepath.com/ideas/essays/archives/000385.php.) Although a simple concept, this approach has revolutionized the user experience and birthed a new era of Rich Internet Applications.

Figure 4-1. The difference between synchronous and asynchronous communication for a web application

Using JavaScript's XMLHttpRequest object directly isn't exactly rocket science, but like anything else, there are often tricky implementation details involved and boilerplate that must be written in order to cover the common-use cases. For example, asynchronous requests are never guaranteed to return a value (even though they almost always do), so you'll generally need to implement logic that determines when and how to timeout a request; you may want to have some facilities for automatically vetting and transforming JSON strings into JavaScript objects; you'll probably want to have a concise way of separating the logic that handles a successful request versus a request that produces an error; and so forth.

JSON

JSON bears a brief mention before we move on to a discussion of AJAX because it has all but become the universally accepted norm for lightweight data exchange in AJAX applications. You can read about the formalities of JSON at http://json.org, but basically, JSON is nothing more than a string-based representation of JavaScript objects. Base provides two simple functions for converting String values and JavaScript objects back and forth. These functions handle the mundane details of escaping special characters like tabs and new lines, and even allow you to pretty-print if you feel so inclined:

dojo.fromJson(/*String*/ json) //Returns Object
dojo.toJson(/*Object*/ json, /*Boolean?*/ prettyPrint) //Returns String

Tip

By default, a tab is used to indent the JSON string if it is pretty-printed. You can change the tab to whatever you'd like by switching the value of the built-in attribute dojo.toJsonIndentStr.

Here's a quick example that illustrates the process of converting an Object to a JSON string that is suitable for human consumption:

var o = {a:1, b:2, c:3, d:4};
dojo.toJson(o, true); //pretty print
/* produces ...
'{
  "a": 1,
  "b": 2,
  "c":3,
  "d":4
}'

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^[12] Even though the "X" in AJAX specifically stands for XML, the term AJAX now commonly refers to virtually any architecture that employs the XMLHttpRequest object to perform asynchronous requests, regardless of the actual type of data that's returned. Although opting to use the umbrella term XHR would technically be more accurate, we'll follow common parlance and use AJAX in the broader context.