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13.1.2 JavaScript in Web Applications

13.1.2 JavaScript in Web Applications

Web applications use all of the JavaScript DHTML features that web documents do, but they also go beyond these content, presentation, and behavior manipulation APIs to take advantage of other fundamental services provided by the web browser environment.

To really understand web applications, it is important to realize that web browsers have grown well beyond their original role as tools for displaying documents and have transformed themselves into simple operating systems. Consider: a traditional operating system allows you to organize icons (which represent files and applications) on the desktop and in folders. A web browser allows you to organize bookmarks (which represent documents and web applications) in a toolbar and in folders. An OS runs multiple applications in separate windows; a web browser displays multiple documents (or applications) in separate tabs. An OS defines low-level APIs for networking, drawing graphics, and saving files. Web browsers define low-level APIs for networking (Chap ter 18 ), saving data ( Chapter 20 ), and drawing graphics ( Chapter 21 ).

With this notion of web browser as simplified OS in mind, we can define web applications as web pages that use JavaScript to access the more advanced services (such as networking, graphics, and data storage) offered by browsers. The best known of these advanced services is the XMLHttpRequest object, which enables networking through scripted HTTP requests. Web apps use this service to obtain new information from the server without a page reload. Web applications that do this are commonly called Ajax applications and they form the backbone of what is known as “Web 2.0.” XMLHttpRequest is covered in detail in Chapter 18 .

The HTML5 specification (which, at the time of this writing, is still in draft form) and related specifications are defining a number of other important APIs for web apps. These include the data storage and graphics APIs of Chapters 21 and 20 as well as APIs for a number of other features, such as geolocation, history management, and background threads. When implemented, these APIs will enable a further evolution of web application capabilities. They are covered in Chapter 22 .

JavaScript is more central to web applications than it is to web documents, of course. JavaScript enhances web documents, but a well-designed document will continue to work with JavaScript disabled. Web applications are, by definition, JavaScript programs that use the OS-type services provided by the web browser, and they would not be expected to work with JavaScript disabled.1

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