As mentioned in Chapter 1, JavaScript is the heart of AJAX. JavaScript has a similar syntax to the good old C language. JavaScript is a parsed language (not compiled), and it has some Object-Oriented Programming (OOP) capabilities. JavaScript wasn't meant for building large powerful applications, but for writing simple scripts to implement (or complement) a web application's client-side functionality (however, new trends are tending to transform JavaScript into an enterprise-class language—it remains to be seen how far this will go). JavaScript is fully supported by the vast majority of web browsers. Although it is possible to execute JavaScript scripts by themselves, they are usually loaded on the client browsers together with HTML code that needs their functionality. The fact that the entire JavaScript code must arrive unaltered at the client is a strength and weakness at the same time, and you need to consider these aspects before deciding upon a framework for your web solution. You can find very good introductions to JavaScript at the following web links: • http://www.echoecho.com/javascript.htm •http://www.devlearn.com/javascript/jsvars.html • http://www.w3schools.com/js/default.asp Part of JavaScript's power on the client resides in its ability to manipulate the parent HTML document, and it does that through the DOM interface. The DOM is available with a multitude of languages and technologies, including JavaScript, Java, PHP, C#, C++, and so on. In this chapter, you will see how to use the DOM with both JavaScript and PHP. The DOM has the ability to manipulate (create, modify, parse, search, etc.) XML-like documents, HTML included. On the client side, you will use the DOM and JavaScript to: • Manipulate the HTML page while you are working on it • Read and parse XML documents received from the server • Create new XML documents On the server side, you can use the DOM and PHP to: • Compose XML documents, usually for sending them to the client • Read XML documents received from various sources Two good introductions to DOM can be found at http://www.quirksmode.org/dom/intro.html andhttp://www.javascriptkit.com/javatutors/dom.shtml. Play a nice DOM game here:http://www.topxml.com/learning/games/b/default.asp. A comprehensive reference of the JavaScript DOM can be found at http://krook.org/jsdom/. The Mozilla reference for the JavaScript DOM is available athttp://www.mozilla.org/docs/dom/reference/javascript.html. In the first example of this chapter, you will use the DOM from JavaScript to manipulate the HTML document. When adding JavaScript code to an HTML file, one option is to write the JavaScript code in a element within the <body> element. Take the following HTML file for example, which executes some simple JavaScript code when loaded. Notice the document object, which is a default object in JavaScript that interacts with the DOM of the HTML page. Here we use its write method to add content to the page:
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