AJAX In Action [25]
var x=eval('7*5');
Here we have defined our calculation as a piece of text, rather than two numbers and an arithmetic operator. Calling eval()on this text interprets the JavaScript it contains, and returns the value of the expression. In most cases, this simply slows the program execution down, but at times the extra flexibility that it brings can be useful.
General purpose means that the language is suitable for use with most algorithms and programming tasks. The core JavaScript language contains support for numbers, strings, dates and times, arrays, regular expressions for text processing, and mathematical functions such as trigonometry and random number generation. It is possible to define structured objects using JavaScript, bringing design principles and order to more complex code.
Within the web browser environment, parts of the browser’s native functionality, including CSS, the DOM, and the XMLHttpRequest objects, are exposed to the JavaScript engine, allowing page authors to programmatically control the page to a greater or lesser degree. Although the JavaScript environment that we encounter in the browser is heavily populated with browser-specific objects, the underlying language is just that, a programming language. This isn’t the time or place for a detailed tutorial on JavaScript basics. In appendix B we take a closer look at the language and outline the fundamental differences between JavaScript and the C family of languages, including its Licensed to jonathan zheng 36 CHAPTER 2 First steps with Ajax namesake, Java. JavaScript examples are sprinkled liberally throughout this book, and several other books already exist that cover the language basics (see our Resources section at the end of this chapter). Within the Ajax technology stack, JavaScript is the glue that binds all the other components together. Having a basic familiarity with JavaScript is a prerequisite for writing Ajax applications. Being fluent in JavaScript and understanding its strengths will allow you to take full advantage of Ajax. We’ll move on now to Cascading Style Sheets, which control the visual style of elements on a web page. 2.3 Defining look and feel using CSS Cascading Style Sheets are a well-established part of web design, and they find frequent use in classic web applications as well as in Ajax. A stylesheet offers a centralized way of defining categories of visual styles, which can then be applied to individual elements on a page very concisely. In addition to the obvious styling elements such as color, borders, background images, transparency, and size, stylesheets can define the way that elements are laid out relative to one another and simple user interactivity, allowing quite powerful visual effects to be achieved through stylesheets alone. In a classic web application, stylesheets provide a useful way of defining a style in a single place that can be reused across many web pages. With Ajax, we don’t think in terms of a rapid succession of pages anymore, but stylesheets still provide a helpful repository of predefined looks that can be applied to elements dynamically with a minimum of code. We’ll work through a few basic CSS examples in this section, but first, let’s look at how CSS rules are defined. CSS styles a document by defining rules, usually in a separate file that is referred to by the web page being styled. Style rules can also be defined inside a web page, but this is generally considered bad practice. A style rule consists of two parts: the selector and the style declaration. The selector specifies which elements are going to be styled, and the style declaration declares which style properties are going to be applied. Let’s say that we want to make all our level-1 headings in a document (that is, the tags) appear red. We can declare a CSS rule to