Operators Are Standing By: Connecting with JavaScript Operators in Java

Generating Data Matrix in Java Operators Are Standing By: Connecting with JavaScript Operators

In this digital age, the role of the telephone operator is long forgotten for all except special needs However, if you think back to the early years of the telephone, the operator played an important role in connecting the caller to another phone line A JavaScript operator acts something like the old phone operator, by connecting different JavaScript pieces of code You use operators to add numbers, connect two strings, assign values to variables, or evaluate expressions, for example

In the early days of JavaScript, scripters needed to account for browsers that didn t support scripting Therefore, to prevent JavaScript code from screwing up an older browser s processing of the document, a common practice was to enclose all the scripting code with HTML comments:

All browsers now deal with the script element What s more, using HTML comments inside a script tag violates XHTML standards Avoid this practice when you re writing your own JavaScript

Tables 2-1, 2-2, 2-3, and 2-4 list the major operators in JavaScript The ones you most often use are shown in bold

Table 2-1

= += = *= /= %=

The value of y is assigned to x Same as x=x+y Same as x=x-y Same as x=x*y Same as x=x/y Same as x=x%y (modulus)

Table 2-2

== != === < <= > >= :

x is equal to y x is not equal to y Evaluates both for value and data type (for example, if x = 5 and y = 5, then x==y is true, but x===y is false) x is less than y x is less than or equal to y x is greater than y x is greater than or equal to y If y is less than 5, then assign -5 to x; otherwise, assign y to x (known as the conditional operator)

Table 2-3

&& || !

if ( x > 3 && y=0 ) if ( x>3 || y=0 ) if !( x=y)

Table 2-4

+ * / % ++ --

x+2 x-3 x*2 x/2 x%2 x++ x--

Addition Subtraction Multiplication Division Modulus (division remainder) Increment (same as x=x+1) Decrement (same as x=x 1)

Understanding the DOM Accessing objects in the DOM Working with properties and methods Adding and removing DOM objects Exploring DOM objects

s we start introducing scripting to you, we dance around the Document Object Model (DOM) in two chapters We allude to it We used it in some scripting examples, but now we dive head first into the DOM, which is the heart of scripting Web pages In this chapter, you explore the DOM and how to work with it to create interactive scripts for your Web pages

The Document Object Model (DOM) is a scripting interface to HTML and XML documents That s a geeky way of saying that the DOM allows you to access, tap into, and even modify the structure of your Web page by using scripting As its name suggests, the DOM is a modeled structure that describes the relationships of all elements on a document through a hierarchy The DOM also defines which properties, methods, and events are available for each of its objects The DOM is a standard set by the W3C, the Web standards body All newer browsers (Internet Explorer, Firefox, Safari, and Opera, for example) support the Level 1 and Level 2 versions of the DOM (Level 1 focused on HTML, and Level 2 refined the standard to support XML and XHTML documents)

The cross-browser support of the DOM that you can now enjoy is a relatively new phenomenon When the DOM was introduced in the late 1990s, different browsers provided various levels of support and often had their own idiosyncratic additions to the standard Fortunately, these problems have largely disappeared in newer generations of browsers Think of the DOM as a tree-like structure that organizes the parts of the HTML document as a hierarchy of object nodes A node can be an element, an attribute, some content, or any part of a Web page A node can contain other nodes The DOM uses family terminology to describe the relationships of these nodes A parent contains child nodes, and two nodes on the same hierarchical level are considered siblings The document object serves as the trunk of this document tree Because this object has no HTML element equivalent, you can think of it as a superobject of sorts, as it contains every document element, including html, head, and body All remaining elements and other parts of the Web page are descendent nodes of document For most common purposes, you work exclusively with HTML elements in the DOM But, technically speaking, the nodes in a document tree consist of elements and content (The content of an element is a child node of the container element) Attributes aren t considered part of the document tree, but are accessed as properties of the element object Consider, for example, the following code snippet for a basic Web page:

<html> <head> <title>Great Novelists Online</title> </head> <body> <h1>Best Opening Lines</h1> <p id= p1 >It was the best of times It was the worst of times</p> </body> </html>