Webmaster in a Nutshell, Third Edition

Dynamic Content

One of the most important features provided by web page scripting is the ability to detect and react to events that occur while a document is loaded, rendered, and used. Web page authors can set up scripts that will be triggered by such events as a cursor passing over an image, clicking on a link, or even leaving a page. The scripting code that responds to these events may be placed within the <script> element or loaded from a separate file. A special set of common element attributes point to a script when their event is triggered. These attributes are called event handlers.

For example, you might want to invoke a JavaScript function when the user passes the mouse over a hyperlink in a document. You simply add a “mouse over” event handler for the <a> tag called onMouseOver

<a href=doc.html DEFANGED_OnMouseOver="document.status='Click me!'; return 
true">

When the mouse passes over the example link, the browser executes the JavaScript statements. (Here, the JavaScript itself is embedded in the attribute, since it is so simple. Notice that the statements are enclosed in quotes and separated by a semicolon, and that single quotes surround the text-message portion of the first statement.)

While a complete explanation of this code is beyond our scope, the net result is that the browser places the message “Click me!” in the status bar of the browser window. Commonly, HTML authors use this simple JavaScript function to display a more descriptive explanation of a hyperlink, ...