Handling Events

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Often, JavaScript statements create or manipulate graphical user interface elements such as forms or windows. 

Event Handlers 

JavaScript programs are typically event-driven. Events are actions that occur on the Web page, usually as a result of something the user does, although not always. For example, a button click is an event, as is giving focus to a form element; resizing the page is an event, as is submitting a form. It is these events, which cause JavaScript programs to spring into action. For example, if you move your mouse over this phrase, a message will pop-up, courtesy of JavaScript. 

An event, then, is the action, which triggers an event handler. The event handler specifies which JavaScript code to execute. Often, event handlers are placed within the HTML tag, which creates the object on which the event acts: 

          <tag attributel attribute2 onEventName="javascript code;"> 

For example, a hyperlink is subject to a MouseOver event, meaning that its event handler will be triggered when the mouse passes over the link. Therefore, you place the event handler for a hyperlink's MouseOver inside the A tag: 

<a href="" onMouseOver="popupFunc( ) ;"> 

The JavaScript which is called by the event handler may be any valid JavaScript code: a single statement or a series of statements, although most often it is a function call. 

The set of all events, which may occur, and the particular page elements on which they can occur, is part of the Document Object Model (DOM), and not JavaScript itself. As a result, Netscape and Microsoft do not share the exact same set of events, nor are all page elements subject to the same events between browsers. For example, Internet Explorer4 supports a MouseOver event for an image while Navigator 4 does not. 

The table below illustrates some of the most commonly used events supported in both DOM's. Because the DOM's differ in their event support, the following documents are recommended as an overview of each browser's event support: 

Common Events 

EventOccurs when...

Event Handler.


User clickson form element or link onClick


User changes value of text, textarea, or select element onChange


User gives form element input focus onFocus


User removes input focus from form element onBlur


User moves mouse pointer over a link or anchor onMouseOver


User moves mouse pointer off of link or anchor onMouseOut


User selects form element's input field onSelect


User submits a form onSubmit


User resizes the browser window onResize


User loads the page in the Navigator onLoad


User exits the page onUnload


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