JavaScript - Events and Event Handling

Events and Event Handling

We saw at the beginning of this section that one way to embed client-side JavaScript into HTML documents is to use event handler attributes of HTML tags. The following table lists the standard event handler attributes and the HTML tags to which they may be applied. The first column of the table gives the event handler attribute name: these names always begin with "on". The second column of the table lists the HTML tags to which these attributes can be applied, and explains, when necessary, what events trigger the handler code to be executed.

Handler	Supported by/Triggered when
`onabort`	`<img>`; image load aborted
`onblur`	`<body>` and form elements; window or element loses keyboard focus
`onchange`	Form elements; displayed value changes
`onclick`	All elements; mouse press and release; return `false` to cancel
`ondblclick`	All elements; mouse double-click
`onerror`	`<img>`; image loading fails
`onfocus`	`<body>` and form elements; window or element gets keyboard focus
`onkeydown`	`<body>` and form elements; key pressed; return `false` to cancel
`onkeypress`	`<body>` and form elements; key pressed and released; return `false` to cancel
`onkeyup`	`<body>` and form elements; key released
`onload`	`<body>`, `<frameset>`, `<img>`, `<iframe>`, `<object>`; document, image, or object completely loaded
`onmousedown`	All elements; mouse button pressed
`onmousemove`	All elements; mouse pointer moved
`onmouseout`	All elements; mouse moves off element
`onmouseover`	All elements; mouse moves over element; return `true` to prevent link URL display in status bar
`onmouseup`	All elements; mouse button released
`onreset`	`<form>`; form reset requested; return `false` to prevent reset
`onresize`	`<body>`, `<frameset>`; window size changes
`onsubmit`	`<form>`; form submission requested; return `false` to prevent submission
`onunload`	`<body>`, `<frameset>`; document unloaded

Note that when the browser triggers certain event handlers, such as onclick, onmouseover and onsubmit, it examines the return value of the handler (if there is one) to determine whether it should perform the default action associated with the event or not. Typically, if an event handler returns false, the default action (such as following a hyperlink or submitting a form) is not performed. The one exception is for the onmouseover handler: when the mouse moves over a hyperlink, the browser displays the link's URL in the status line unless the event handler returns true.

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