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onUnload event handler

onunload executes JavaScript immediately after someone leaves the page. A common use (though not that great) is to thank someone as that person leaves your page for coming and visiting. 

<body onunload="alert('Thank you. Please come back to this site and visit us soon, ok?')">

There are other event handlers, many belonging to forms. These event handlers are discussed in the tutorial Accessing and validating forms using Javascript.

Here is a list of commonly used event handlers in JavaScript:

Event Handlers   Applicable inside:
onAbort   <img> tags
onBlur   window object, all form objects (ie: <input>), and <frame>.
onClick   Most visible elements such as <a>, <div>, <body> etc.
onChange   Use this to invoke JavaScript if the mouse goes pass some link
onError   Text fields, textareas, and select lists.
onFocus   Most visible elements such as <a>, <div>, <body> etc.
onLoad   <body>, <img>, and <frame>
onMouseover   Most visible elements such as <a>, <div>, <body> etc.
onMouseout   Most visible elements such as <a>, <div>, <body> etc.
onReset   <form> tag, triggered when the form is reset via <input type="reset">.
onSelect   Elements with textual content. Most commonly used inside text fields and textareas.
onSubmit   <form> tag, triggered when the form is submitted.
onUnload   <body>