JavaScript: The Definitive Guide, Fourth Edition

Name

Window.onload — the handler invoked when a document finishes loading

Availability

JavaScript 1.0

Synopsis

<body onload=”handler" ... > <frameset onload=”handler" ... > window.onload

Description

The onload property of a Window specifies an event handler function that is invoked when a document or frameset is completely loaded into its window or frame.

The initial value of this property is a function that contains the semicolon-separated JavaScript statements specified by the onload attribute of the <body> or <frameset> tags.

When the onload event handler is invoked, you can be certain that the document has fully loaded, and therefore that all scripts within the document have executed, all functions within scripts are defined, and all forms and other document elements have been parsed and are available through the Document object.

Usage

If any of your document’s event handlers depend on the document being fully loaded, you should check that it is loaded before executing those handlers. If the network connection were to stall out after a button appeared in the document but before the parts of the document that the button relied on were loaded, the user would get unintended behavior or an error message after clicking the button. One good way to verify that the document is loaded is to use the onload handler to set a variable -- loaded, for example -- to true and to check the value of this variable before doing anything that depends on the complete document being loaded.

See Also