You can register an error handler function by assigning a function object to the window's onerror property so that when an error occurs in the JavaScript within the window, that function will be called.

This can be a useful way of trapping and completely inhibiting the display of error messages to the user. When the error handling function is called, it is passed three arguments:

• The textual message that explains the error

• The URL of the document that contains the error

• The line number within that document where the error occurred

The line number is the physical line number within the document, not a line number within a script, so you should be able to open the document in a text editor and go to the indicated line and identify the problem.

The only way to activate this error handling capability is to assign a function to the onerror property. There is no HTML tag attribute mechanism for defining an error handler for a window although you can associate one with the <IMG> tag with an attribute value.

You can return either true or false as a result of calling your handler:

true - Inhibit any further error processing and abort the script.

false - Hand control back to the browser to deal with the error in the normal way.

Deactivate error handling altogether by calling a function that simply returns true. To restore default error handling to the window later, assign a function handler to the property that merely returns a false value as its result without intervening in the error process in any way.

Warnings:

• Beware of the return values. Returning true from an error handler inhibits the browser from carrying out any further action. This is exactly opposite to the return value from a form element event handler, which requires that a false value be returned to inhibit any further action by the browser.

• This works on version 4 of Netscape Navigator. It is also fully supported by MSIE version 5. Version 4 of MSIE for Macintosh (and possibly other platforms) allows the error to be trapped but does not pass any meaningful argument values. They are all undefined.

• This is not supported on the WebTV platform.

• There is a problem with this event handler hook in the Netscape 6.0 browser. The values are not passed to the handler correctly although the handler is called.

Property attributes:

DontEnum.