Availability

JavaScript 1.1; buggy in Netscape 6/6.1

Synopsis

You register an onerror event handler like this:

window.onerror=handler-func

The browser invokes the handler like this:

window.onerror(message, url, line)

Arguments

message

A string that specifies the error message for the error that occurred.

url

A string that specifies the URL of the document in which the error occurred.

line

A number that specifies the line number at which the error occurred.

Returns

true if the handler has handled the error and JavaScript should take no further action; false if JavaScript should post the default error message dialog box for this error.

Description

The onerror property of the Window object specifies an error handler function that is invoked when a JavaScript error occurs in code executing in that window. By default, JavaScript displays an error dialog box when an error occurs. You can customize error handling by providing your own onerror event handler.

You define an onerror event handler for a window by setting the onerror property of a Window object to an appropriate function. Note that unlike other event handlers in JavaScript, the onerror handler cannot be defined in an HTML tag.

When the onerror handler is invoked, it is passed three arguments: a string specifying the error message, a string specifying the URL of the document in which the error occurred, and a number that specifies the line number at which the error occurred. An error handling function may do anything it wants with these arguments: it may display its own error dialog box or log the error in some way, for example. When the error handling function is done, it should return true if it has completely handled the error and wants JavaScript to take no further action or false if it has merely noted or logged the error in some fashion and still wants JavaScript to display the error message in its default dialog box.

Note that while this event handler returns true to tell the browser to take no further action, most Form and form element event handlers return false to prevent the browser from performing some action, such as submitting a form. This inconsistency can be confusing.

You can turn off error handling entirely for a window by setting the onerror property of the window to a function that returns true and does nothing else. You can restore the default error-handling behavior (the dialog box) by setting onerror to a function that returns false and does nothing else.

Bugs

This event handler is correctly triggered by errors in Netscape 6 and Netscape 6.1, but the values passed as the message, URL, and line number arguments are incorrect, so although you can use it to detect the occurrence of an error, you cannot use it to obtain any useful information about the error.