JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

you can replace or suppress the error messages shown in Netscape 3 and Internet Explorer
(with debugging turned on) and the output to the JavaScript Console in Netscape 4+. The
values to which window.onerror can be set and the effects of doing so are outlined in Table
23-3.
Table 23-3: window.onerror Values and Effects
Value of
window.onerror
Effect
Null Suppresses reporting of runtime errors in Netscape 3+.
A function The function is executed whenever a runtime error occurs. If the
function returns true, then the normal reporting of runtime errors is
suppressed. If it returns false the error is reported in the browser
as usual.
Note The onerror handler is also available for objects other than Window in many browsers,
most notably the and elements.
For example, to suppress error messages in older browsers you might use
function doNothing() { return true; }
window.onerror = doNothing;
window.noSuchProperty() // throw a runtime error
Since modern browsers don‘t typically display script errors unless users specifically configure
them to do so, the utility of the return value is limited.
The truly useful feature of onerror handlers is that they are automatically passed three values
by the browser. The first argument is a string containing an error message describing the error
that occurred. The second is a string containing the URL of the page that generated the error,
which might be different from the current page if, for example, the document has frames. The
third parameter is a numeric value indicating the line number at which the error occurred.
Note Early versions of Netscape 6 did not pass these values to onerror handlers.
You can use these parameters to create custom error messages, such as
function reportError(message, url, lineNumber)
{
if (message && url && lineNumber)
alert("An error occurred at "+ url + ", line " + lineNumber +
"\nThe error is: " + message);
}
return true;
window.onerror = reportError; // assign error handler