Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

13.5 Error Handling

The onerror property of

a Window object is special. If you assign a
function to this property, the function will be invoked whenever a
JavaScript error occurs in that window: the function you assign
becomes an error handler for the window.

Three arguments are passed to an error handler. The first is a
message describing the error that occurred. This may be something
like "missing operator in expression", "self is
read-only", or "myname is not defined". The second
argument is a string that contains the URL of the document containing
the JavaScript code that caused the error. The third argument is the
line number within the document where the error occurred. An error
handler can use these arguments for any purpose it desires. A typical
error handler might display the error message to the user, log it
somewhere, or force the error to be ignored.

In addition to those three arguments, the return value of the
onerror handler is significant. Browsers typically
display an error message in a dialog box or in the status line when
an error occurs. If the onerror handler returns
true, it tells the system that the handler has
handled the error and that no further action is necessary -- in
other words, the system should not display its own error message. For
example, if you do not want your users to be pestered by error
messages, no matter how buggy the code you write is, you could use a
line of code like this at the start of all your JavaScript programs:

self.onerror = function(  ) { return true; } 

Of course, doing this will make it very difficult for users to give
you feedback when your programs fail silently without producing error
messages.

We'll see a sample use of an onerror handler
in Example 14-1. That example uses the
onerror handler to display the error details to
the user and allow the user to submit a bug report containing those
details.

Note that the onerror error handler is buggy in
Netscape 6. Although the function you
specify is triggered when an error occurs, the three arguments that
are passed are incorrect and unusable. Netscape 6 and other browsers
that support JavaScript 1.5 have an alternative means of catching and
handling errors, however: they can use the
try/catch statement. (See Chapter 6 for details.)