Catch a javascript error in an external script file

Question

I have a bit of JavaScript (Jquery Tools' Overlay) which may throw an exception when dropped on a page that uses it incorrectly, and I'm trying to handle it gracefully.

I have a general window.onerror handler to rescue these errors and report them back to the server, however that's not getting triggered.

I also cannot wrap a try/catch around this code, as it's being included as a remote script in HTML.

Any ideas on how you can rescue errors that an external script throws?

UPDATE: Here's the example. I should correct myself, window.onerror does get triggered, however the script does not continue running (in the example, the alert never alerts).

<html>
<script type="text/javascript">
window.onerror = function(e){ console.log("caught error: "+e); return true;}
</script>
<body>

<!-- this is the line in the dom that causes the script to throw -->
<a rel="nofollow"></a>

<script type="text/javascript" src="http://www.google.com/jsapi"></script>
<script type="text/javascript">google.load("jquery", "1.4.1");</script>
<script src="http://cdn.jquerytools.org/1.2.5/tiny/jquery.tools.min.js"></script>

<script type="text/javascript">
//this code will throw an error in jquery tools
$("a[rel]").overlay();

alert("it's ok, I still ran.");
</script>

</body>

</html>

Is your window.onerror defined before the error is thrown? If not, it won't work. — Korvin Szanto
– Korvin Szanto, Commented Sep 21, 2011 at 20:50

Flimm · Accepted Answer · 2022-06-20 18:56:12Z

15

Define the error handler before any other script is loaded/executed.

<script>window.onerror = function(e){alert(e);}</script>
<script src="external.js"></script>
<script>
function ole(){alert("Hi!")}
ole();
</script>

When your script contains a syntax error, the error handler won't be called though (edit: in some older browsers):

<script>window.onerror=function(e){alert(e);}</script>
<script>
!@ //The error won't be caught (at least not in older browsers)
</script>

edited Jun 20, 2022 at 18:56

Flimm

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answered Sep 21, 2011 at 20:56

Rob W

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5 Comments

Tom Lianza Over a year ago

Thanks, I just updated the question. It actually is triggering onerror, however control flow also stops, instead of continuing.

Rob W Over a year ago

That's expected. The error is thrown by $, and not caught by any local error handler. When an error is thrown, any code after that line isn't executed any more (the code defined in the global scope, in this case).

Yonn Trimoreau Over a year ago

Best JavaScript hack of the year x) you made my day

CertainPerformance Over a year ago

Maybe browsers have changed since the answer was posted - window.onerror looks to capture syntax errors now, at least in Chrome

softmarshmallow Over a year ago

will error from requirejs's loaded script also be caught with this? 'cuase i can't.

Flimm · Accepted Answer · 2022-01-11 18:28:38Z

Here's a solution that should work in modern browsers. Older browsers may not catch syntax errors.

<script>
  window.addEventListener('error', function(e) {
   alert(e.message);
   console.error(e.message, e.filename, e.lineno, e.colno, e.error);
  });
</script>
<script>
  this is a syntax error in JavaScript
</script>

This works as well for external scripts <script src="foobar.js"></script>, but browsers will hide detailed error information by default for scripts hosted in different origins. You can fix it by using crossorigin, like this:

<script crossorigin="anonymous" src="http://example.com/foobar.js"></script>

More useful information:

Is assigning a function to window.onerror preferable to window.addEventListener('error', callback)?
Sentry blog post about getting detailed error information from externally hosted scripts
Catching errors when a resource fails to load because of a 404 or a network error

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