Friday 14 December 2012

Catching Cross-Domain JS Errors

As I've mentioned before, most modern browsers do not provide access to error information in window.onerror for scripts loaded from across domains. This is a very severe restriction, and has limited the usefulness of Errorception to some extent.

Fortunately, a couple of months ago, Firefox landed a patch to add this feature, and this has already been shipped with the latest versions of Firefox. Chrome is expected to follow suit very soon, since this has already landed in Webkit.

Unfortunately, this doesn't work out of the box, and will require some tweaking of your server and markup. Fortunately, the changes you need to make are minimal.

On the server

You will need to enable CORS for the external JS file you load. The most minimal way to do this is to set the following HTTP header in the response for your JS file.

Access-Control-Allow-Origin: *

That's the only server-side change you need to make!

In the markup

Script tags have now got a new non-standard attribute called crossorigin. The most secure value for this would be anonymous. So, you'll have to modify your script tags to look like the following.

<script src="" crossorigin="anonymous"></script>

Browser support

As of this writing, only Firefox supports reporting errors for cross-domain scripts. All WebKit browsers including Chrome is expected to support this very soon. This isn't a problem with IE at all, since IE already reports errors to window.onerror irrespective of the domain (yay, security!). Standardisation for the new attribute has been proposed though it hasn't gotten anywhere.

Update: Thanks to Matthew Schulkind for pointing out in the comments below: It appears that Firefox insists that if you are using the cross-origin attribute, the script file must be served with the access control HTTP header. If the access control header isn't present, the script simply doesn't get evaluated. This is a minor annoyance at development time, so I've filed a bug with Mozilla about this.