javascript - When should XMLHttpRequest's onerror handler fire

Question

I have a little problem understanding XMLHttpRequest's handlers. The specification says this about the onerror handler:

error [Dispatched ... ] When the request has failed.

load [Dispatched ... ] When the request has successfully completed.

The problem is, what does it mean that "the request has failed". That could be

• the request couldn't be issued at all (eg. Connection refused and such errors), or
• the above plus the server returned an error code (eg. 404)

Also, I'd like to know whether it means onerror and onload should never fire simultaneously.

This reference indicates the onerror handler should be executed depending on the status code and onload depending on readyState. That would indicate they are not mutually exclusive, however, I don't think this is an authoritative information.

I'm asking because using the latest Opera snapshot, I found onload is fired even on 404 status code. I know testing status is a sure bet, but I'd like to know whether it's something I have to do per specification or just a workaround for a bug in Opera.

Answer 1

As mentioned in the comments, onerror fires when there is a failure on the network level. If the error only exists on the application level, e.g., an HTTP error code is sent, then onload still fires. You need to test the returned status code explicitly in your onreadystatechange handler.

Note that a denied cross-domain request will also fire the onerror handler.