jpalecek jpalecek - 3 months ago 17
Ajax Question

When should XMLHttpRequest's onerror handler fire

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

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 onreadystatuschange handler.

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

Comments