Zhenbin Xu wrote:
The issue of return "null or an exception" is simply a compromise here. IE would throw an exception for state violations. Accessing responseXML before open() is a state violation so it would trigger exception. Other browsers may return null in such situation. In order to accommodate all browsers, the spec would have to be rewritten in some way.
Please note that it is not a goal for the spec to be written in such a way that all existing browsers are conforming to the spec. It turned out that it was impossible to write a spec with that goal while still keeping the spec useful. So we no longer try to "accomodate all browsers", but instead write a spec that leads to interoperability between browsers.
We would certainly love to have the spec change to "MUST throw INVALID_STATE_ERR exception", which is consistent with other INVALID_STATE_ERR cases. For instance, the spec says if send() is called before OPENED, it should trigger INVALID_STATE_ERR exception. Another example is that user agent must raise INVALID_STATE_ERR if "status" is not available. responseText and responseXML are the outlier in the spec.
Personally I think it makes more sense to return 'null' from .responseXML. We at mozilla have not had any interoperability problems with this behavior. Exceptions are better left for exceptional circumstances.
However I can't say that I think the behavior is very important to me one way or another, as long as it's usefully defined.
Best Regards, Jonas Sicking
