Why does setting XMLHttpRequest responseType before calling open throw?

问题

Running new XMLHttpRequest().responseType = "json" in the console throws an "InvalidStateError" exception in Firefox 26 and IE11 but not in Chrome 31.

Why?

The Spec states that setting responseType

throws an "InvalidStateError" exception if the state is LOADING or DONE.

but in this case the state is UNSENT.

What's going on?

回答1:

It's a working draft, so it's normal if there are small differences or bugs in the implementations. I think Firefox still follows the behavior described in the draft of August 2011, where is specified that the exception is thrown if the state is not OPENED or HEADERS_RECEIVED.

回答2:

Note that setting responseType after I call open on the xhr object solved my problem (using IE11)

来源：https://stackoverflow.com/questions/20760635/why-does-setting-xmlhttprequest-responsetype-before-calling-open-throw