3、我经常看到的一种情况就是，用户没有在回调中检查对象的readyState或status字段。
1、The first ready state, signified by the readyState property 0 (readyState == 0), represents an uninitialized request.
2、One that I often see is when users don't inspect the readyState or status fields of the object in the callback.
