1、The first ready state, signified by the readyState property 0 (readyState == 0), represents an uninitialized request.
3、我經常看到的一種情況就是,用户沒有在回調中檢查對象的readyState或status字段。
2、One that I often see is when users don't inspect the readyState or status fields of the object in the callback.