The readyState value of 4 means that the transaction is complete.
readyState的值4表示事务已经完成。
When the server-response function is invoked, the readyState is checked to make sure it's Loaded.
当调用服务器响应函数时，会相应检查 readyState以确保它是 Loaded。
One that I often see is when users don't inspect the readyState or status fields of the object in the callback.
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。
The first ready state, signified by the readyState property 0 (readyState == 0), represents an uninitialized request.
第一种就绪状态的特点是readyState属性为0（readyState == 0），表示未初始化状态。
Each time the readystate changes, the readystatechange event fires and the handler function attached via the onreadystatechange property is called.
每次readystate变化时，readystatechange事件就触发，由onreadystatechange属性指定的事件处理函数就被调用。
This callback function is invoked several times during the life cycle of the Ajax request whose current state can be obtained from the readyState property.
此回调函数在Ajax请求的整个生命周期进行了多次调用，而请求的当前状态可从readyState属性获得。
This callback function is invoked several times during the life cycle of the Ajax request whose current state can be obtained from the readyState property.
此回调函数在Ajax请求的整个生命周期进行了多次调用，而请求的当前状态可从readyState属性获得。
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