xmlhttp.onreadystatechange = function()
{
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
    {
        document.getElementById("myDiv").innerHTML = xmlhttp.responseText;
    }
}

上面的代码来自：http
:
//www.w3schools.com/ajax/ajax_xmlhttprequest_onreadystatechange.asp。

题：

根据本教程：

readyState: 4: request finished and response is ready

status: 200: "OK"

When readyState is 4 and status is 200, the response is ready:

从什么时候开始xmlhttp.readyState == 4，响应就准备好了，我们为什么还需要xmlhttp.status == 200？xmlhttp.readyState == 4和之间有什么区别xmlhttp.status == 200？