How to transfer "this" in onreadystatechange with an anonymous function

Question

When I use AJAX to post data to the server, I want to use this in the onreadystatechange function.
For example:

HTML

<div onclick=send.call(this)></div>

JavaScript

function send () {
    this.style.background = "#ccc";
    var xml = new XMLHttpRequest();
    xml.open("post", "/post.php", true);
    xml.send("data");
    xml.onreadystatechange = function () {
        if (xml.readyState == 4 && xml.status == 200) {
            this.style.background = "#fff";
        }
    }.call(this);
}

I think it works, but when I use call, the onreadystatechange function only run once. When I use the code such as:

xml.onreadystatechange = function () {
    console.log(xml.state);
}

the output is: 1 2 3 4, but when I use

xml.onreadystatechange = function () {
    console.log(xml.state);
}.call(this)

the output is: 1 Why?