What is difference between using **this** keyword vs variable name in XMLHttpRequest callback?

```
var req = new XMLHttpRequest();
req.open("GET", encodeURI(uri), true);
req.onreadystatechange = function () {
    if (this.readyState == 4) {
        req.onreadystatechange = null; //avoid memory leaks
        if (this.status == 200) {
            var res = JSON.parse(req.responseText).d;
            console.log(res);
        }
    }
};
req.send();

```

Can I just use **req** instead of **this**, like `if (req.readyState == 4)`?