onreadystatechange is a event listener in AJAX,  this will be automatically invoked
for all the actions such as submiting data, finishing call etc., of any Ajax call,
and these would change the readyState property of the XMLHttpRequest object,
that will inform you if the given Ajax request is successed or failed.

So by using readyState value we can see check if a given Ajax call is successful or not,
if readyState Value is 4, we could consider that is successful. You could also check
"xmlhttp.status==200"

xmlhttp.onreadystatechange = function() {
        if(xmlhttp.readyState == 4 && xmlhttp.status==200){
            alert(xmlhttp.readyState);
        }
};
xmlhttp.open("GET","somepage.xml",true);
xmlhttp.send(null);

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