an async AJAX call without jQuery

doAsyncAjax.js

/*!
 * an async AJAX call without jQuery
 * stackoverflow.com/questions/8567114/how-to-make-an-ajax-call-without-jquery
 * gist.github.com/englishextra/79df33f0b9eb8828c12b2a760f74211c
 */
function doAsyncAjax(url, callback) {
    var xmlhttp = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");

    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            callback(xmlhttp.responseText);
        }
    }

    xmlhttp.open("GET", url, true);
    xmlhttp.send();
}