Vanilla JS HTTP GET Request

HTTPGet.js

function httpGet(theUrl, callback, callbackContext, errorCallback) {
    var req = new XMLHttpRequest();
    req.open("GET", theUrl, true);
    req.onreadystatechange = (function(){
        if (req.readyState == 4 && (req.status >= 200 && req.status < 300)){
            callback.call(callbackContext);
        } else if (req.readyState == 4) {
            errorCallback();
        }
    });
    req.send();
};