JavaScript Ajax Request in Salesforce Lightning

makeAjaxRequest.js

callAjax : function(method, url, async, callback) {
    var request = new XMLHttpRequest();
    request.onreadystatechange = function(component) {
        if (request.readyState == 4 ) {
            callback.call(this, request);
        }
    };
    request.open(method, url, async);
    request.send();
},
makeAjaxRequest : function(component, url) {
    this.callAjax("GET", url, true,
        function(response)
        {
            if (response.status == 200) {
                var data = response.responseText;
            }
            else if (response.status == 400) {
                console.log("AjaxRequest: 400 Error");
            }else {
                console.log("AjaxRequest: Error");
            }
        }
    );
}