XMLHttpRequest ajax功能

function createxhr()
    {
        var xhr=null;
        if(window.XMLHttpRequest)
        {
            var xhr = new XMLHttpRequest();
        }
        else if(window.ActiveXObject)
        {
            xhr = new ActiveXObject("Microsoft.XMLHttp");
        }
        return xhr;
    }

function state_change(url){
        var xhr = createxhrjson();
        console.log(xhr);
        xhr.open("GET",url,true);
        xhr.onreadystatechange = function(){
            if(xhr.readyState==4 &&  xhr.status == 200){
                document.getElementById("res").innerHTML = xhr.responseText;
            }
        }
    }

xhr.open(‘get‘, url + ‘?time=‘ + time + ‘&username=’ + username, true);
xhr.send(null);

var queryVal = ‘time=‘ + time + ‘&username = ‘ + username:
xhr.open(‘post‘, url, true);
xhr.setRequstHeader(‘Content-Type‘, ‘application/x-www-form-urlencoded‘);
xhr.send(queryVal);