SOURCE

//1.创建连接
var xhr = null;
xhr = new XMLHttpRequest()
//2.连接服务器
xhr.open('get',url,true)
//3.发送请求
xhr.send(null);
//4.接受请求
xhr.onreadystatechange = function () {
    if(xhr.readyState == 4) {
        if(xhr.status == 200) {
            success(xhr.responseText);
        } else {
            fail && fail(xhr.status)
        }
    }
}