ajax,response查看不到，真能看到console.log里面能打印出来

request.onreadystatechange = cb(request)
直接把结果赋值给了onreadystatechange,等于没有了毁掉函数，当然什么都看不到了
改进的方法是
request.onreadystatechange = function(){cb(request)};

var Ajax

(function(){

var request = false

function createRequest() {

try {
request = new XMLHttpRequest();
} catch (trymicrosoft) {
//两种微软的XMLHttpRequest
try {
request = new ActiveXObject("Msxml2.XMLHTTP");
} catch (othermicrosoft) {
try {
request = new ActiveXObject("Microsoft.XMLHTTP");
} catch (failed) {
request = false;
}
}
}
if (!request)
alert("Error initializing XMLHttpRequest!");
}

createRequest();

Ajax.Get = function (url,cb){
request.open("GET",url);
//XHR对象的readyState有很多值，表示不同的状态，每次状态改变都会触发onreadystatechange这个函数
//所以不能确保是发送完了以后的request
request.onreadystatechange = function(){
if(request.readyState==4){
cb(request)
}
};
request.send(null);
}
//一json的格式发送
Ajax.Post = function(){
//var len = arguments.length;
url  = arguments[0]
cb   = arguments[1]
form = arguments[2]
request.open("POST",url);
request.onreadystatechange = function(){
if(request.readyState==4){
request.setRequestHeader("Content-Type","application/x-www-form-urlencoded;charset=UTF-8");

cb(request);
}
}
if form !=null{
var postStr = JSON.stringify(form);
request.send(postStr);
}else{
request.send();
}
}

})();