Ajax 实现？

1 var xhr = new XMLHttpRequest();
2 xhr.onreadystatechange = function(){
3 if(xhr.readyState == 4){
4 if((xhr.status >= 200 && xhr.status <= 300) || xhr.status == 304){
5 alert(xhr.responseText);
6 }else{
7 alert("Request was unsuccessful:" + xhr.status);
8 }
9 }
10 //get
11 xhr.open("get", "example.php", true);
12 xhr.setRequestHeader("MyHeader", "MyValue");
13 xhr.send(null);
14 //post
15 // xhr.open("post", "postexample.php", true);
16 // xhr.setRequestHeader("Content-Type", "applicatoin/x-www-form-urlencoded");
17 // var form = document.getElementById("user-info");
18 // xhr.send(serialize(form));
19 };