关于AJAX 的交互模型、交互流程及代码示范

 1 (function () {
 2             var btn = document.getElementById(‘test‘);
 3             btn.onclick = function () {
 4                 ajax(‘0821-a.txt‘, function (str) {
 5                     alert(str);
 6                 });
 7             };
 8             function ajax(url, success, fail) {
 9                 var xhr = null;
10                 if(window.XMLHttpRequest) {
11                     xhr = new XMLHttpRequest();
12                 }else {
13                     xhr = new ActiveXObject(‘Microsoft.XMLHTTP‘);
14                 }
15                 xhr.open(‘get‘, url, true);
16                 xhr.send(null);
17                 xhr.onreadystatechange = function () {
18                     if(xhr.readyState == 4) {
19                         if(xhr.status == 200) {
20                             success(xhr.responseText);
21                         }else {
22                             fail && fail(xhr.status);
23                         }
24                     }
25                 };
26             }
27         })();