1. 创建新的 XMLHttpRequest 对象(Ajax 应用程序的核心):
2. 用 JavaScript 代码捕获和设置字段值
   //获取 id 值为 first_name 的表单域var name = document.getElementById('first_name').value;//修改 id 值为 test 的表单域的值document.getElementById('test').value = response[0];

   　　

3. 在 Microsoft 浏览器上创建 XMLHttpRequest 对象
   var xmlHttp = false;try {  xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");} catch (e) {  try {    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");  } catch (e2) {    xmlHttp = false;  }}

   　　

4. 以支持多种浏览器的方式创建 XMLHttpRequest 对象
   /* Create a new XMLHttpRequest object to talk to the Web server */var xmlHttp = false;/*@cc_on @*//*@if (@_jscript_version >= 5)try {  xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");} catch (e) {  try {    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");  } catch (e2) {    xmlHttp = false;  }}@end @*/if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {  xmlHttp = new XMLHttpRequest();}

   　　

5. 开始工作：发送Ajax请求
   function callServer() {  // Get the city and state from the web form  var city   = document.getElementById("city").value;  var state = document.getElementById("state").value;  // Build the URL to connect to  var url = "/scripts/getZipCode.php?city=" + escape(city) + "&state=" + escape(state);  // Open a connection to the server  xmlHttp.open("GET", url, true);  // Setup a function for the server to run when it's done  xmlHttp.onreadystatechange = updatePage;  // Send the request  xmlHttp.send(null);}