Ajax的关键在于XMLHttpRequest对象，如下基本用法：
<!DOCTYPE html><html> ???<head> ???????<meta charset="utf-8" /> ???????<title></title> ???</head> ???<body> ???????<div> ???????????<button>Apples</button> ???????????<button>Cherries</button> ???????????<button>Bananas</button> ???????</div> ???????<div id="target"> ???????????press a button ???????</div> ???????<script type="text/javascript"> ???????????var buttons=document.getElementsByTagName("button") ???????????for(var i=0;i<buttons.length;i++){ ???????????????buttons[i].onclick=handleButtonPress; ???????????} ???????????function handleButtonPress(e){ ???????????????var httpRequest=new XMLHttpRequest(); ???????????????httpRequest.onreadystatechange=handleResponse; ???????????????httpRequest.open("GET",e.target.innehttpRequest=new XMLHttpRequest();rHTML+".html") ???????????????httpRequest.send() ???????????} ???????????function handleResponse(e){ ???????????????if(e.target.readyState==XMLHttpRequest.DONE&&e.target.status==200){ ???????????????????document.getElementById("target").innerHTML=e.target.responseText; ???????????????} ???????????} ???????</script> ???</body></html> 第一步是创建一个新的XMLHttpRequest对象。与之前在DOM中见过的大多数对象不同，你并非通过浏览器定义的某个全局变量来访问这类对象，而是使用关键词new如：var httpRequest=new XMLHttpRequest();
下一步是给readystatechange事件设置一个事件处理器。这个事件会在请求过程中被多次触发，向你提供事情的进展情况。将onreadystatechange属性的值设为handleResponse;httpRequest.onreadystatechange=handleResponse;可以告诉XMLHttpRequest对象你想要做什么，使用open方法来指定HTTP方法（在这里是GET）和需要请求的URL:httpRequest.open("GET",e.target.innerHTML+".html")注：