Ajax 是异步的JavaScript和XML的简称，是一种更新页面某部分的机制。它赋予了你从服务器获取数据后，更新页面某部分的权力，从而避免了刷新整个页面。另外，以此方式实现页面局部更新，不仅能有效地打造流畅的用户体验，而且减轻了服务器的负载。
下面是对一个基本的 Ajax 请求进行剖析：
var xhr = new XMLHttpRequest();
xhr.open('GET', 'send-ajax-data.php');
xhr.send(null);
在这里， 我们创建了一个能向服务器发出 HTTP 请求的类的实例。然后调用其 open 方法，其中第一个参数是 HTTP 请求方法，第二个参数是请求页面的 URL。最后，我们调用参数为 null 的 send 方法。假如使用 POST 请求方法（这里我们使用了 GET），那么 send 方法 的参数应该包含任何你想发送的数据。
下面是我们如何处理服务器的响应：
xhr.onreadystatechange = function(){
var DONE = 4; // readyState 4 代表已向服务器发送请求
var OK = 200; // status 200 代表服务器返回成功
if(xhr.readyState === DONE){
if(xhr.status === OK){
console.log(xhr.responseText); // 这是返回的文本
} else{
console.log("Error: "+ xhr.status); // 在此次请求中发生的错误
}
}
}
onreadystatechange 是异步的，那么这就意味着它将可在任何时候被调用。这种类型的函数被称为回调函数——一旦某些处理完成后，它就会被调用。在此案例中，这个处理发生在服务器。
示例
便捷的 Ajax 方法也是不少人依赖 jQuery 的原因，但实际上原生 JavaScript 的 Ajax api 也很强大，并且基本的使用在各个浏览器中的差异不大，因此完全可以自行封装一个 Ajax 对象，下面是封装好的 Ajax 对象：
// Ajax 方法
// 惰性载入创建 xhr 对象
function createXHR(){
if ( 'XMLHttpRequest' in window ){
createXHR = function(){
return new XMLHttpRequest();
};
} else if( 'ActiveXObject' in window ){
createXHR = function(){
return new ActiveXObject("Msxml2.XMLHTTP");
};
} else {
createXHR = function(){
throw new Error("Ajax is not supported by this browser");
};
}
return createXHR();
}
// Ajax 执行
function request(ajaxData){
var xhr = createXHR();
ajaxData.before && ajaxData.before();
// 通过事件来处理异步请求
xhr.onreadystatechange = function(){
if( xhr.readyState == 4 ){
if( xhr.status == 200 ){
if( ajaxData.dataType == 'json' ){
// 获取服务器返回的 json 对象
jsonStr = xhr.responseText;
json1 = eval('(' + jsonStr + ')'),
json2 = (new Function('return ' + jsonStr))();
ajaxData.callback(json2);
// ajaxData.callback(JSON.parse(xhr.responseText)); // 原生方法，IE6/7 不支持
} else
ajaxData.callback(xhr.responseText);
} else {
ajaxData.error && ajaxData.error(xhr.responseText);
}
}
};
// 设置请求参数
xhr.open(ajaxData.type, ajaxData.url);
if( ajaxData.noCache == true ) xhr.setRequestHeader('If-Modified-Since', '0');
if( ajaxData.data ){
xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
xhr.send( ajaxData.data );
} else {
? ?
xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
xhr.send( null );
}
return xhr;
}
function post(ajaxData){
ajaxData.type = 'POST';
var _result = request(ajaxData);
return _result;
}
function get(ajaxData){
ajaxData.type = 'GET';
ajaxData.data = null;
var _result = request(ajaxData);
return _result;
}
下面给出一个使用例子：
index.html
<!DOCTYPE HTML>
<html lang="zh-CN">
<head>
<meta charset="UTF-8">
<title>原生 JavaScript 实现 Ajax</title>
<link rel="stylesheet" type="text/css" media="all" href="./common/common.css" />
<style>
#content {text-align: center; font-family: 'lucida Grande', 'Microsoft Yahei'}
#content .btn_ctr {display: block; width: 120px; height: 30px; margin: 0 auto 20px; background: #53a7bb; color: #fff; font-weight: bold; font-size: 14px; line-height: 30px; text-decoration: none;
border-radius: 4px;
}
#test {width: 280px; height: 130px; margin: 0 auto; padding: 15px; background: #fff; border-radius: 4px; text-align: left; }
</style>
</head>
<body>
<div id="header">
<div id="header-content">
<div id="header-inside">
<p class="go-to-article"><a href="http://kayosite.com/css3-animation.html" title="打开原文" target="_blank" >Back To Article</a></p>
<p class="go-to-blog"><a href="http://kayosite.com" title="进入我的博客 Kayo's Melody" target="_blank" >My Blog</a></p>
<p class="copyright">Demo By Kayo © Copyright 2011-2013</p>
</div>
<h1 id="CSS-Animation">CSS3 Animation</h1>
</div>
</div>
<div id="content">
<a class="btn_ctr" href="javascript:;" onclick="get({url: './ajax.html', callback: function(out){document.getElementById('test').innerHTML = out;}})">Ajax 获取内容</a>
<div id="test"></div>
</div>
<script>
// Ajax 方法，这里不在重复列出
</script>
</body>
</html>
<!DOCTYPE HTML> <html lang="zh-CN"> <head> <meta charset="UTF-8"> <title>ajax</title> </head> <body> <p>成功获取到这段文本</p> </body> </html>
具体的效果可以浏览完整 Demo 。
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