一般我们写网页的时候，如果用到 Ajax 请求服务器，都是使用 JQuery 等已经封装好的库来调用，比较简单。

但是一般这些库的功能很多，引入了太多我们用不到的东西，如果我们需要写一个功能单一，简单的页面，完全用不到引用如此庞大的库文件。

我们可以简单实现一个自己的 Ajax 请求功能，具体的代码如下：

var ajax = {};
ajax.x = function () {
 if (typeof XMLHttpRequest !== 'undefined') {
 return new XMLHttpRequest();
 }
 var versions = [
 "MSXML2.XmlHttp.6.0",
 "MSXML2.XmlHttp.5.0",
 "MSXML2.XmlHttp.4.0",
 "MSXML2.XmlHttp.3.0",
 "MSXML2.XmlHttp.2.0",
 "Microsoft.XmlHttp"
 ];
 var xhr;
 for (var i = 0; i < versions.length; i++) {
 try {
 xhr = new ActiveXObject(versions[i]);
 break;
 } catch (e) {
 }
 }
 return xhr;
};
ajax.send = function (url, method, data, success,fail,async) {
 if (async === undefined) {
 async = true;
 }
 var x = ajax.x();
 x.open(method, url, async);
 x.onreadystatechange = function () {
 if (x.readyState == 4) {
 var status = x.status;
 if (status >= 200 && status < 300) {
 success && success(x.responseText,x.responseXML)
 } else {
 fail && fail(status);
 }
 }
 };
 if (method == 'POST') {
 x.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
 }
 x.send(data)
};
ajax.get = function (url, data, callback, fail, async) {
 var query = [];
 for (var key in data) {
 query.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
 }
 ajax.send(url + (query.length ? '?' + query.join('&') : ''), 'GET', null, callback, fail, async)
};
ajax.post = function (url, data, callback, fail, async) {
 var query = [];
 for (var key in data) {
 query.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
 }
 ajax.send(url,'POST', query.join('&'), callback, fail, async)
};

使用方法： GET

ajax.get('/test.php', {foo: 'bar'}, function(response,xml) {
 //success
},
function(status){
 //fail
});
POST
ajax.post('/test.php', {foo: 'bar'}, function(response,xml) {
 //succcess
},function(status){
 //fail
});

这里需要注意一个问题，如果我们想要发送类似

var sendCmd = "?op_code=" + code + "&op_cmd=" + cmd;
ajax.post('/control' + sendCmd,'',function(response,xml) {
 console.log('success');
},
function(status){
 console.log('failed:' + status);
});