转载自：，为了防备面试，备份一份吧！！

虽然现在有了框架，许多AJAX调用直接调用它们的API就可用。但有些极端情况，比如面试，比如第三方应用不想加载这些库，我们就只有自己写。这时想必有许多人要疯狂google，百度了。网上太多垃圾信息，我还是在自己博客上保存一份吧。

我的实现：

var ajax = new(self.XMLHttpRequest||ActiveXObject)("Microsoft.XMLHTTP")ajax.onreadystatechange = function(){  if (ajax.readyState==4 && ajax.status==200){      alert(ajax.responseText)  }}ajax.open("POST", url, true);ajax.send("key=val&key1=val2");

群里黑暗骑士的实现：

Ajax = function(){    function request(url,opt){        function fn(){}        var async   = opt.async !== false,            method  = opt.method    || 'GET',            data    = opt.data      || null,            success = opt.success   || fn,            failure = opt.failure   || fn;            method  = method.toUpperCase();        if(method == 'GET' && data){            url += (url.indexOf('?') == -1 ? '?' : '&') + data;            data = null;        }        var xhr = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');        xhr.onreadystatechange = function(){            _onStateChange(xhr,success,failure);        };        xhr.open(method,url,async);        if(method == 'POST'){            xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded;');        }        xhr.send(data);        return xhr;     }    function _onStateChange(xhr,success,failure){        if(xhr.readyState == 4){            var s = xhr.status;            if(s>= 200 && s < 300){                success(xhr);            }else{                failure(xhr);            }        }else{}    }    return {request:request};   }();