一个简单的AJAX示例(转)

转载自：http://www.cnblogs.com/rubylouvre/archive/2013/01/08/2851051.html，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};   
}();