javascript Ajax 类实现代码

                            http = new ActiveXObject( "Msxml2.XMLHTTP." + i + ".0" );
                        }
                        break;
                    }catch(e){
                        //alert(i);
                        http = false;
                    }
                }
            }else if(window.XMLHttpRequest){
                http = new XMLHttpRequest();
                if(http.overrideMimeType){
                    http.overrideMimeType("text/xml");
                }
            }
        }catch(e){
            http = false;
        }

        if(!http){
            Alert("不能创建XMLHttpRequest对象实例");
            return http;
        }

        this.HttpRequest = http;
        return http;
    }

    //检测 this.HttpRequest
    this.checkHttpRequest = function(){
        if(!this.HttpRequest){
            return this.InitHttpRequest();
        }
        return this.HttpRequest;