创建兼容的XHR对象

    function createXHR () {//创建XMLHttpRequest对象
        var xhr=null;
        if(window.XMLHttpRequest){
            createXHR=createStandardXHR; // Firefox, Opera 8.0+,Safari，IE7+
        }
        else{
            createXHR=createCompatibleXHR; // IE5、IE6
        }

        try{
            xhr=createXHR();
        }
        catch(e){
            createXHR = createErrorXHR;
            xhr=createXHR();
        }

        return xhr;
    }

    function createStandardXHR(){
        return new XMLHttpRequest();
    }

    function createCompatibleXHR(){
        return new ActiveXObject("Microsoft.XMLHTTP");
    }

    function createErrorXHR(){
        alert("Your browser does not support XMLHTTP.");
        return null;
    }