var count = 0;
var XHR = {}
XHR.create = function(){
    if (window.XMLHttpRequest) {
        count++;
        XHR.create = function(){
            return new XMLHttpRequest();
        }
    }
    else {
        try {
            count++;
            //为了判断Msxml2和Microsoft版本的ActiveXObject，不得不实例化一个
            XHR.create = new ActiveXObject("Msxml2.XMLHTTP") &&
            function(){
                return new ActiveXObject("Msxml2.XMLHTTP");
            }
        } catch (e) {
            count++;
            XHR.create = function(){
                return new ActiveXObject("Microsoft.XMLHTTP");
            }
        }
    }
    XHR.create();
};

var xhr1 = XHR.create();
var xhr2 = XHR.create();
var xhr3 = XHR.create();
var xhr4 = XHR.create();
var xhr5 = XHR.create();
var xhr6 = XHR.create();
var xhr7 = XHR.create();

alert(count);
alert(xhr3);