js 获取类变量所在的对象

function RequestInterface(stack, date, type) {
            //attributes.
            XMLHttpRequest.prototype.placeObj = this;
            this.xmlHttpReq;
            this.responseData;

            //methods.
          this.doReadyStateChange = doReadyStateChange;
        }

// address must relative address
        function requestData(address, requestCmd) {
            //alert('requestCmd: ' + requestCmd);
            this.xmlHttpReq = new XMLHttpRequest();
            this.xmlHttpReq.open("POST", address, true);
            this.xmlHttpReq.send(requestCmd);
            this.xmlHttpReq.onreadystatechange = 

                         this.doReadyStateChange;
        }