js禁用回退鍵

       function forbidBackSpace(e) {
            var ev = e || window.event; //獲取event對象   
            var obj = ev.target || ev.srcElement; //獲取事件源   
            var t = obj.type || obj.getAttribute('type'); //獲取事件源類型   
            //獲取作為判斷條件的事件類型   
            var vReadOnly = obj.readOnly;
            var vDisabled = obj.disabled;
            //處理undefined值情況   
            vReadOnly = (vReadOnly == undefined) ? false : vReadOnly;
            vDisabled = (vDisabled == undefined) ? true : vDisabled;
            //當敲Backspace鍵時，事件源類型為密碼或單行、多行文本的，   
            //並且readOnly屬性為true或disabled屬性為true的，則退格鍵失效   
            var flag1 = ev.keyCode == 8 && (t == "password" || t == "text" || t == "textarea") && (vReadOnly == true || vDisabled == true);
            //當敲Backspace鍵時，事件源類型非密碼或單行、多行文本的，則退格鍵失效   
            var flag2 = ev.keyCode == 8 && t != "password" && t != "text" && t != "textarea";
            //判斷   
            if (flag2 || flag1) return false;
        }
        //禁止后退鍵 作用於Firefox、Opera  
        document.onkeypress = forbidBackSpace;
        //禁止后退鍵  作用於IE、Chrome  
        document.onkeydown = forbidBackSpace;