js禁用回退键

       function forbidBackSpace(e) {
            var ev = e || window.event; //获取event对象   
            var obj = ev.target || ev.srcElement; //获取事件源   
            var t = obj.type || obj.getAttribute('type'); //获取事件源类型   
            //获取作为判断条件的事件类型   
            var vReadOnly = obj.readOnly;
            var vDisabled = obj.disabled;
            //处理undefined值情况   
            vReadOnly = (vReadOnly == undefined) ? false : vReadOnly;
            vDisabled = (vDisabled == undefined) ? true : vDisabled;
            //当敲Backspace键时，事件源类型为密码或单行、多行文本的，   
            //并且readOnly属性为true或disabled属性为true的，则退格键失效   
            var flag1 = ev.keyCode == 8 && (t == "password" || t == "text" || t == "textarea") && (vReadOnly == true || vDisabled == true);
            //当敲Backspace键时，事件源类型非密码或单行、多行文本的，则退格键失效   
            var flag2 = ev.keyCode == 8 && t != "password" && t != "text" && t != "textarea";
            //判断   
            if (flag2 || flag1) return false;
        }
        //禁止后退键 作用于Firefox、Opera  
        document.onkeypress = forbidBackSpace;
        //禁止后退键  作用于IE、Chrome  
        document.onkeydown = forbidBackSpace;