JS disable browser backspace key

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;