解决点击backspace,返回上一页问题

代码如下： 
/** 
* 
* 禁止点击backspace 返回上一页 
*/ 
window.onload=function(){   
   document.getElementsByTagName("body")[0].onkeydown =function(){  
         
        //获取事件对象  
        var elem = event.relatedTarget || event.srcElement || event.target ||event.currentTarget;   
          
        if(event.keyCode==8){//判断按键为backSpace键  
          
                //获取按键按下时光标做指向的element  
                var elem = event.srcElement || event.currentTarget;   
                  
                //判断是否需要阻止按下键盘的事件默认传递  
                var name = elem.nodeName;  
                  
                if(name!='INPUT' && name!='TEXTAREA'){  
                    return _stopIt(event);  
                }  
                var type_e = elem.type.toUpperCase();  
                if(name=='INPUT' && (type_e!='TEXT' && type_e!='TEXTAREA' && type_e!='PASSWORD' && type_e!='FILE')){  
                        return _stopIt(event);  
                }  
                if(name=='INPUT' && (elem.readOnly==true || elem.disabled ==true)){  
                        return _stopIt(event);  
                }  
            }  
        }  
    }  
function _stopIt(e){  
        if(e.returnValue){  
            e.returnValue = false ;  
        }  
        if(e.preventDefault ){  
            e.preventDefault();  
        }                 
  
        return false;  
}