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;  
}