js--屏蔽Backspace，输入内容不屏蔽

<script type="text/javascript">  
    /*屏蔽Backspace，输入内容不屏蔽*/
    function document.onkeydown()
    {
        if(document.activeElement.type == "text")
        {
            if ((event.keyCode==116)||            //屏蔽 F5 刷新键
           (event.ctrlKey && event.keyCode==82))//Ctrl + R
           {
               event.keyCode=0;
               event.returnValue=false;
           }
        }
       else
       {
           if ((event.keyCode==8)   ||             //屏蔽退格删除键
           (event.keyCode==116)||                  //屏蔽 F5 刷新键
           (event.ctrlKey && event.keyCode==82)){ //Ctrl + R
          event.keyCode=0;
          event.returnValue=false;
          }
        }
    }
</script>