js小键盘箭头分页翻页回车键

<script>
document.onkeydown=nextpage;
//var npagege='1';
//上一页链接
//var prep=npagege-1;
//var nexp=npagege+1;
var prevpage='/1.html';
//下一页链接
var nextpage='/2.html';
//目录页链接
var catalog='/436.html';
function nextpage(event) 
{
   event = event ? event : (window.event ? window.event : null); 
   if (event.keyCode==39) 
   {
     //alert("Next Page!")
     location=nextpage
   }
  if (event.keyCode==37) 
  {
    //alert("Prevpage Page!");
    location=prevpage;
  }
  if (event.keyCode==13)
  {
      location=catalog
  }
}
</script>