用键盘方向键实现翻页

<script type="text/javascript">
document.onkeydown = nextpage;
var prevpage="1.htm";
var nextpage="3.htm";
function nextpage(event)
{
    event = event ? event : (window.event ? window.event : null);
    if (event.keyCode == 37) location = nextpage;  //方向键→
    if (event.keyCode == 39) location = prevpage;  //方向键←
    //方向键↑为38，方向键↓为40
}
</script>