js键盘上下左右键怎么触发function(实例讲解)

代码如下:
<script type="text/javascript">
 function jumpPage() {
    if (event.keyCode==37)//左
fun_a();
if (event.keyCode==38)//上
 fun_b();
 if (event.keyCode==39)//右
fun_c();
if (event.keyCode==40)//下
fun_d();
 }

function fun_a(){
window.alert('你按下了左')
 }

function fun_b(){
window.alert('你按下了上')
}

function fun_c(){
window.alert('你按下了右')
}

function fun_d(){
 window.alert('你按下了下')
}
document.onkeydown=jumpPage;

</script>

HTML code
代码如下:
<table width="100%" border="1" cellspacing="1" cellpadding="0" id=table> <tr id="lineone" style="background-color:#00cccc;"> <td id=aa height="23"> </td> <td id=aa >按向下键</td> <td id=cc >按向下键</td> <td id=dd >按向下键</td> <td id=ee >按向下键</td> </tr> <tr id="linetwo" tyle="background-color:#ffffff;"> <td id=a height="23"> </td> <td id=b >按向上键</td> <td id=c >按向上键</td> <td id=d >按向上键</td> <td id=e >按向上键</td> </tr></table><script language="javascript"> function keyDown(e) { var iekey=event.keyCode; action(iekey); } document.onkeydown = keyDown; function action(iekey) { if(iekey==40) { lineone.style.backgroundColor = ""; linetwo.style.backgroundColor = "#00cccc"; } if(iekey==38) { lineone.style.backgroundColor = "#00cccc"; linetwo.style.backgroundColor = ""; } } </script><input name="id" type="hidden" id="id" value