onclick时，怎么知道是左键还是右键？

在这个事件是无论是左键点还是右键点都会激发这个事件，现想把它们区分出来，不知道该怎么弄keycode好像都是0
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event.button可以判断
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event.button

2 右键
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 1<script language="JavaScript1.2">   
 2if (window.Event)   
 3document.captureEvents(Event.MOUSEUP)   
 4function myclick(e){   
 5if (window.Event)   
 6{   
 7alert(e.which);   
 8}   
 9else   
10alert(event.button)   
11}   
12document.onmousedown = myclick;   
13</script>