keydown兼容性

document.onkeydown = keyDown;//IE處理按鍵事件
//Firefox，若document的屬性layers不存在，則window.evnet
if(document.layers){
  document.captureEvents(Event.KEYDOWN);
}
//keyDown()函數原型,IE:冒泡事件，event對象一個keyCode;Firefox:charCode屬性
function keyDown(e){
   var ev = (e)?e:(window.event)?window.event:null;
   if(ev){
     return (ev.charCode)?ev.charCode:
	 ((ev.keyCode)?ev.keyCode:((ev.which)?ev.whick:null));
   }
   return -1；
}