javascript下计数器每秒自动加1

<script type="text/javascript"> var a = 0; function tran(){ var time = document.getElementById('time'); time.innerHTML = a++; window.setTimeout('tran()',1000); } window.setTimeout('tran()',1000); </script>
[Ctrl+A 全选 注:如需引入外部Js需刷新才能执行]