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                        /JavaScript進階篇

關于變量賦值的問題

<html>
<head>
<meta?http-equiv="Content-Type"?content="text/html;?charset=utf-8">
<title>計時器</title>
<script?type="text/javascript">
???var?i=setInterval(clock,100);???//位置1
???function?clock(){
??????var?time=new?Date();?????????????????????
??????document.getElementById("clock").value?=?time.getHours()+":"+time.getMinutes()+":"+time.getSeconds();
???}
???
???function?start(){
?????i=setInterval("clock()",100);??//位置2
???}
???function?stop(){
???????clearInterval(i);
???}
</script>
</head>
<body>
??<form>
????<input?type="text"?id="clock"?size="50"??/>
????<input?type="button"?value="Start"??onclick="start()"/>
????<input?type="button"?value="Stop"??onclick="stop()"/>
??</form>
</body>
</html>

代碼中位置1 i變量已經賦值為什么位置2的function中仍然要賦值?