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                        /JavaScript进阶篇

请帮我看看我的定时器哪里出错了，感激不尽

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>定时器</title>
<script type="text/javascript">
  var attime,h,m,s;
  function clock(){
    var time=new Date();    
    h=time.getHours();
    m=time.getMinutes();
    s=seconds.getSeconds();
    var week=["星期日","星期一","星期二","星期三","星期四","星期五","星期六"];
    attime= h+":"+m+":"+s+":"+week[time.getDay()] ;
    document.getElementById("clock").value = attime;
  }
  
   var int=setInterval(clock,100);
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50"  />
</form>
</body>
</html>

还有就是我看了下这位同学的答案，为什么他的setInterval没有写间隔时间也可以运行，不是说必须要写的吗？

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>定时器</title>
<script type="text/javascript">
  var attime,hh,mm,ss,ww,xx;
  function clock(){
    var time=new Date(); 
    hh=time.getHours();
    mm=time.getMinutes();
    ss=time.getSeconds();
    if(mm<10){hh="0"+mm};
    if(hh<10){hh="0"+hh};
    if(ss<10){ss="0"+ss};
    var week=["星期日","星期一","星期二","星期三","星期四","星期五","星期六"]
    ww=time.getDay()
    xx=week[ww]
    attime= hh+":"+mm+":"+ss+xx;
    document.getElementById("clock").value = attime;
  }
  var int=setInterval(clock)
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50"  />
</form>
</body>
</html>