JavaScript与jQuery实现注册协议倒计时功能的三种方案

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>www.jb51.net 注册倒计时</title>
  <script type="text/javascript">
    //用户有十秒钟的时间看协议，超过十秒钟，“同意”按钮将生效
    var Seconds = 10;
    //计时器ID
    var setIntervalID;
    function ok() {
      var getid = document.getElementById("but");
      if (Seconds <= 0) {
        getid.value="同意";
        getid.disabled = false; //或者写成getid.disabled=""; 启用getid控件。
        //停止计时器
        clearInterval(setIntervalID);
      }
      else {
        getid.value = "请仔细阅读协议还剩下【" + Seconds + "】秒";
      }
      Seconds--;
    }
   setIntervalID=setInterval("ok()", 1000);
  </script>
</head>
<body>
<textarea cols="20" rows="8"></textarea><br />
<!--disabled="disabled" 默认submit表单是禁用的，也就是只读的，不能点击-->
<input type="submit" id ="but" value="同意" disabled="disabled" />
</body>
</html>

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  <title>www.jb51.net 注册倒计时</title>
  <script src="jquery-1.7.2.min.js"></script>
</head>
<body>
  <form action="https://www.jb51.net/" method="post" name="agree">
    <input type="submit" class="button" value="" name="agreeb">
  </form>
</body>
</html>
<script>
  var secs = 5;
  var si;
  $(function () {
    $(".button").attr("disabled", "disabled");
    $(".button").val("请认真查看<服务条款和声明>(" + secs + ")")
    si = setInterval(a, 1000);
  })
  function a() {
    $(".button").val("请认真查看<服务条款和声明>(" + (secs-1) + ")");
    if (secs == 0) {
      clearInterval(si);
      $(".button").val("我同意" ).removeAttr("disabled");
    }
    secs--;
  }
</script>

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
  <title>www.jb51.net 注册倒计时</title>
</head>
<body>
  <form action="https://www.jb51.net/" method="post" name="agree">
    <input type="submit" class="button" value="请认真查看<服务条款和声明> (30)" name="agreeb">
  </form>
</body>
</html>
<script>
  var secs = 30;
  document.agree.agreeb.disabled = true;
  for (var i = 1; i <= secs; i++) {
    window.setTimeout("update(" + i + ")", i * 1000);
  }
  function update(num) {
    if (num == secs) {
      document.agree.agreeb.value = " 我 同 意 ";
      document.agree.agreeb.disabled = false;
    }
    else {
      var printnr = secs - num;
      document.agree.agreeb.value = "请认真查看<服务条款和声明> (" + printnr + ")";
    }
  }
</script>