离开网站时不执行功能

Question

我想要实现的是，如果用户点击关闭按钮，会触发一个函数Exit，在用户离开网站之前会显示一个表单。但我的表单没有显示

的Javascript

<script type="text/javascript">

  window.onbeforeunload = Exit;

      function Exit()
      {
          setTimeout(function() {
            setTimeout(function() {
             document.getElementById('form').style.display="block";
               window.location.href="register.html";
            }, 1000);
        },1);
        return "Message";
      }
    </script>

Html代码

<div class="container">

  <form role="form" id="form" method="post" action="register.php" style="display:none">
    <div class="form-group">
      <label for="username">Name:</label>
      <input type="text" name="username" class="form-control" id="username" placeholder="Enter name">
    </div>
    <div class="form-group">
      <label for="email">Email:</label>
      <input type="email" name="email" class="form-control" id="email" placeholder="Enter email">
    </div>
    <div class="form-group">
      <label for="phone">Phone Number:</label>
      <input type="number" name="phone" class="form-control" id="phone" placeholder="Enter phonenumber">
    </div>
  <div class="form-group">
      <label for="reason">Reason to leave:</label>
      <textarea class="form-control" name="reason" rows="5" id="comment"></textarea>
    </div>
    <button type="submit" name="submit" class="btn btn-default">Submit</button>
  </form>
</div>