<form name="loginForm" action="login.aspx" method="post">
 <button type="submit" value="Submit" id="submit">Submit</button>
</form>

<script>
 var submitBtn = document.getElementById("submit");

 submitBtn.onclick = function (event) {
  alert("preventDefault!");
  return false;
 };
</script>

<form name="loginForm" action="login.aspx" method="post">
 <button type="submit" value="Submit" id="submit">Submit</button>
</form>

<script>
 var submitBtn = document.getElementById("submit");

 submitBtn.onclick = function (event) {
  alert("preventDefault!");
  var event = event || window.event;
  event.preventDefault(); // 兼容标准浏览器
  window.event.returnValue = false; // 兼容IE6~8
 };
</script>