fireEvent, onsubmit, 动态设置某对象的事件方法

<script language="javascript">
 
 function bbb()
 {
  alert("ccc");
  return false;
 }
 function aaa()
 {
  //  form1.onsubmit=bbb;
  form1.Submit3.onfocus=bbb;
  form1.Submit3.fireEvent("onfocus");
  }
</script>
<form name="form1" method="post" onsubmit="return bbb()" >
              用户名：<input name="username" type="text" size="18">
              密码：<input name="password" type="password" size="18">
              <input type="submit" name="Submit" value="登录">
              <input type="button" name="Submit2" onclick="aaa();" value="注册">
              <input type="button" name="Submit3"  value="忘记密码">
</form>