Untitled

<!DOCTYPE html>
<html>
<head>
 <title></title>
</head>
<body>
 <form>
  usuario <input type="text" id="user" name=""> <br>
  password <input type="password" id="pass"> <br>
  <input type="submit" name="" id="boton" onclick="mostrar()">
 </form>
 <script type="text/javascript">
  var user = document.getElementById('user')
  var pass = document.getElementById('pass')
  var boton = document.getElementById('boton')
  function mostrar(){
   alert(user.value + "\n" + pass.value);
  }
 </script>
</body>
</html>