selectのonchangeメソッドvalue値の取得

<select class="aa" onchange="select-plat()">

          <option value=""> </option>

          <option value="1"> </option>

          <option value="2"> </option>

          <option value="3"> </option>

</select>

<script>

    function select-plat(){

        var val = $(".aa").val();

        

    }

</script>