select的onchange方法获得value值

<select class="aa" onchange="select-plat()">
          <option value="">板式选择</option>
          <option value="1">列表展示</option>
          <option value="2">两列</option>
          <option value="3">大图显示</option>
</select>

<script>
    function select-plat(){
        var val = $(".aa").val();
        
    }
</script>