js获取select标签的 value 和 text

<select name="" id="test">
    <option value="a1">yi</option>
    <option value="a2">er</option>
    <option value="a3">san</option>
</select>
<button onclick="aa()">huoqushuju</button>
<p id="res_value"></p>
<p id="res_text"></p>


<script>
    function aa(){
        var select = document.getElementById('test');
        var res_value = select.value;
        var res_text = select.options[select.selectedIndex].text;

        document.getElementById('res_value').innerHTML = res_value;
        document.getElementById('res_text').innerHTML = res_text;
    }
</script>