js获取select标签的 value 和 text

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<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>