HTMLSelectElement.selectedIndex

<p id="p">selectedIndex: 0</p>

<select id="select">
  <option selected>Option A</option>
  <option>Option B</option>
  <option>Option C</option>
  <option>Option D</option>
  <option>Option E</option>
</select>

var selectElem = document.getElementById("select");
var pElem = document.getElementById("p");

// 当有新的<option>元素被选中时
selectElem.addEventListener("change", function () {
  var index = selectElem.selectedIndex;
  // 把 index 数据添加到 p 元素中
  pElem.innerHTML = "selectedIndex: " + index;
});