index.html

<select id="resolution">
  <option value="1080p60" label="1080p 60FPS">
  <option value="720p" label ="720p 30FPS">
  <option value="mp3" label="Audio only (mp3)">
</select>
<script src="index.js"></script>

index.js

const valueMap = {
  "1080p60": {
    res: "1080p60",
    type: "video"
  },
  "720p": {
    res: "720p30",
    type: "video"
  },
  "mp3": {
    res: "",
    type: "audio"
  }
};

document.getElementById("resolution").addEventListener("change", function(e) {
  const details = valueMap[this.value];
  /* If "1080p60" is selected, details will be
   * {
   *   "res": "1080p60",
   *   "type": "video"
   * }
   */
});