[javascript] How do I change an HTML selected option using JavaScript?

You could also change select.options.selectedIndex DOM attribute like this:

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function selectOption(index){ _x000D_
  document.getElementById("select_id").options.selectedIndex = index;_x000D_
}
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<p>_x000D_
<select id="select_id">_x000D_
  <option selected>first option</option>_x000D_
  <option>second option</option>_x000D_
  <option>third option</option>_x000D_
</select>_x000D_
</p>_x000D_
<p>_x000D_
  <button onclick="selectOption(0);">Select first option</button>_x000D_
  <button onclick="selectOption(1);">Select second option</button>_x000D_
  <button onclick="selectOption(2);">Select third option</button>_x000D_
</p>
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