Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<select id="mySelect">
  <option>Apple</option>
  <option>Orange</option>
  <option>Pineapple</option>
  <option>Banana</option>
</select>

<p>Click the button to deselect options.</p>

<button onclick="myFunction()">Try it</button>

<script>
function myFunction() {
    document.getElementById("mySelect").selectedIndex = "-1";
}
</script>

</body>
</html>