How to 'observe' when a select option is programmatically set

I'm sure this is easy, but I can't find it.

I want to know when a select is changed (new option selected) through code. Not options added or removed, but just what would be considered a simple onchange of the select. But the 'change' is through javascript.

Here is a snippet:

// find elements
var banner = $("#banner-message");
var button = $("button");
var sel = $('#myselect');

let observer = new MutationObserver(function(mutations) {
  alert('in observer');
});

observer.observe(sel[0], { attributes: true, subtree: true });

let counter = 2;
button.on("click", () => {
  if (counter == 1) {
    sel.val('one');
    counter = 2;
  } else {
    sel.val('two');
    counter = 1;
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>
<div id="banner-message">
  <select id='myselect'>
    <option>one</option>
    <option>two</option>
  </select>
  <button>test</button>
</div>

I cannot get the observer code to fire.

Solution

You can monkeypatch HTMLOptionElement#selected's setter:

// find elements
var banner = $("#banner-message");
var button = $("button");
var sel = $('#myselect').on('change', e => {
  console.log(sel.val());
});

const {get, set} = Object.getOwnPropertyDescriptor(HTMLOptionElement.prototype, 'selected');
Object.defineProperty(HTMLOptionElement.prototype, 'selected', {
  get,
  set(val){
    console.log('select value:', this.value, ' of ', this.parentElement.id, ':', val);
    set.call(this, val);
    $(this.parentElement).change();
  }
});

let counter = 2;
button.on("click", () => {
  if (counter == 1) {
    sel.val('one');
    counter = 2;
  } else {
    sel.val('two');
    counter = 1;
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>
<div id="banner-message">
  <select id='myselect'>
    <option>one</option>
    <option>two</option>
  </select>
  <button>test</button>
</div>