JSFiddle - React, Tailwind, and code Playground

HTML

<select multiple id="myselect">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>
  <option value="audi">Audi</option>
</select>

JavaScript

$('#myselect').on('change', function() {
  var $sel = $(this),
    val = $(this).val(),
    $opts = $sel.children(),
    prevUnselected = $sel.data('unselected');
  // create array of currently unselected 
  var currUnselected = $opts.not(':selected').map(function() {
    return this.value
  }).get();
  // see if previous data stored
  if (prevUnselected) {
    var unselected = currUnselected.reduce(function(a, curr) {
      if ($.inArray(curr, prevUnselected) == -1) {
        a.push(curr)
      }
      return a
    }, []);
    // "unselected" is an array if it has length some were removed
    if (unselected.length) {
      alert('Unselected is ' + unselected.join(', '));
    }
  }
  $sel.data('unselected', currUnselected)
}).change();