Answers for "js .change"

.on change get value

$('select').on('change', function() {
  alert( this.value );
});

handling event changes

<button>Act-once button</button>
<script>
  let button = document.querySelector("button");
  function once() {
    console.log("Done.");
    button.removeEventListener("click", once);
  }
  button.addEventListener("click", once);
</script>