How to change iframe src when a button is pressed

Question

I tried to change src of an iframe, to change trailers, but after trying to utilize the answer of a different post: iframe src change on button click but I still couldn't get it to work, it keeps saying: "newSrc is not defined at HTMLButtonElement.onclick (index)(9:65)"

I want to thank everyone for their comments, I found the problem and solved it, I forgot to link the javascript on the page I was working on, the dumbest mistake that I could've made

Thank you Chris for helping with extra tips and Nermin for making me realise my dumb mistake

function newSrc() {
  var e = document.getElementById("Menu_trailers");
  var newSrc = e.options[e.selectedIndex].value;
  document.getElementById("trailers").src=newSrc;
}

      <figure class="trailer">
        <iframe src="https://www.youtube.com/embed/sfAc2U20uyg" id="trailers"> </iframe>
      </figure>
      <section class="button_trailer">
        <select id="Menu_trailers">
          <option value="https://www.youtube.com/embed/sfAc2U20uyg">Trailer 1</option>
          <option value="https://www.youtube.com/embed/o-L1mMZ31hM">Trailer 2</option>
        </select>
        <button onclick="newSrc();" class="button_next_trailer">Change trailer</button>
      </section>

Answer 1

Here is the JS code to get the src from the select menu, then set it to the iframe element

function newSrc() {
  // Get The Select Menu Element
  var e = document.getElementById("Menu_trailers");
  // Set Select Menu Value (Selected Option) To Be The src Of The iframe
  document.getElementById("trailer").src = e.value;
}