const mobileMenu = document.getElementById('menu-btn');
const mobileLinks = document.getElementById('links')

mobileMenu.addEventListener('click', () => {
  console.log('Clicked')
  if (mobileLinks.style.display === "none") {
    mobileLinks.style.display = "block";
    mobileLinks.style.width = "100%";
    mobileLinks.style.transition = "width 2s ease-in-out";
  } else {
    mobileLinks.style.display = "none";
    mobileLinks.style.width = "0%";
    mobileLinks.style.transition = "width 2s ease-in-out";
  }
})

<div class="menu" id="menu-btn">
  <div class="hamburger"></div>
  <div class="hamburger"></div>
  <div class="hamburger"></div>
</div>

<div class="mobile-links" id="links" style="display:none;">
  <ul>
    <li><a href="#">Item 1</a></li>
    <li><a href="#">Item 2</a></li>
    <li><a href="#">Item 3</a></li>
    <li><a href="#">Item 4</a></li>
    <li>
      <a href="#" Item 5</a></li>
    <li><a href="#">Item 6</a></li>
  </ul>
</div>