what's wrong on code javascript with list item menu?

[].slice.call(document.querySelectorAll('.dropdown .nav-link')).forEach(function(el) {
  el.addEventListener('click', onClick, false);
});

function onClick(e) {
  e.preventDefault();
  var el = this.parentNode;
  el.classList.contains('show-submenu') ? hideSubMenu(el) : showSubMenu(el);
}

function showSubMenu(el) {
  el.classList.add('show-submenu');
  document.addEventListener('click', function onDocClick(e) {
    // Return if target is an anchor tag
    if (e.target.localName === 'a') return;
    
    e.preventDefault();
    if (el.contains(e.target)) {
      return;
    }
    document.removeEventListener('click', onDocClick);
    hideSubMenu(el);
  });
}

function hideSubMenu(el) {
  el.classList.remove('show-submenu');
}

html {
  box-sizing: border-box;
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  background-color: #f00;
}

.nav {
  width: 550px;
  margin: 100px auto 0 auto;
  text-align: center;
}


/* Navigation */

.nav {
  font-family: Georgia, Arial, sans-serif;
  font-size: 14px;
}

.nav-items {
  padding: 0;
  list-style: none;
}

.nav-item {
  display: inline-block;
  margin-right: 25px;
}

.nav-item:last-child {
  margin-right: 0;
}

.nav-link,
.nav-link:link,
.nav-link:visited,
.nav-link:active,
.submenu-link,
.submenu-link:link,
.submenu-link:visited,
.submenu-link:active {
  display: block;
  position: relative;
  font-size: 14px;
  letter-spacing: 1px;
  cursor: pointer;
  text-decoration: none;
  outline: none;
}

.nav-link,
.nav-link:link,
.nav-link:visited,
.nav-link:active {
  color: #fff;
  font-weight: bold;
}

.nav-link::before {
  content: "";
  position: absolute;
  top: 100%;
  left: 0;
  width: 100%;
  height: 3px;
  background: rgba(0, 0, 0, 0.2);
  opacity: 0;
  -webkit-transform: translate(0, 10px);
  transform: translate(0, 10px);
  transition: opacity 0.3s ease, transform 0.3s ease;
}

.nav-link:hover::before,
.nav-link:hover::before {
  opacity: 1;
  -webkit-transform: translate(0, 5px);
  transform: translate(0, 5px);
}

.dropdown {
  position: relative;
}

.dropdown .nav-link {
  padding-right: 15px;
  height: 17px;
  line-height: 17px;
}

.dropdown .nav-link::after {
  content: "";
  position: absolute;
  top: 6px;
  right: 0;
  border: 5px solid transparent;
  border-top-color: #fff;
}

.submenu {
  position: absolute;
  top: 100%;
  left: 50%;
  z-index: 100;
  width: 200px;
  margin-left: -100px;
  background: #fff;
  border-radius: 3px;
  line-height: 1.46667;
  margin-top: -5px;
  box-shadow: 0 0 8px rgba(0, 0, 0, .3);
  opacity: 0;
  -webkit-transform: translate(0, 0) scale(.85);
  transform: translate(0, 0)scale(.85);
  transition: transform 0.1s ease-out, opacity 0.1s ease-out;
  pointer-events: none;
}

.submenu::after,
.submenu::before {
  content: "";
  position: absolute;
  bottom: 100%;
  left: 50%;
  margin-left: -10px;
  border: 10px solid transparent;
  height: 0;
}

.submenu::after {
  border-bottom-color: #fff;
}

.submenu::before {
  margin-left: -13px;
  border: 13px solid transparent;
  border-bottom-color: rgba(0, 0, 0, .1);
  -webkit-filter: blur(1px);
  filter: blur(1px);
}

.submenu-items {
  list-style: none;
  padding: 10px 0;
}

.submenu-item {
  display: block;
  text-align: left;
}

.submenu-link,
.submenu-link:link,
.submenu-link:visited,
.submenu-link:active {
  color: #3498db;
  padding: 10px 20px;
}

.submenu-link:hover {
  text-decoration: underline;
}

.submenu-seperator {
  height: 0;
  margin: 12px 10px;
  border-top: 1px solid #eee;
}

.show-submenu .submenu {
  opacity: 1;
  -webkit-transform: translate(0, 25px) scale(1);
  transform: translate(0, 25px) scale(1);
  pointer-events: auto;
}

<nav role="navigation" class="nav">
  <ul class="nav-items">
    <li class="nav-item dropdown">
      <a href="#" class="nav-link"><span>Drop Down</span></a>
      <nav class="submenu">
        <ul class="submenu-items">
          <li class="submenu-item"><a href="google.com" class="submenu-link">Google</a></li>
          <li class="submenu-item"><a href="youtube.com" class="submenu-link">Yuotube</a></li>
          <li class="submenu-item"><a href="facebook.com" class="submenu-link">Facebook</a></li>
        </ul>
      </nav>
    </li>
  </ul>
</nav>