<!--

Forum question answer only:

https://www.sitepoint.com/community/t/capture-ul-tag-under-li-through-js/407291/15


-->
<ul class="nav first">
  <li class="lfirst"><a href="">Menu</a></li>
  <li class="lfirst">
    <a href="#">Menu</a>
    <ul class="second">
      <li><a href="#">Menu 01</a></li>
      <li><a href="#">Menu 02</a></li>
      <li><a href="#">Menu 03</a></li>
    </ul>
  </li>
  <li class="lfirst"><a href="#">Menu</a></li>

  <li class="lfirst">
    <a href="#">Menu</a>
    <ul class="second">
      <li><a href="#">Menu 01</a></li>
      <li><a href="#">Menu 02</a></li>
      <li><a href="#">Menu 03</a></li>
    </ul>
  </li>
  <li class="lfirst"><a href="#">Menu</a></li>
</ul>

ul {
  margin: 0;
  padding: 0;
  list-style: none;
}
.first {
  display: flex;
  position: relative;
  z-index: 99;
  max-width: 980px;
  border: 1px solid #000;
  border-right: none;
  margin: auto;
  background: cyan;
  border-bottom: 10px solid red;
}
.first > li {
  position: relative;
  flex: 1 0 0;
  border-right: 1px solid #000;
}
.first a {
  padding: 10px;
  display: block;
  text-decoration: none;
  background: cyan;
  z-index: 1;
  transition: 0.2s ease;
}
.first li:hover > a,
.first a:hover {
  background: aquamarine;
}
.first > li > ul {
  position: absolute;
  left: 0;
  top: 0;
  right: 0;
  z-index: -1;
  opacity: 0;
  pointer-events: none;
  transition: 1s ease;
  background: lightblue;
}
.first > li.active > ul,
/*.first > li:hover > ul,*/
.first > li:focus-within > ul {
  top: 100%;
  margin-top: 1px;
  opacity: 1;
  pointer-events: initial;
}
.first li ul a {
  padding: 5px;
  border-bottom: 1px solid red;
}

/* arrows */
.nav li a:first-child:not(:last-child) {
  padding-right: 20px; /* make space for arrows*/
}
.nav li a:first-child:not(:last-child):after {
  content: "";
  position: absolute;
  right: 3px;
  top: 50%;
  margin-top: -6px;
  width: 0;
  height: 0;
  border-left: 6px solid transparent;
  border-right: 6px solid transparent;
  border-top: 6px solid red;
  transition: 0.5s ease;
}
.nav li:hover a:first-child:not(:last-child):after {
  transform: rotate(180deg);
}

/* js adds active class so testing here */

.first > li.active > ul {
  box-shadow: 0 0 40px 40px rgba(0, 0, 0, 0.5);
}

/* none of this js is needed as css :hpver can accomplish all this */

const lFirsts = document.querySelectorAll(".lfirst");

lFirsts.forEach((lFirst) => {
  lFirst.addEventListener("mouseover", () => {
    lFirst.classList.add("active");
  });
  lFirst.addEventListener("mouseout", () => {
    lFirst.classList.remove("active");
  });
});