パカッと開く箱型メニューボタン（上下に分かれて開閉）

<div class="hamburger-wrapper">
  <button class="box-button" id="hamburger-btn">
    <div class="bar top"></div>
    <div class="bar bottom"></div>
  </button>
  <div class="menu-overlay" id="menu-overlay">
    <ul>
      <li><a href="#">TOP</a></li>
      <li><a href="#">ABOUT</a></li>
      <li><a href="#">WORKS</a></li>
      <li><a href="#">CONTACT</a></li>
    </ul>
  </div>
</div>

.hamburger-wrapper {
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 40px 0;
  position: relative;
  z-index: 10;
}

/* ボックス型ボタン */
.box-button {
  width: 60px;
  height: 50px;
  background: none;
  border: none;
  position: relative;
  cursor: pointer;
  z-index: 20;
}

.box-button .bar {
  position: absolute;
  width: 100%;
  height: 6px;
  background-color: #333;
  transition: all 0.4s ease;
  border-radius: 3px;
}

.box-button .top {
  top: 10px;
}

.box-button .bottom {
  bottom: 10px;
}

.box-button.open .top {
  transform: translateY(-15px) rotate(45deg);
}

.box-button.open .bottom {
  transform: translateY(15px) rotate(-45deg);
}

/* メニュー本体 */
.menu-overlay {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(17, 17, 17, 0.95);
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  transform: scale(0);
  opacity: 0;
  transition: transform 0.5s ease, opacity 0.5s ease;
  z-index: 5;
  pointer-events: none;
}

.menu-overlay.open {
  transform: scale(1);
  opacity: 1;
  pointer-events: auto;
}

.menu-overlay ul {
  list-style: none;
  padding: 0;
  margin: 0;
  text-align: center;
}

.menu-overlay ul li {
  margin: 20px 0;
}

.menu-overlay ul li a {
  color: #fff;
  text-decoration: none;
  font-size: 2rem;
  font-weight: bold;
  transition: color 0.3s;
}

.menu-overlay ul li a:hover {
  color: #d60000;
}

const btn = document.getElementById("hamburger-btn");
const menu = document.getElementById("menu-overlay");

btn.addEventListener("click", function () {
  btn.classList.toggle("open");
  menu.classList.toggle("open");
});

menu.addEventListener("click", function (e) {
  if (e.target === menu) {
    btn.classList.remove("open");
    menu.classList.remove("open");
  }
});