多階層ドロップダウン（レスポンシブ）

デモ

See the Pen nav 18 by logsuke (@design-list) on CodePen.

コード

<!-- Boxicons headタグ内へ -->
<link href='https://unpkg.com/boxicons@2.1.2/css/boxicons.min.css' rel='stylesheet'>

     <header class="header" id="header">
          <nav class="nav">
               <a href="#" class="logo">
                    <svg viewBox="0 0 496 512" width="100" title="meh-rolling-eyes">
                         <path
                              d="M248 8C111 8 0 119 0 256s111 248 248 248 248-111 248-248S385 8 248 8zM88 224c0-24.3 13.7-45.2 33.6-56-.7 2.6-1.6 5.2-1.6 8 0 17.7 14.3 32 32 32s32-14.3 32-32c0-2.8-.9-5.4-1.6-8 19.9 10.8 33.6 31.7 33.6 56 0 35.3-28.7 64-64 64s-64-28.7-64-64zm224 176H184c-21.2 0-21.2-32 0-32h128c21.2 0 21.2 32 0 32zm32-112c-35.3 0-64-28.7-64-64 0-24.3 13.7-45.2 33.6-56-.7 2.6-1.6 5.2-1.6 8 0 17.7 14.3 32 32 32s32-14.3 32-32c0-2.8-.9-5.4-1.6-8 19.9 10.8 33.6 31.7 33.6 56 0 35.3-28.7 64-64 64z" />
                    </svg>
                    logo
               </a>
               <div class="nav-menu" id="navMenu">
                    <ul class="nav-links">
                         <li>
                              <a href="#section1" class="nav-link">Section1</a>
                              <i class='bx bx-plus'></i>                       
                              <ul class="parent drop-down">
                                   <li><a href="#section1-1" class="nav-link">Section1-1</a></li>
                                   <li><a href="#section1-2" class="nav-link">Section1-2</a></li>
                                   <li>
                                        <a href="#section1-3" class="nav-link">Section1-3</a>
                                        <i class='bx bx-plus'></i>
                                        <ul class="child drop-down">
                                             <li><a href="#section1-3-1" class="nav-link">Section1-3-1</a></li>
                                             <li><a href="#section1-3-2" class="nav-link">Section1-3-2</a></li>
                                             <li><a href="#section1-3-3" class="nav-link">Section1-3-3</a></li>
                                        </ul>
                                   </li>
                              </ul>
                         </li>
                         <li ><a href="#section2" class="nav-link">Section2</a></li>
                         <li ><a href="#section3" class="nav-link">Section3</a></li>
                    </ul>
               </div>

               <div class="nav-toggle" id="navToggle">
                    <i id="navIcon" class='bx bx-menu'></i>
               </div>
          </nav>
     </header>

/*=== Base ===*/

* {
     margin: 0;
     padding: 0;
     box-sizing: border-box;
     font-family: sans-serif;
}

:root {
     --nav-height: 6rem;
     --max-width: 1024px;
     --container-padding: 0 4rem;
     --gap: 2rem;
     --bg-color: rgb(240, 240, 240);
}

/*=== Font ratio ===*/

html {
     scroll-behavior: smooth;
     font-size: 62.5%;
}

body {
     font-size: 1.6rem;
}

ul {
     list-style: none;
}

a {
     text-decoration: none;
}

/* header */
header {
     width: 100%;
     position: fixed;
     top: 0;
     left: 0;
     z-index: 100;
     background-color: var(--bg-color);
}

.nav svg {
     width: 25px;
     padding: 2px;
     margin-right: 5px;
}

/* nav */
.nav {
     width: 100%;
     max-width: var(--max-width);
     height: var(--nav-height);
     display: flex;
     justify-content: space-between;
     align-items: center;
     margin: 0 auto;
     padding: 0 3rem;
}

.logo,
.nav-toggle {
     color: var(--text-color);
     cursor: pointer;
     z-index: 200;
}

.logo {
     font-weight: bold;
     font-size: 2rem;
     display: flex;
     justify-content: center;
     align-items: center;
}

.nav-toggle {
     font-size: 3rem;
     display: flex;
     align-items: center;
}

.nav-toggle i {
     display: none;
}

.nav-links {
     display: flex;
     gap: 2rem;
     line-height: var(--nav-height);
}

.nav-links li {
     position: relative;
}

.nav-links .nav-link {
     color: var(--text-color);
     /* display: inline-block; */
}

.nav-link:hover {
     color: rgb(134, 170, 207);
}

.nav-links i {
     display: inline-block;
     cursor: pointer;
     padding: .5rem;
}

.drop-down {
     position: absolute;
     background: var(--bg-color);
     display: none;
}

.drop-down li {
     width: 150px;
     text-align: center;
}

/* border */
.drop-down>li {
     border-bottom: 1px solid rgb(88, 87, 87);
}

.drop-down>li:last-child {
     border: none;
}

.parent {
     top: var(--nav-height);
     left: 50%;
     transform: translateX(-50%);
}

.child {
     top: 0;
     right: -100%;
}

.nav-links li:hover>.drop-down {
     display: block;
}

/*スマホサイズナビゲーションバー*/
@media screen and (max-width: 1024px) {

     .nav-menu {
          position: fixed;
          top: 0;
          left: -250px;
          background: var(--bg-color);
          width: 250px;
          height: 100vh;
          padding-top: var(--nav-height);
          transition: transform .5s ease-in-out;
          z-index: 120;
     }

     .nav-menu.toggle {
          transform: translateX(100%);
     }

     .nav-toggle i {
          display: block;
     }

     .nav-links {
          display: block;
          padding: 2rem;
     }

     .nav-links li {
          border: none;
          text-align: start;
     }

     .drop-down {
          position: static;
          opacity: 1;
          transform: none;
          width: 100%;
          padding-left: 2rem;
          display: none;
     }

     .nav-links li:hover>.drop-down {
          display: none;
     }

     .nav-links i.toggle~.drop-down {
          display: block;
     }



}

const navMenu = document.getElementById('navMenu'),
    navToggle = document.getElementById('navToggle'),
    navIcon = document.getElementById('navIcon'),
    navLink = document.querySelectorAll('.nav-link'),
    dropDown = document.querySelectorAll('.drop-down')

navToggle?.addEventListener('click', function () {
    if (navIcon.classList.contains('bx-menu')) {
        navIcon.classList.replace('bx-menu', 'bx-x');
        navMenu.classList.toggle('toggle');
    } else {
        navIcon.classList.replace('bx-x', 'bx-menu');
        navMenu.classList.toggle('toggle');
    }
})

navLink.forEach(function (li) {
    li.addEventListener('click', function () {
        navIcon.classList.replace('bx-x', 'bx-menu');
        navMenu.classList.remove('toggle');
    })
})

dropDown.forEach(function (i) {
    i.previousElementSibling.addEventListener('click', function() {
        this.classList.toggle('toggle');
    })
})