JavaScript 侧边栏，二级菜单示例

<!DOCTYPE html>
<html>
<head>
    <style>
        body {
            margin: 0;
            padding: 0;
        }

        #sidebar {
            position: fixed;
            top: 0;
            left: -250px; /* 控制初始隐藏侧边栏 */
            width: 250px;
            height: 100vh;
            background: #f2f2f2;
            transition: left 0.3s ease-in-out;
            z-index: 9999;
        }

        #sidebar.active {
            left: 0; /* 显示侧边栏 */
        }

        #content {
            margin-left: 250px; /* 侧边栏宽度 */
            padding: 20px;
        }

        #sidebar ul {
            list-style-type: none;
            padding: 0;
            margin: 0;
        }

        #sidebar ul li {
            position: relative;
            padding: 10px 0;
        }

        #sidebar ul li a {
            color: #333;
            text-decoration: none;
            padding: 10px 20px;
            display: block;
        }

        #sidebar ul li a:hover {
            background-color: #ddd;
        }

        #sidebar ul li ul {
            position: absolute;
            left: 100%;
            top: 0;
            width: 250px;
            display: none; /* 初始隐藏二级菜单 */
        }

        #sidebar ul li:hover ul {
            display: block; /* 鼠标悬停时显示二级菜单 */
        }

        #sidebar ul li ul li {
            padding: 5px 0;
        }

        #sidebar ul li ul li a {
            padding-left: 40px; /* 调整二级菜单的缩进 */
        }

        #overlay {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 100vh;
            background-color: rgba(0, 0, 0, 0.5);
            display: none; /* 初始隐藏遮罩层 */
            z-index: 9998;
        }

        .toggle-btn {
            display: inline-block;
            padding: 10px;
            cursor: pointer;
        }
    </style>
</head>
<body>
    <div id="sidebar">
        <ul>
            <li><a href="#">菜单1</a></li>
            <li>
                <a href="#">菜单2</a>
                <ul>
                    <li><a href="#">子菜单1</a></li>
                    <li><a href="#">子菜单2</a></li>
                </ul>
            </li>
            <li><a href="#">菜单3</a></li>
        </ul>
    </div>

    <div id="content">
        <div class="toggle-btn" onclick="toggleSidebar()">打开/关闭侧边栏</div>
        <h1>主要内容区域</h1>
    </div>

    <div id="overlay" onclick="toggleSidebar()"></div>

    <script>
        var sidebar = document.getElementById('sidebar');
        var overlay = document.getElementById('overlay');

        function toggleSidebar() {
            sidebar.classList.toggle('active');
            overlay.style.display = sidebar.classList.contains('active') ? 'block' : 'none';
        }
    </script>
</body>
</html>