二级下拉菜单结构图,ul嵌套ul

<ul class="cd-main-nav">
    <li><a href="https://jy.tp.yuanmeng.life/">首页</a></li>
    <li><a href="https://jy.tp.yuanmeng.life/js">jQuery</a></li>
    <li><a href="https://jy.tp.yuanmeng.life/php">PHP</a></li>
    <li><a href="https://jy.tp.yuanmeng.life/templates">模板</a></li>
    <li>
        <a href="#0" class="cd-subnav-trigger"><span>分类</span></a>

        <ul>
            <li class="go-back"><a href="#0">Menu</a></li>
            <li><a href="#0">Category 1</a></li>
            <li><a href="#0">Category 2</a></li>
            <li><a href="#0">Category 3</a></li>
            <li><a href="#0">Category 4</a></li>
            <li><a href="#0">Category 5</a></li>
            <li><a href="#0" class="placeholder">Placeholder</a></li>
        </ul>
    </li>
</ul>

自适应宽度main.js

moveNavigation();
$(window).on('resize', function() {
    (!window.requestAnimationFrame) ? setTimeout(moveNavigation, 300) : window.requestAnimationFrame(moveNavigation);
});

点击展开和关闭下拉菜单样式

$('.cd-nav-trigger').on('click', function(event) {
    event.preventDefault();
    if ($('header').hasClass('nav-is-visible'))
        $('.moves-out').removeClass('moves-out');

    $('header').toggleClass('nav-is-visible');
    $('.cd-main-nav').toggleClass('nav-is-visible');
    $('.cd-main-content').toggleClass('nav-is-visible');
});

关闭二级下拉菜单

$('.go-back').on('click', function(event) {
    event.preventDefault();
    $('.cd-main-nav').removeClass('moves-out');
});

显示二级下拉菜单

$('.cd-subnav-trigger').on('click', function(event) {
    event.preventDefault();
    $('.cd-main-nav').toggleClass('moves-out');
});