How to Implement Submenus in Bootstrap Dropdowns?

Resolving Missing Sub Menu in Bootstrap Dropdown

Introduction

Bootstrap 3, an older version of the popular front-end framework, lacked a dedicated class for implementing sub menus in dropdowns. This absence can be attributed to evolving web design trends favoring simpler interfaces and reduced use of sub menus. However, there are ways to achieve sub menu functionality using additional CSS.

Bootstraps 5 (Updated 2023)

Implement JavaScript to disable the automatic closing of submenus when the parent dropdown is open:

let dropdowns = document.querySelectorAll('.dropdown-toggle')
dropdowns.forEach((dd) => {
    dd.addEventListener('click', function (e) {
        var el = this.nextElementSibling
        el.style.display = el.style.display === 'block' ? 'none' : 'block'
    })
})

Alternatively, use CSS for Navbar dropdowns:

.dropdown-submenu {
    position: relative;
}
.dropdown-submenu > .dropdown-menu {
    top: 0;
    left: 100%;
    margin-top: -1px;
}
.navbar-nav li:hover > ul.dropdown-menu {
    display: block;
}

Bootstraps 4 (Updated 2018)

Due to the removal of the dropdown-submenu class in Bootstrap 3, a workaround using CSS is necessary:

.navbar-nav li:hover > ul.dropdown-menu {
    display: block;
}

.dropdown-submenu {
    position: relative;
}

.dropdown-submenu > .dropdown-menu {
    top: 0;
    left: 100%;
    margin-top: -6px;
}

Bootstraps 3

CSS:

.dropdown-submenu {
    position: relative;
}

.dropdown-submenu > .dropdown-menu {
    top: 0;
    left: 100%;
    margin-top: -6px;
    margin-left: -1px;
    border-radius: 0 6px 6px 6px;
}

.dropdown-submenu:hover > .dropdown-menu {
    display: block;
}

.dropdown-submenu > a:after {
    content: " ";
    float: right;
    width: 0;
    height: 0;
    border-color: transparent;
    border-style: solid;
    border-width: 5px 0 5px 5px;
    border-left-color: #cccccc;
    margin-top: 5px;
}

.dropdown-submenu:hover > a:after {
    border-left-color: #ffffff;
}

Markup:

<ul class="nav navbar-nav">
    <li class="menu-item dropdown">
        <a href="#" class="dropdown-toggle" data-toggle="dropdown">Drop Down<b class="caret"></b></a>
        <ul class="dropdown-menu">
            <li class="menu-item dropdown dropdown-submenu">
                <a href="#" class="dropdown-toggle" data-toggle="dropdown">Level 1</a>
                <ul class="dropdown-menu">
                    <li class="menu-item">
                        <a href="#">Link 1</a>
                    </li>
                    <li class="menu-item dropdown dropdown-submenu">
                        <a href="#" class="dropdown-toggle" data-toggle="dropdown">Level 2</a>
                        <ul class="dropdown-menu">
                            <li><a href="#">Link 3</a></li>
                        </ul>
                    </li>
                </ul>
            </li>
        </ul>
    </li>
</ul>

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