JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="menu-system">
<li class="active">
<a href="#">Location</a>
</li>
<li class="dropdown">
    <a href="#" class="dropdown-toggle menu-item" data-toggle="dropdown"> Top level Option 1</a>
    <ul class="dropdown-menu">
        <li><a href="#opt1"> Option 1</a></li>
        <li><a href="#opt2"> Option 2</a></li>
        <li><a href="#opt3"> Option 3</a></li>
        <li><a href="#opt4"> Option 4</a></li>
    </ul>
</li>

CSS

@import url('http://twitter.github.com/bootstrap/assets/css/bootstrap.css');

body {
    padding-top: 60px;
    padding-bottom: 40px;
}

.sidebar-nav {
    padding: 9px 0;
}

.dropdown-menu .sub-menu {
    left: 100%;
    position: absolute;
    top: 0;
    visibility: hidden;
    margin-top: -1px;
}

.dropdown-menu li:hover .sub-menu {
    visibility: visible;
}

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

.nav-tabs .dropdown-menu, .nav-pills .dropdown-menu, .navbar .dropdown-menu {
    margin-top: 0;
}

.navbar .sub-menu:before {
    border-bottom: 7px solid transparent;
    border-left: none;
    border-right: 7px solid rgba(0, 0, 0, 0.2);
    border-top: 7px solid transparent;
    left: -7px;
    top: 10px;
}
.navbar .sub-menu:after {
    border-top: 6px solid transparent;
    border-left: none;
    border-right: 6px solid #fff;
    border-bottom: 6px solid transparent;
    left: 10px;
    top: 11px;
    left: -6px;
}

JavaScript

$('.dropdown-menu li a').each(function() {
    if($(location).attr('href').indexOf($(this).attr('href')) > 0) {
         $('.dropdown-menu').show();
    } 
});