Nav fiddle

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<div class="nav">
<span data-action="toggle-nav" class="action nav-toggle">
         <span>Menu</span>
</span>
</div>

CSS

.nav-toggle {
  position: absolute;
  transform: translateY(50%);
  display:block;
}

.nav-open .nav-toggle:after {
  background: rgba(0, 0, 0, 0.5);
  content: '';
  display: block;
  height: 100vh;
  position: fixed;
  left: 0;
  top: 0;
  width: 100vw;
  z-index: 1;
}

JavaScript

$('.nav-toggle').click(function(){
$('.nav').toggleClass('nav-open');
});