mega submenu

basic.html

<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#main_nav">
  <span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="main_nav">
<ul class="navbar-nav">
  <li class="nav-item"> <a class="nav-link" href="#"> Home </a> </li>
  <li class="nav-item dropdown"> <a class="nav-link dropdown-toggle" href="#" data-toggle="dropdown"> Mega submenu </a>
	<ul class="dropdown-menu">
		<li><a class="dropdown-item" href="#"> Dropdown item 1 </a></li>
		<li><a class="dropdown-item" href="#"> Dropdown item 2 </a></li>
		<li class="has-submenu">
		   <a class="dropdown-item dropdown-toggle" href="#"> Dropdown item 3 </a>
		   <div class="megasubmenu dropdown-menu">
		    Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
		    tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
		    quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
		    consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
		    cillum dolore eu fugiat nulla pariatur. 
		   </div>
		</li>
		<li class="has-submenu">
		   <a class="dropdown-item dropdown-toggle" href="#"> Dropdown item 4 </a>
		   <div class="megasubmenu dropdown-menu">
		      Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
		      tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
		      quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
		      consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
		      cillum dolore eu fugiat nulla pariatur. 
		   </div>
		</li>
		<li><a class="dropdown-item" href="#"> Dropdown item 6 </a></li>
       </ul>
    </li>
    <li class="nav-item"> <a class="nav-link" href="#">Menu item </a> </li>
    <li class="nav-item"><a class="nav-link" href="#"> About </a></li>
  </ul>
</div> <!-- navbar-collapse.// -->
</nav>

script.js

// Prevent closing from click inside dropdown
  $(document).on('click', '.dropdown-menu', function (e) {
    e.stopPropagation();
  });

// clickable on mobile view
  if ($(window).width() < 992) {
      $('.has-submenu a').click(function(e){
        e.preventDefault();
          $(this).next('.megasubmenu').toggle();

          $('.dropdown').on('hide.bs.dropdown', function () {
             $(this).find('.megasubmenu').hide();
          })
      });
  }

style.css

.megasubmenu{ padding: 20px; }

@media (min-width: 992px){
  .dropdown-menu .dropdown-toggle:after{
    border-top: .3em solid transparent;
      border-right: 0;
      border-bottom: .3em solid transparent;
      border-left: .3em solid;
  }
  .dropdown-menu{
    margin:0; 
  }
  .megasubmenu{ 
    left:100%; top:0; min-height: 100%; min-width:500px;
  }
  .dropdown-menu > li:hover .megasubmenu{
    display: block;
  }
}