Fixed on scroll

basic.html

<header>
<div class="bg-warning py-2">
	<div class="container">
		Some top header info
	</div>
</div>
<nav id="navbar_top" class="navbar navbar-expand-lg navbar-dark bg-primary">
<div class="container">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#main_nav" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="main_nav">	
    <ul class="navbar-nav ml-auto">
      <li class="nav-item"><a class="nav-link" href="#"> Menu item </a></li>
      <li class="nav-item"><a class="nav-link" href="#"> Menu item </a></li>
    </ul>
  </div> <!-- navbar-collapse.// -->
</div> <!-- container.// -->
</nav>
</header>

script.js

///////////////// fixed menu on scroll for desktop
if ($(window).width() > 992) {
  $(window).scroll(function(){  
     if ($(this).scrollTop() > 40) {
        $('#navbar_top').addClass("fixed-top");
        // add padding top to show content behind navbar
        $('body').css('padding-top', $('.navbar').outerHeight() + 'px');
      }else{
        $('#navbar_top').removeClass("fixed-top");
         // remove padding top from body
        $('body').css('padding-top', '0');
      }   
  });
} // end if