Shrinking navigation bar when scrolling down (bootstrap3)

Sticky navbar:

To make a sticky nav you need to add the class navbar-fixed-top to your nav

Official documentation:
https://getbootstrap.com/docs/5.0/components/navbar/#placement

Official example:
http://getbootstrap.com/examples/navbar-fixed-top/

A simple example code:

<nav class="navbar navbar-default navbar-fixed-top" role="navigation">
  <div class="container">
    ...
  </div>
</nav>

with related jsfiddle: http://jsfiddle.net/ur7t8/

Resize the navbar:

If you want the nav bar to resize while you scroll the page you can give a look to this example: http://www.bootply.com/109943

JS

$(window).scroll(function() {
  if ($(document).scrollTop() > 50) {
    $('nav').addClass('shrink');
  } else {
    $('nav').removeClass('shrink');
  }
});

CSS

nav.navbar.shrink {
  min-height: 35px;
}

Animation:

To add an animation while you scroll, all you need to do is set a transition on the nav

CSS

nav.navbar{
  background-color:#ccc;
   // Animation
   -webkit-transition: all 0.4s ease;
   transition: all 0.4s ease;
}

I made a jsfiddle with the full example code: http://jsfiddle.net/Filo/m7yww8oa/

toggleClass works too:

$(window).on("scroll", function() {
    $("nav").toggleClass("shrink", $(this).scrollTop() > 50)
});

For those not willing to use jQuery here is a Vanilla Javascript way of doing the same using classList:

function runOnScroll() {
    var element = document.getElementsByTagName('nav')  ;
    if(document.body.scrollTop >= 50) {
        element[0].classList.add('shrink')
    } else {
        element[0].classList.remove('shrink')
    }
    console.log(topMenu[0].classList)

};

There might be a nicer way of doing it using toggle, but the above works fine in Chrome

You can combine "scroll" and "scrollstop" events in order to achieve desired result:

$(window).on("scroll",function(){
   $('nav').addClass('shrink');
}); 
$(window).on("scrollstop",function(){
   $('nav').removeClass('shrink');
});