activeLink.js

(function() {
  var href = window.location.href,
      parser = document.createElement('a');

  parser.href = href;

  var pageUrl = parser.pathname,
      navLinks = document.querySelectorAll('nav.docs-nav ul li a');

  [].slice.call(navLinks).forEach(function(link, i) {
    var currentHref = link.getAttribute('href');

    if (currentHref == pageUrl) {
      link.classList.add('active');
    }
  });       
})();