crimx/index.html

## index.html
<!DOCTYPE html>
<html>
<head>
<script src="//code.jquery.com/jquery-2.1.1.min.js"></script>
  <meta charset="utf-8">
  <title>Test by Jesse</title>
</head>
<body>
<nav id = 'sidenav'>
  <ul>
    <li><a href="#1">First</a></li>
    <li><a href="#2">Second</a></li>
    <li><a href="#3">Third</a></li>
    <li><a href="#4">Fourth</a></li>
    <li><a href="#5">Fifth</a></li>
  </ul>
</nav>

<div class="sections">
  <section id="1"><h1>First</h1></section>
  <section id="2"><h1>Second</h1></section>
  <section id="3"><h1>Third</h1></section>
  <section id="4"><h1>Fourth</h1></section>
  <section id="5"><h1>Fifth</h1></section>
</div>
</body>
</html>

## scroll.js
$(window).on('scroll', function () {

  var scrollTop = $(document).scrollTop(),
      activeLi = $('#sidenav li.active').first();

  if (activeLi.length <= 0) {
     $('#sidenav li').first().addClass('active');
  } else {
    var activeSec = $(activeLi.find('a').attr('href'));
    if (activeSec.offset().top + activeSec.outerHeight() <= scrollTop) {
      activeLi.removeClass('active');
      activeLi.next().addClass('active');
    } else {
      var preActiveLi = activeLi.prev();
      if (preActiveLi.length > 0) {
        var preActiveSec = $(preActiveLi.find('a').attr('href'));
        if (preActiveSec.offset().top - scrollTop >= 0) {
          activeLi.removeClass('active');
          preActiveLi.addClass('active');
        }
      }
    }
  }

});

## style.css
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

nav {
  position: fixed;
  height: 200px;
  margin: auto;
  top: 0;
  bottom: 0;
  right: 0;
  background: #1ABC9C;
}

nav ul {
  margin: 0 auto;
  list-style: none;
  text-align: center;
}

nav ul li {
  display: block;
  padding: 10px 20px;
}

nav ul li a {
  padding: 10px 0;
  color: #fff;
  font-size: 1rem;
  text-decoration: none;
  font-weight: bold;
  transition: all 0.2s ease;
}

li:hover > a {
  color: #34495E;
}

li.active {
  background-color: white;
}

li.active > a {
  color: #1ABC9C;
}

section {
  height: 900px;
}
	<!DOCTYPE html>
	<html>
	<head>
	<script src="//code.jquery.com/jquery-2.1.1.min.js"></script>
	<meta charset="utf-8">
	<title>Test by Jesse</title>
	</head>
	<body>
	<nav id = 'sidenav'>
	<ul>
	<li><a href="#1">First</a></li>
	<li><a href="#2">Second</a></li>
	<li><a href="#3">Third</a></li>
	<li><a href="#4">Fourth</a></li>
	<li><a href="#5">Fifth</a></li>
	</ul>
	</nav>

	<div class="sections">
	<section id="1"><h1>First</h1></section>
	<section id="2"><h1>Second</h1></section>
	<section id="3"><h1>Third</h1></section>
	<section id="4"><h1>Fourth</h1></section>
	<section id="5"><h1>Fifth</h1></section>
	</div>
	</body>
	</html>
	$(window).on('scroll', function () {

	var scrollTop = $(document).scrollTop(),
	activeLi = $('#sidenav li.active').first();

	if (activeLi.length <= 0) {
	$('#sidenav li').first().addClass('active');
	} else {
	var activeSec = $(activeLi.find('a').attr('href'));
	if (activeSec.offset().top + activeSec.outerHeight() <= scrollTop) {
	activeLi.removeClass('active');
	activeLi.next().addClass('active');
	} else {
	var preActiveLi = activeLi.prev();
	if (preActiveLi.length > 0) {
	var preActiveSec = $(preActiveLi.find('a').attr('href'));
	if (preActiveSec.offset().top - scrollTop >= 0) {
	activeLi.removeClass('active');
	preActiveLi.addClass('active');
	}
	}
	}
	}

	});
	* {
	margin: 0;
	padding: 0;
	box-sizing: border-box;
	}

	nav {
	position: fixed;
	height: 200px;
	margin: auto;
	top: 0;
	bottom: 0;
	right: 0;
	background: #1ABC9C;
	}

	nav ul {
	margin: 0 auto;
	list-style: none;
	text-align: center;
	}

	nav ul li {
	display: block;
	padding: 10px 20px;
	}

	nav ul li a {
	padding: 10px 0;
	color: #fff;
	font-size: 1rem;
	text-decoration: none;
	font-weight: bold;
	transition: all 0.2s ease;
	}

	li:hover > a {
	color: #34495E;
	}

	li.active {
	background-color: white;
	}

	li.active > a {
	color: #1ABC9C;
	}

	section {
	height: 900px;
	}