Description :
scrollspy is a plugin made by using bootstrap classes which is used to update the navigation bar automatically based on the position of scroll.
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<style>
body {
position: relative;
}
#part1 {
padding-top: 50px;
height: 400px;
color: black;
background-color: yellow;
}
#part2 {
padding-top: 70px;
height: 400px;
color: black;
background-color: yellowgreen;
}
#part3 {
padding-top: 70px;
height: 400px;
color: black;
background-color: red;
}
#part5 {
padding-top: 70px;
height: 400px;
color: black;
background-color: orange;
}
#part6 {
padding-top: 70px;
height: 400px;
color: black;
background-color: deeppink;
}
</style>
</head>
<body data-spy="scroll" data-target=".navbar">
<nav class="navbar navbar-inverse navbar-fixed-top">
<div class="container-fluid">
<div class="navbar-header">
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#myNavbar">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">Parts</a>
</div>
<div>
<div class="collapse navbar-collapse" id="myNavbar">
<ul class="nav navbar-nav">
<li><a href="#part1">Part 1</a></li>
<li><a href="#part2">Part 2</a></li>
<li><a href="#part3">Part 3</a></li>
<li class="dropdown">
<a class="dropdown-toggle" data-toggle="dropdown" href="#">part 4 <span class="caret"></span></a>
<ul class="dropdown-menu">
<li><a href="#part5">part 4-1</a></li>
<li><a href="#part6">part 4-2</a></li>
</ul>
</li>
</ul>
</div>
</div>
</div>
</nav>
<div id="part1" class="container-fluid">
<h1>Part 1</h1>
<p>Please looked at navigation bar while scrolling.</p>
</div>
<div id="part2" class="container-fluid">
<h1>Part 2</h1>
<p>Please looked at navigation bar while scrolling.</p>
</div>
<div id="part3" class="container-fluid">
<h1>Part 3</h1>
<p>Please looked at navigation bar while scrolling.</p>
</div>
<div id="part5" class="container-fluid">
<h1>Part 4 Submenu 1</h1>
<p>Please looked at navigation bar while scrolling.</p>
</div>
<div id="part6" class="container-fluid">
<h1>Part 4 Submenu 2</h1>
<p>Please looked at navigation bar while scrolling.</p>
</div>
</body>
</html>