Scrollspy + jquery

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">

<!-- add to the body tag scroll spy AND main-nav id to nav bar AND the id tags to the links AND those id's to the section -->

<body id="home" data-spy="scroll" data-target="#main-nav">
  <nav class="navbar navbar-expand-md navbar-light fixed-top py-4" id="main-nav">
    <div class="container">
      <a href="#home" class="navbar-brand">
        <img src="img/mlogo.png" width="50" height="50" alt="logo">
        <h3 class="d-inline align-middle">Bootstrap-Sass</h3>
      </a>
      <button class="navbar-toggler" data-toggle="collapse" data-target="#navbarCollapse">
        <span class="navbar-toggler-icon"></span>
      </button>
      <div class="collapse navbar-collapse" id="navbarCollapse">
        <ul class="navbar-nav ml-auto"><!-- ml-auto nustumia li i desine puse -->
          <li class="nav-item">
            <a href="#home" class="nav-link">Home</a>
          </li>
          <li class="nav-item">
            <a href="#about" class="nav-link">About</a>
          </li>
          <li class="nav-item">
            <a href="#authors" class="nav-link">Meet The Authors</a>
          </li>
          <li class="nav-item">
            <a href="#contact" class="nav-link">Contact</a>
          </li>
        </ul>
      </div><!-- end og collapse navbar-collapse -->
    </div><!-- end of container -->
  </nav>

<section id="about" class="py-5 text-center bg-light"> ....


<!-- under the body -->

  <script src="http://code.jquery.com/jquery-3.3.1.min.js" integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
  crossorigin="anonymous"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49"
  crossorigin="anonymous"></script>
  <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js" integrity="sha384-smHYKdLADwkXOn1EmN1qk/HfnUcbVRZyYmZ4qpPea6sjB/pTJ0euyQp0Mk8ck+5T"
  crossorigin="anonymous"></script>


  <script>
    // Get the current year for the copyright
    $('#year').text(new Date().getFullYear());

    // Init Scrollspy
    $('body').scrollspy({target: '#main-nav'});

    // Smooth Scrolling
    $("#main-nav a").on('click', function (event) {
      if (this.hash !== "") {
        event.preventDefault();

        const hash = this.hash;

        $('html, body').animate({
          scrollTop: $(hash).offset().top
        }, 800, function () {

          window.location.hash = hash;
        });
      }
    });

  </script>