Highlighting links based on scroll position

Question

I'm trying to get the my links to highlight if the user is scrolling over the page that is for that link. But for some reason it isn't working properly. I Have commented out my first try in jquery and tried again but link two is highlighting when link one should.

<nav>
  <ul>
    <li><a href="" id="link_1">Link 1</a></li>
    <li><a href="" id="link_2">Link 2</a></li>
    <li><a href="" id="link_3">Link 3</a></li>
  </ul>
   <p></p>
</nav>

<div id="sec_one" class="sections">

</div>

<div id="sec_two" class="sections">

</div>

<div id="sec_three" class="sections">

</div>

<script
  src="https://code.jquery.com/jquery-3.2.1.min.js"
  integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4="
  crossorigin="anonymous"></script>

*{
  margin: 0;
  padding: 0;
}
nav{
  width: 100%;
  background-color: black;
  position: fixed;
  top: 0;
}

nav ul{
  width: 50%;
  margin: 0 auto;
  list-style-type: none;
  text-align: center;
}

nav ul li{
  display: inline;
  width: 100%;
}

nav ul li a{
  font-size: 40px;
  color: white;
  text-decoration: none;
}

nav ul li a{

}

.sections{
  width: 100%;
  height: 2000px;
}

#sec_one{
  background-color: blue;
}

#sec_two{
  background-color: red;
}

#sec_three{
  background-color: yellow;
}

.active{
  background-color: #666666;
}

p{
  color: white;
}

$(window).scroll(function(){
  var scrollPos = $(window).scrollTop();
  var page1Top = $("#sec_one").scrollTop();
  var page1Bot = $("#sec_one").outerHeight();

  var page2Top = $("#sec_two").scrollTop();
  var page2Bot = $("#sec_two").outerHeight();

  var page3Top = $("#sec_three").scrollTop();
  var page3Bot = $("#sec_three").outerHeight();

 /*if(scrollPos >= page1Top && scrollPos < page1Bot){
    $("#link_1").addClass("active");
    $("#link_2").removeClass("active");
    $("#link_3").removeClass("active");
  }else if(scrollPos >= page2Top && scrollPos < page2Bot){
    $("#link_1").removeClass("active");
    $("#link_3").removeClass("active");
    $("#link_2").addClass("active");
  }else if(scrollPos >= page3Top && scrollPos < page3Bot){
    $("#link_3").addClass("active");
    $("#link_1").removeClass("active");
    $("#link_2").removeClass("active");
  }*/

  if(scrollPos >= page1Top && scrollPos < page1Bot){
    $("#link_1").addClass("active");
    $("#link_2").removeClass("active");
    $("#link_3").removeClass("active");
    }else {
      $("#link_1").removeClass("active");
    }

  if(scrollPos >= page2Top && scrollPos < page2Bot){
    $("#link_2").addClass("active");
    $("#link_1").removeClass("active");
    $("#link_3").removeClass("active");
    }else {
      $("#link_2").removeClass("active");
    }

});

Answer 1

Your main problem is you're not using .offset() - With your code you're just getting the position relative to itself so the top always becomes 0 and bottom becomes 2000 - using offset would mean you're getting the position relative to the document so that it takes into account other elements as well.

Also you don't need to check the bottom location. You can just use the top location of the next section.

$(document).ready(function() {
  $(window).scroll(function() {
    var scrollPos = $(window).scrollTop();
    
    var page1Top = $("#sec_one").offset().top;
    var page2Top = $("#sec_two").offset().top;
    var page3Top = $("#sec_three").offset().top;

    if (scrollPos >= page1Top && scrollPos < page2Top) {
      $("#link_1").addClass("active");
      $("#link_2").removeClass("active");
      $("#link_3").removeClass("active");
    } else {
      $("#link_1").removeClass("active");
    }

    if (scrollPos >= page2Top && scrollPos < page3Top) {
      $("#link_2").addClass("active");
      $("#link_1").removeClass("active");
      $("#link_3").removeClass("active");
    } else {
      $("#link_2").removeClass("active");
    }
    
    if (scrollPos >= page3Top) {
      $("#link_3").addClass("active");
      $("#link_1").removeClass("active");
      $("#link_2").removeClass("active");
    } else {
      $("#link_3").removeClass("active");
    }

  });
});

* {
  margin: 0;
  padding: 0;
}

nav {
  width: 100%;
  background-color: black;
  position: fixed;
  top: 0;
}

nav ul {
  width: 50%;
  margin: 0 auto;
  list-style-type: none;
  text-align: center;
}

nav ul li {
  display: inline;
  width: 100%;
}

nav ul li a {
  font-size: 40px;
  color: white;
  text-decoration: none;
}

nav ul li a {}

.sections {
  width: 100%;
  height: 2000px;
}

#sec_one {
  background-color: blue;
}

#sec_two {
  background-color: red;
}

#sec_three {
  background-color: yellow;
}

.active {
  background-color: #666666;
}

p {
  color: white;
}

<nav>
  <ul>
    <li><a href="" id="link_1">Link 1</a></li>
    <li><a href="" id="link_2">Link 2</a></li>
    <li><a href="" id="link_3">Link 3</a></li>
  </ul>
  <p></p>
</nav>

<div id="sec_one" class="sections"></div>
<div id="sec_two" class="sections"></div>
<div id="sec_three" class="sections"></div>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Answer 2

Avoid the target of individual elements and the use of the rough ID's, you can evaluate the scrollTop and the offset().top of the element and based on the index of the section highlight the item you need:

$(window).scroll(function() {
  var scrollPos = $(window).scrollTop(),
      navH     = $('nav').height();
  $('.sections').each(function(i){
    var offT = $(this).offset().top;
    if((offT-scrollPos-navH) <= 0) {
      $('.active').removeClass('active')
      $('nav a').eq(i).addClass('active')
    }
  })
});

* { margin: 0; padding: 0;} nav { width: 100%; background-color: black; position: fixed; top: 0;} nav ul { width: 50%; margin: 0 auto; list-style-type: none; text-align: center;} nav ul li { display: inline; width: 100%;} nav ul li a { font-size: 40px; color: white; text-decoration: none;} .sections { width: 100%; height: 2000px;} #sec_one { background-color: blue;} #sec_two { background-color: red;} #sec_three { background-color: yellow;} .active { background-color: #666666;} p { color: white;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<nav>
  <ul>
    <li><a href="" id="link_1" class="active">Link 1</a></li>
    <li><a href="" id="link_2">Link 2</a></li>
    <li><a href="" id="link_3">Link 3</a></li>
  </ul>
  <p></p>
</nav>
<div id="sec_one" class="sections"></div>
<div id="sec_two" class="sections"></div>
<div id="sec_three" class="sections"></div>