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HTML

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About the <head>

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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Packages

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HTML 

              
                <html>
  <head>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
  </head>

  <main>
    <aside>
      <div  id="post__sidebar-left">
        <h4>Paragraphs</h4>
      </div>
    </aside>

    <div class="post__content">
      <h2 id="hello">Hello</h2>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eu dui sit amet diam mollis facilisis sed sed lectus. Mauris vel risus maximus, molestie eros eu, lacinia massa. Ut quis ornare risus. Praesent vestibulum commodo dolor eu lacinia.
      <h3 id="subtitle-1">Subtitle 1</h3>
      Sed faucibus, purus quis laoreet commodo, ante tellus auctor felis, faucibus tincidunt sapien nisi eu leo.
      <h3 id="subtitle-2">Subtitle 2</h3>
      Sed faucibus, purus quis laoreet commodo, ante tellus auctor felis, faucibus tincidunt sapien nisi eu leo.
      <h2 id="another-title">Another title</h2>
      Sed vulputate turpis massa, eget hendrerit nunc ullamcorper sed. Phasellus porttitor aliquet faucibus. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque gravida diam id erat interdum, eu bibendum massa interdum. Maecenas cursus nulla quis enim mollis accumsan. Aenean id blandit libero.
    </div>
  </main>
</html>
!

CSS 

              
                html {
  background-color: rgb(25, 25, 25);
  color: rgb(220, 220, 220);
}

a {
  color: paleturquoise;
}

main {
  display: flex;
  max-width: 400px;
}

#post__sidebar-left {
  padding: 10px;
  position: sticky;
  top: 0;
}

#post__sidebar-left a.current {
  color: gold;
}
!

JS 

              
                let headers;
let sidebarLinks;

$(document).ready(function () {
    headers = $(".post__content h2, .post__content h3").toArray();
    const sidebar = $("#post__sidebar-left");

    for (let i = 0; i < headers.length; i++) {
        const currentHeader = headers[i];
        const headerText = $(currentHeader).text();
        const headerId = $(currentHeader).attr("id");
        const headerType = $(currentHeader).prop("nodeName").toLowerCase();

        // <h2> <a href="header-linke"> Header Name </a> </h2>
        const newLine = "<" + headerType + "> <a href=\"#" + headerId + "\">" + headerText + "</a> </" + headerType + ">";
        sidebar.append(newLine);
    }

    sidebarLinks = $("#post__sidebar-left a").toArray();
    
    $(window).scroll(function () {
        let bottomScroll = $(window).scrollTop() + ($(window).height() / 2);
        let found = false;

        for (let i = headers.length - 1; i >= 0; i--) {
            const currentHeader = headers[i];
            const currentSidebarLink = sidebarLinks[i];
            $(currentSidebarLink).removeClass("current");

            const headerVPPosition = $(currentHeader).offset().top;

            if (!found && bottomScroll > headerVPPosition) {
                $(currentSidebarLink).addClass("current");
                found = true;
            }
        }
    });
});
!
999px

Console