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How to create Javascript Scrollspy (Vanilla Js Tutorial)

Nikhil Chandra Roy
Front-End Web Developer with JavaScript, ReactJs and Backend Python(Django)
・3 min read

Hi guys,
In this tutorial we are going to create javascript scrollspy with vanilla js.

HTML Structure

<menu>
  <ul>
    <li class="active"> <a href="#section1"> Section 1 </a> </li>
    <li> <a href="#section2"> Section 2 </a> </li>
    <li> <a href="#section3"> Section 3 </a> </li>
    <li> <a href="#section4"> Section 4 </a> </li>
    <li> <a href="#section5"> Section 5 </a> </li>

    </ul>

  </menu>

<main>
  <section id="section1"> 
  <h1> Section 1 </h1>
    <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. </p>

  </section>

  <section id="section2"> 
  <h1> Section 2 </h1>
    <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. </p>

  </section>

  <section id="section3"> 
  <h1> Section 3 </h1>
    <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. </p>

  </section>

  <section id="section4"> 
  <h1> Section 4 </h1>
    <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. </p>

  </section>

  <section id="section5"> 
  <h1> Section 5 </h1>
    <p> Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.</p>

  </section>

  </main>
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We have created html structure and now we are going to design it by css.

Css Design(SCSS)

:root{
  scroll-behavior: smooth;
}
body{
  display: flex;
  flex-wrap: wrap;
  @mixin mp-0{
    margin: 0;
    padding: 0;
  }
  @include mp-0;
  menu{

    $w: 200px;
    display: block;
    @include mp-0;
    $h: 100vh;
    width: $w;
    ul{
      @include mp-0;
      margin: 0;
      padding: 0;
      list-style: none;
      position: fixed;
      width: $w;
      height: $h;
      background: #ccc;
      top: 0;
      left: 0;
      li{
        transition: .3s;
        padding: .5rem;
       &.active{
          background: deeppink;
         a{
           color: white;
         }

        }
        a{
        color: #222;
        text-decoration: none;

        }

      }
    }
  }

  main{
    width: 100%;
  @include mp-0;
  margin-left: 200px;
    padding: .5rem;
    section{
      padding:1rem;
      background: #eee;
      line-height: 28px;
      box-shadow: 0 3px 7px #222;
      margin-bottom: .5rem;
      h1{
      text-align: center;
    }
    }

  }


}


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and

Javascript

let menuSection = document.querySelectorAll('menu li');

// for clickable event
menuSection.forEach(v=> {
  v.onclick = (()=> {
   setTimeout(()=> {
      menuSection.forEach(j=> j.classList.remove('active'))
    v.classList.add('active')
  },300)
   })
})

// for window scrolldown event

window.onscroll = (()=> {
  let mainSection = document.querySelectorAll('main section');

  mainSection.forEach((v,i)=> {
    let rect = v.getBoundingClientRect().y
    if(rect < window.innerHeight-200){
      menuSection.forEach(v=> v.classList.remove('active'))
      menuSection[i].classList.add('active')
    }
  })
})
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This is our simple scrollspy tutorial.
for codpen

Discussion (3)

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yellow1912 profile image
yellow1912

You should use the new observer api instead for better performance. If you use scroll event perhaps consider denouncing it?

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nikhilroy2 profile image
Nikhil Chandra Roy Author

hi, can you explain a little more what do you mean new observer api here?

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yellow1912 profile image
yellow1912

I mean this one:

developer.mozilla.org/en-US/docs/W...

It's a better way to detect when an element comes into view.

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