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HTML

CSS

CSS Base

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JavaScript

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HTML

              
                <div g-component="SampleComponent">
  <div class="indicator">
    <div g-ref="indicatorItems" class="indicator-item">Section 1</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 2</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 3</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 4</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 5</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 6</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 7</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 8</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 9</div>
    <div g-ref="indicatorItems" class="indicator-item">Section 10</div>
  </div>
  <div g-ref="items" class="section">Section 1</div>
  <div g-ref="items" class="section">Section 2</div>
  <div g-ref="items" class="section">Section 3</div>
  <div g-ref="items" class="section">Section 4</div>
  <div g-ref="items" class="section">Section 5</div>
  <div g-ref="items" class="section">Section 6</div>
  <div g-ref="items" class="section">Section 7</div>
  <div g-ref="items" class="section">Section 8</div>
  <div g-ref="items" class="section">Section 9</div>
  <div g-ref="items" class="section">Section 10</div>
</div>


              
            
!

CSS

              
                .section
  height 200px
  background #ddd
  border none
  display flex
  justify-content center
  align-items center
  font-size 30px
  
  &:nth-child(odd)
    background #eee
    
.indicator
  position fixed
  font-size 16px
  
  &-item    
    border none
    animation blink .3s 
    
    &.is-in-viewport
      animation blink2 .3s 
      color #0a0
      
@keyframes blink
  0%
    color #f00
    
  100%
    color #000
    
@keyframes blink2
  0%
    color #f00
    
  30%
    color #f00  
    
  100%
    color #0a0
  
              
            
!

JS

              
                class SampleComponent extends Component {
    constructor(element) {
        super(element);

        this.ref = {
            items: [],
            indicatorItems: [],
        }
    }

    mount() {      
        this.scrollHandler = this.scroll.bind(this);
        this.resizeHandler = this.resize.bind(this);
        window.addEventListener('scroll', this.scrollHandler);
        window.addEventListener('resize', this.resizeHandler);

        this.setState({
            displaySections: {},
            borders: [],
        });

        this.resize();
    }

    resize(event) {
        // calculate item top/bottom borders
        const borders = this.ref.items.map(item => {
            const rect = item.getBoundingClientRect();
            return {
                top: rect.top + window.pageYOffset,
                bottom: rect.top + window.pageYOffset + item.offsetHeight
            }
        });

        this.setState({
            borders: borders,
        });
      
       this.scroll();
    }

    scroll() {
        let displaySections = {};
        this.state.borders.forEach((item, index) => {
            // is in viewport
            if (item.top < window.pageYOffset + window.innerHeight && item.bottom > window.pageYOffset) {
                displaySections[index] = true;
            } else {
                displaySections[index] = false;
            }
        });

        this.setState({
            displaySections: displaySections,
        });
    }

    stateChange(stateChanges) {
        if('displaySections' in stateChanges) {
            Object.keys(stateChanges.displaySections).forEach(key => {
                this.ref.indicatorItems[key].classList.toggle('is-in-viewport', stateChanges.displaySections[key]);
            });
        };
    }
}

loadComponents({ "SampleComponent": SampleComponent });
              
            
!
999px

Console