Scroll-triggered Parallax Slider In JavaScript

An awesome image slider that slides smoothly between pictures when you scroll up or down the page.

It also applies a subtle parallax effect to images when they’re scrolled.

How to use it:

1. The HTML structure for the parallax slider.

<main>
  <div class="slider">
    <div class="slider-inner">
      <div class="item">
        <div class="image"></div>
      </div>
      <div class="item">
        <div class="image"></div>
      </div>
      <div class="item">
        <div class="image"></div>
      </div>
      <div class="item">
        <div class="image"></div>
      </div>
      <div class="item">
        <div class="image"></div>
      </div>
      <div class="item">
        <div class="image"></div>
      </div>
    </div>
  </div>
</main>

2. The necessary CSS styles.

main {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100vh;
}
.slider {
  position: absolute;
  top: 0;
  left: 0;
  width: 2800px;
  height: 100%;
}
.slider-inner {
  position: absolute;
  top: 15%;
  height: 70%;
  width: 100%;
  display: flex;
  justify-content: space-around;
}
.item {
  position: relative;
  width: 400px;
  height: 100%;
  overflow: hidden;
}
.image {
  position: absolute;
  left: -100px;
  width: 600px;
  height: 100%;
  background-size: cover;
  background-position: center;
}

3. Add background images to the parallax slider. Note that you can also add background images to each slide using CSS background property.

images.forEach((image, index) => {
  image.style.backgroundImage = `url(./images/${index+1}.jpg)`
})

4. The main JavaScript to activate the parallax slider.

let images = [...document.querySelectorAll('.image')];
let slider = document.querySelector('.slider');
let sliderWidth;
let imageWidth;
let current = 0;
let target = 0;
let ease = 0.05;
window.addEventListener('resize', init)
// Linear interpolation function
function lerp(start, end, t) {
  return start * (1-t) + end * t;
}
// Transform function
function setTransform(el, transform) {
  el.style.transform = transform;
}
// Init function
function init() {
  sliderWidth = slider.getBoundingClientRect().width;
  imageWidth = sliderWidth/images.length;
  document.body.style.height = `${sliderWidth - (window.innerWidth - window.innerHeight)}px`
}
// Animating slider
function animate() {
  current = parseFloat(lerp(current, target, ease)).toFixed(2);
  target = window.scrollY;
  setTransform(slider, `translateX(${-current}px)`)
  animateImages()
  requestAnimationFrame(animate)
}
// Animating images for parallax effect
function animateImages() {
 let ratio = current / imageWidth;
 let intersectionRatioValue;
 images.forEach((image, index) => {
   intersectionRatioValue = ratio - (index * 0.7)
   setTransform(image, `translateX(${intersectionRatioValue * 70}px)`)
 })
}
init()
animate()