Simple scroll parallax

Recently, a designer asked me to add a parallax background image on a website. I researched the web to find a suitable solution. Regrettably, I found only massive libraries that would do the thing but bring in a lot of unnecessary javascript code. That is no go for me. I wanted a lightweight and straightforward solution. It took me a while, but I came up with the following code that works perfectly for my needs and hopefully will serve you well too. Check Codepen for a working example. 

The first thing we need is HTML and CSS

• DIV with the data-parallax attribute is a container that holds the parallax image and content
• DIV with the data-parallax-target attribute stands for the parallax image. It is positioned absolutely to the parent element and moves as a user scrolls thanks to javascript code (explained below). The key here is that the data-parallax-target element has the bottom property set to be higher than its parent element.
• DIV with the content class name is positioned relatively and contains everything that we want to render on top of the parallax image.

<div data-parallax="hey">
  <div data-parallax-target="hey"></div>
  <div class="content">
    <h1>Hello world</h1>
  </div>
</div>

[data-parallax] {
  position: relative;
  overflow: hidden;
}

[data-parallax-target] {
    position: absolute;
    top: 0;
    right: 0;
    left: 0;
    bottom: -200px;
    background-image: url('https://www.milanlund.com/img/splash/about-me--480.jpg');
    background-size: cover;
    background-position: center center;
    transform: translate3d(0, 0, 0);
    transition: transform .1s linear;
}

h1 {
  padding: 3em 1em;
}

.content {
  position: relative;
}

And of course a bit of javascript code

On the scroll event, the code checks whether the data-parallax element is in the viewport. If positive, the translate position gets computed for the data-parallax-target so that the image moves. For better performance, you can use a passive event listener for the scroll event.

(function () {
    var parallax = document.querySelectorAll('[data-parallax]');

    window.addEventListener('scroll', function () {
        for (var i = 0; i < parallax.length; i++) {
            var parallaxTarget = document.querySelector('[data-parallax-target="' + parallax[i].getAttribute('data-parallax') + '"]');
    
            var viewportOffset = parallax[i].getBoundingClientRect();
            var visibilityIndex = viewportOffset.top / window.innerHeight * 100;
    
            if (visibilityIndex >= 0 && visibilityIndex <= 100) {
                var parallaxOffset = parseInt(getComputedStyle(parallaxTarget).bottom);
                parallaxTarget.style.transform = 'translate3d(0, ' + Math.floor(parallaxOffset - (visibilityIndex / 100 * parallaxOffset)) + 'px, 0)';
            }
        }
    });
})();

Check Codepen for a working example.