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Animate on scroll with Tailwind CSS

One of the requirements of a classic brochureware website these days is animated elements that fade into view as the visitor scrolls to the website. There are a number of Javascript libraries to help you do this, like Michał Sajnoóg’s AOS — Animate on Scroll Library… but how do you do this kind of thing with Tailwind CSS and as little JavaScript as you can get away with?!

Tailwind 1.6

Tailwind 1.6 introduced animation as a core plugin, so the first step is to make sure that you’re running the latest and greatest version of Tailwind. In your package.json file make sure your dependancies are up to date:

"dependencies": {
  "tailwindcss": "^1.6.0"
}

Remember to run npm install

Configuring Tailwind

We’re going to keep things simple, and apply a fade in on scroll on an element. To do this we need to jump into the tailwind.config.js file and extend our theme with the new animations. This is all wonderfully documented on the Tailwind CSS site. Read the animation documentation.

extend: {
  animation: {
    fadeIn: "fadeIn 2s ease-in forwards"
  },
  keyframes: {
    fadeIn: {
      "0%": { opacity: 0 },
      "100%": { opacity: 1 }
    }
  }

If you’ve written CSS animations before, you’ll recognise this syntax from CSS animations. As always the MDN documentation for CSS animations is a great place to start for a primer on this.

Here we’re defining the animation name, fadeIn, setting it to take 2 seconds, slowly easing in, and then run only once so the element doesn’t vanish straight away. The keyframes reference sets the start and end state, going from an opacity: 0 to opacity: 1.

Before we leave the tailwind.config.js one extra tweak to make is to add the motion-safe to the animation variants:

variants: {
    animation: ["motion-safe"]
}

You can read a little bit more about it on the pull request in the Tailwind repo, but the summary is that for accessibility users can opt-out of animations, and that is exposed by a the prefers-reduced-motion media query.

If you jump into your HTML you can add your new class behind the variant and get your new animation:

<div class="h-8 w-8 bg-blue-500 motion-safe:animate-fadeIn">
  FadeIn
</div>

Magic! Your element will fade in… but not on scroll. Not quite the dream. Into the Javascript!

Entering on scroll

There are a couple of ways to add or remove a class on an element as the user scrolls… adding an eventListener to the scroll event, running a loop over requestAnimationFrame, but my preference is to use the Intersection Observer API which to quote MDN:

The Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document’s viewport.

Or… simply put — check if the element in the viewport.

// Get all the elements you want to show on scroll
const targets = document.querySelectorAll(".js-show-on-scroll");
 
// Set up a new observer
const observer = new IntersectionObserver(callback);
 
// Loop through each of the target
targets.forEach(function(target) {
  // Hide the element
  target.classList.add("opacity-0");
 
  // Add the element to the watcher
  observer.observe(target);
});
 
// Callback for IntersectionObserver
const callback = function(entries) {
  entries.forEach(entry => {
 
    // Is the element in the viewport?
    if (entry.isIntersecting) {
 
      // Add the fadeIn class:
      entry.target.classList.add("motion-safe:animate-fadeIn");
    } else {
 
      // Otherwise remove the fadein class
      entry.target.classList.remove("motion-safe:animate-fadeIn");
    }
  });
};

Those comments should explain what is happening through the code, as the element enters or leaves the viewport the callback will be fired, and then the Tailwind class will be toggled accordingly.

Note: Older browsers will need a polyfill in place for the Intersection Observer, happily Poly​fill​.io has one built in for you to use. Thanks FT.

With that bit of Javascript in place, all that remains is for you to update your HTML:

<div class="h-8 w-8 bg-blue-200 js-show-on-scroll">
  FadeIn
</div>

See a demo on Codepen

Extending this…

There is a lot more you could do, from more complicated animations, through to customising which animations are used by using a data-attribute on the HTML.

With this method, it becomes customise which animation you are using in the JS (just make sure you whitelist the classes for PurgeCSS because you have enabled PurgeCSS right… right?!)

<div … data-animate-type="motion-safe:animate-fadeIn">…</div>
const animationType = entry.target.dataset.animateType;
 
if (entry.isIntersecting) {
    entry.target.classList.add(animationType);
}

Posted on Jul 31, 2020 by James Doc

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