1.0.1 • Public • Published

io-Animate - Intersection Observer Animate

A pure, vanilla JavaScript, lightweight solution using Intersection Observer to animate HTML elements based on their presence in the viewport. This package comes with no built-in CSS animations, instead you can use any CSS animation library, or make your own custom animations. io-Animate simply applies those animations when the specified elements are visible. The polyfill for the Intersection Observer API is also included, so it should function properly on all devices and modern browsers.

I created this package after running into many issues with CSS animations not properly being triggered on scroll, particularily when using mobile devices or web browsers other than Chrome and Firefox. I found that using the Intersection Observer API was the most accurate when determining if an element was present in the viewport, across all devices and web browsers.

Getting Started


As io-Animate comes with no CSS animations of its own, you must have existing CSS animations. The default CSS prefix for animations is animated, which is based off of animate.css by Daniel Eden.

  <link rel="stylesheet" href="path/to/your/animations.css">


Install with npm:

npm install io-animate

or yarn:

yarn add io-animate


For explanation purposes, this documentation will assume that animate.css has been pre-loaded.


You must first initialize io-Animate inside your code, with a few configurable options:

import ioAnimate from 'io-animate'

  '.io-animate', // The document query selector for elements to be observed
  'animated', // The CSS prefix to add to elements when they are visible.
  false // a boolean that determines if the animations should only play once


Add the document query selector specified when initalizing io-Animate to the elements. In this example it is the CSS class io-animate

Each element must specify the animation CSS class to trigger. Optionally, you can add more classes for delay and speed times.

<div class="io-animate" data-ioa-in="fadeIn" data-ioa-out="fadeOut" data-ioa-delay="delay-1s" data-ioa-speed="slow">Once visible, this div will fade in slowly after a 1 second delay, and will fade out while scrolling out.</div>

There are 4 data attributes to configure:

Attribute Description Example
data-ioa-in Animation when the element becomes visible. fadeIn
data-ioa-out Animation when the element is not visible. fadeOut
data-ioa-delay CSS class used to add animation delay. delay-1s
data-ioa-speed CSS class that determines the animation speed. slow

Using every data attribute, the resulting CSS class of an element would look like this once visible:

<div class="io-animate animated fadeIn delay-2s slow">I will fade in.</div>


  • Wesley Moses - Initial work - wes337

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  • wes337