@newswire/scroller

@newswire/scroller is a super-tiny library for your scrollytelling needs.

Key features

• 🐜 Less than 600 bytes gzipped
• 👀 Uses a highly-performant Intersection Observer to monitor scrolling changes
• 🙅🏽‍ No dependencies (unless you need an Intersection Observer polyfill - check what browsers you support!)

Examples

• Basic
• Sticky graphic

Installation

@newswire/scroller is available via npm.

npm install @newswire/scroller

You can also use it directly via unpkg.com.

<script src="https://unpkg.com/@newswire/scroller/dist/index.umd.js"></script>
<!-- Now available at `window.Scroller` -->

You can also import it as a module via unpkg!

<script type="module">
  import Scroller from 'https://unpkg.com/@newswire/scroller/dist/index.mjs';

  const scroller = new Scroller({
    scenes: document.querySelectorAll('.scene'),
  });
</script>

Usage

Assume for the following examples that our HTML is as follows:

<div class="container">
  <div class="scene"></div>
  <div class="scene"></div>
  <div class="scene"></div>
  <div class="scene"></div>
  <div class="scene"></div>
</div>

To begin tracking the progression of the scenes, we need to set up our Scroller.

import Scroller from '@newswire/scroller';

// sets up the scroller instance, pass in an array of all the scenes
const scroller = new Scroller({
  scenes: document.querySelectorAll('.scene'),
});

// Scroller has a tiny event emitter embedded in it!

// the `enter` event is triggered every time a scene crosses the threshold
scroller.on('scene:enter', d => {
  d.element.classList.add('active');
});

// the `exit` event is triggered every time a scene exits the threshold
scroller.on('scene:exit', d => {
  d.element.classList.remove('active');
});

scroller.on('init', () => {
  console.log('Everything is ready to go!');
});

// starts up the IntersectionObserver
scroller.init();

If you need to additionally track the "container" of your scenes, Scroller can track and alert you about that too.

import Scroller from '@newswire/scroller';

// sets up the scroller instance, pass in the container and an array of all the scenes
const scroller = new Scroller({
  container: document.querySelector('.container'),
  scenes: document.querySelectorAll('.scene'),
});

// the `enter` event works as you expect with scenes...
scroller.on('scene:enter', d => {
  d.element.classList.add('active');
});

scroller.on('scene:exit', d => {
  d.element.classList.remove('active');
});

// ...but if a container is passed, events fire for it as well!
scroller.on('container:enter', d => {
  console.log("Let's go!");
});

scroller.on('container:exit', d => {
  console.log('We are done here.');
});

scroller.init();

Known quirks

Intersection Observer pre-check

This is less a quirk (and in some ways a feature) and more how Intersection Observer works. Whenever an element gets added to an Intersection Observer instance, it is immediately checked for intersection. In the context of Scroller, this means that if it is instantiated on load of a page and none of its elements are currently intersecting, scene:exit (and possibly container:exit if you provided one) are going to all fire as they fail that initial check. The good thing is this is also true for the *:enter events, so nothing special is necessary for detecting if someone loads in the middle of your interactive. Make sure the code in your event listeners are prepared for this and have some way to determine whether anything in your *:exit listeners are needed yet!

Intersection Observer polyfill?

To keep the library lean, @newswire/scroller intentionally does not attempt to polyfill IntersectionObserver and leaves that task up to the user if necessary. Browser support is pretty good! There's no hard and fast rule here - check what browsers your users are using and make a call.

There are a few good ways to ensure the polyfill is in place.

Load it within your bundle

The spec-based Intersection Observer polyfill is available on npm.

npm install intersection-observer

Once that is installed, you need to make sure that it is loaded before any code that'd depends on it will run. The polyfill is smart - it won't affect browsers who already have support!

import 'intersection-observer';
// or
require('intersection-observer');

// your awesome code here!

Load it with polyfill.io

Before loading your scripts, you can include a link to polyfill.io. This service uses signals from the browser to determine what polyfills are needed and loads them in the environment. You can set flags on the URL to limit what polyfill.io attempts to load.

<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
<script src="<your-code>"></script>

Load it with unpkg.com

This is similar to the polyfill.io method, but without a service in-between determining whether it is necessary. (Every browser will get the code! But it still checks if the browser supports IntersectionObserver before activating.)

<script src="https://unpkg.com/intersection-observer/intersection-observer"></script>
<script src="<your-code>"></script>

API

Table of Contents

• Scroller
  • Parameters
  • Properties
  • Examples
  • on
    • Parameters
    • Examples
  • off
    • Parameters
    • Examples
  • init
    • Examples
• Scroller#container:enter
  • Properties
• Scroller#scene:enter
  • Properties
• Scroller#container:exit
  • Properties
• Scroller#scene:exit
  • Properties
• Scroller#init

Scroller

Uses Intersection Observer to monitor the page location of a series of elements for scrollytelling.

Parameters

• options object
  • options.container Element? Optionally pass in what should be considered the containing element of all the scenes - this gets added to the Intersection Observer instance and additionally fires its own events
  • options.offset Number? How far from the top/bottom of the viewable area to trigger enters/exits of scenes, represented as a value between 0 and 1 (optional, default 0.5)
  • options.scenes Array<Element> An array of all the Elements to be considered scenes of this Scroller

Properties

• observer (IntersectionObserver | null) Once initialized, a reference to the Scroller's instance of IntersectionObserver

Examples

import Scroller from '@newswire/scroller';

const scroller = new Scroller({
  scenes: document.querySelectorAll('.scenes'),
});

scroller.init();

on

Adds a callback to the queue of a given event listener.

Parameters

• type string Name of the event
• handler Function Callback function added to the listener

Examples

const scroller = new Scroller({
  scenes: document.querySelectorAll('.scenes')
});

const fn = (...) => {...};

// adds callback to listener
scroller.on('scene:enter', fn);

Returns void

off

Removes a callback from the queue of a given event listener.

Parameters

• type string Name of the event
• handler Function Callback function removed from the listener

Examples

const scroller = new Scroller({
  scenes: document.querySelectorAll('.scenes')
});

const fn = (...) => {...};

// adds callback to listener
scroller.on('scene:enter', fn);

// removes callback from listener
scroller.off('scene:enter', fn);

Returns void

init

Initializes a Scroller's IntersectionObserver on a page and begins sending any intersection events that occur.

Examples

const scroller = new Scroller({
  scenes: document.querySelectorAll('.scenes'),
});

scroller.init();

Returns void

Scroller#container:enter

Container enter event. Fires whenever the container begins intersecting.

Type: object

Properties

• bounds DOMRectReadOnly The bounds of the active element
• element Element The element that intersected
• index number This is always -1 on the container
• isScrollingDown boolean Whether the user triggered this element while scrolling down or not

Scroller#scene:enter

Scene enter event. Fires whenever a scene begins intersecting.

Type: object

Properties

• bounds DOMRectReadOnly The bounds of the active element
• element Element The element that intersected
• index number The index of the active element
• isScrollingDown boolean Whether the user triggered this element while scrolling down or not

Scroller#container:exit

Container exit event. Fires whenever the container has exited.

Type: object

Properties

• bounds DOMRectReadOnly The bounds of the exiting element
• element Element The element that exited
• index number This is always -1 on the container
• isScrollingDown boolean Whether the user triggering the exit while scrolling down or not

Scroller#scene:exit

Scene enter event. Fires whenever a scene has exited.

Type: object

Properties

• bounds DOMRectReadOnly The bounds of the exiting element
• element Element The element that exited
• index number The index of the exiting element
• isScrollingDown boolean Whether the user triggering the exit while scrolling down or not

Scroller#init

Init event. Fires once Scroller has finished setting up.

License

MIT