Scroll Spy

A library for subscribing to scroll events in the browser. Runs all your events in one requestAnimationFrame.

Installation

$ npm install @earthpeople/scroll-spy

or

$ yarn add @earthpeople/scroll-spy

Usage

// some-component.js
import { startScrollSpy } from "@earthpeople/scroll-spy";

const subscribe = startScrollSpy();

let scrolledPast100 = false;

subscribe("scroll", event => {
  if (event.scrollTop > 100) {
    scrolledPast100 = true;
  }
});

startScrollSpy will only start the event loop if it's not already running, but it will always return the subscription function.

Events

Event	Returns	Fires
scroll	EventObject	On document scroll
scrollY	scrollTop, scrollDelta	On vertical scroll
scrollX	scrollLeft, scrollXDelta	On horizontal scroll
scrollEnd	EventObject	When document stops scrolling
scrollReachedEnd	atBottom, scrollTop, scrollDirection	When scroll reaches end of document
scrollReachedTop	atTop, scrollTop, scrollDirection	When scroll reaches top
scrollDirectionChange	EventObject	When scroll direction changes
resize	EventObject	On window resize
resizeEnd	EventObject	When window stops resizing

EventObject

key	type	value
scrollTop	number	Pixels from top
scrollLeft	number	Pixels from left
scrollDirection	string	"up" or "down"
atTop	bool	true if scroll position is 0 or less*
atBottom	bool	true if scroll position is full scroll height or more*
scrollHeight	number	Full scroll height of the document
scrollDelta	number	Difference in scroll since last tick
scrollXDelta	number	Difference in horizontal scroll since last tick
vh	number	Viewport height in pixels
vw	number	Viewport width in pixels
orientation	string	"landscape" or "portrait"

* This is to prevent false answers in browsers with elastic scroll, like Safari

Subscribing

startScrollSpy returns a subscribe function when called. Use that to add your listeners.

const subscribe = startScrollSpy();

You can also import the subscribe method directly.

import { subscribe } from '@earthpeople/scroll-spy`

You can listen to multiple event types in the same listener, but be aware that some event types only returns a select collection of values.

subscribe("scroll resize", ({ orientation }) => {
  console.log(orientation);
});

Unsubscribing

The subscribe method returns a new function when called. Use that function to unsubscribe.

const unsubscribe = subscribe("scrollReachedEnd", ({ scrollTop }) => {
  alert(`Wow! You scrolled ${scrollTop} pixels and reached the end`);

  // No need to listen to this event anymore
  unsubscribe();
});

You can also unsubscribe to all events or remove all listeners on a specific event

import { startScrollSpy, unsubscribeAll } from '@earthpeople/scroll-spy'

// add a few listeners
...

// unsubscribe from just the resize events
unsubscribeAll('resize')

// unsubscribe from ALL events
unsubscribeAll()

ES5

If you want to use a pre compiled es5-module, import from @earthpeople/scroll-spy/es5 instead.