React Scroll Agent

Tiny, 6 KB (pre-gzip) scrollspy component that performantly runs at 60 FPS thanks to requestAnimationFrame and @reach/observe-rect.

Install

npm i --save react-scroll-agent @reach/observe-rect

Example

import ScrollAgent from 'react-scroll-agent';

<ScrollAgent
  nav={({ current, positions }) => (
    <menu>
      <a href="#section-1" className={current === 0 ? 'is-active' : ''}>
        Section 1
      </a>
      <a href="#section-2" className={current === 1 ? 'is-active' : ''}>
        Section 2
      </a>
      <a
        onClick={() => window.scrollTo(0, positions[2])}
        className={current === 2 ? 'is-active' : ''}
      >
        Section 3
      </a>
    </menu>
  )}
  selector="section[data-scrollspy]"
  threshold="center"
>
  <section id="section-1" data-scrollspy>
    <h1>Section 1</h1>
  </section>
  <section id="section-2" data-scrollspy>
    <h1>Section 2</h1>
  </section>
  <section id="section-3" data-scrollspy>
    <h1>Section 3</h1>
  </section>
</ScrollAgent>;

Props

Name	Type	Required	Default	Description
selector	String	✅		Any CSS selector to specify which elements in children to attach the scrollspy to.
children	React Node			Standard child passthrough. This is where it watches for scroll events.
detectEnd	Boolean		true	If true, the last index will be highlighted when scrolled to the bottom. If false, then when scrolled to the bottom, current will return whichever container is currently at threshold.
nav	React Node			Render prop that returns current index in view and all positions of items.
threshold	String		"top"	Trigger point at which current watches. Accepts "top", "center", or "bottom" (if a specific threshold is needed, simply add padding to the top or bottom of a container).

Nav

The nav render prop returns the following items:

<ScrollAgent
  nav={({ current, positions }) => (
    <menu>
      <a href="#section-1" className={current === 0 ? 'is-active' : ''}>Section 1</a>
      <a
        href="#"
        onClick={e => {
          e.preventDefault();
          window.scrollTo(0, positions[1]);
        }}
        className={current === 1 ? 'is-active' : ''}
      >
        Section 2
      </a>
    </menu>
  )}

Name	Type	Description
current	Number	The index of the current item in view, in visual descending order on the page (may not necessarily be DOM order).
positions	[Number]	An array of all absolute Y values on the page, ordered from top to bottom. Useful for animating scroll to a certain item.

IIFE & ESM

This module ships with ESM and IIFE versions! To load it just request:

ESM (9 KB unminified)

import ScrollAgent from 'react-scroll-agent/esm';

IIFE (4 KB, for use in <script> tag)

<script src="//unpkg.com/react-scroll-agent/dist/iife.js"></script>

Notes

• If the first item isn’t in view, then current will return -1. This is expected, and allows more flexibility in styling. If you always want the first item to be highlighted, then check that current === 0 || current === -1.
• This component achieves performance by letting you handle all animations. If you need smooth scrolling from your nav, you can easily add another library to handle that using positions.
• requestAnimationFrame won’t fire more than 60FPS, so it’s a perfect native debouncing function for managing scroll events and reflows.
• This component won’t update current or positions unless it actually changes, preventing unnecessary re-renders in React.