react-intersection-observer

What is react-intersection-observer?

The react-intersection-observer package is a React implementation of the Intersection Observer API, making it easier to perform actions based on how an element intersects with the viewport. It's useful for lazy loading, infinite scroll, or triggering animations when the user scrolls.

What are react-intersection-observer's main functionalities?

Observing visibility of an element

This feature allows you to track the visibility of a component or element. The `useInView` hook returns a `ref` that you attach to the element you want to observe. It also returns a state `inView` that tells you whether the element is in the viewport or not.

import { useInView } from 'react-intersection-observer';

function Component() {
  const { ref, inView } = useInView();

  return (
    <div ref={ref}>
      {inView ? 'In view!' : 'Not in view!'}
    </div>
  );
}

Triggering animations when entering the viewport

This code sample demonstrates how to use the `useInView` hook to trigger animations or add classes to an element once it enters the viewport. The `threshold` option specifies how much of the element should be visible before `inView` becomes true.

import { useInView } from 'react-intersection-observer';
import { useEffect } from 'react';

function AnimatedComponent() {
  const { ref, inView } = useInView({ threshold: 0.1 });

  useEffect(() => {
    if (inView) {
      // Trigger animation or add class
    }
  }, [inView]);

  return <div ref={ref}>Animate me!</div>;
}

Other packages similar to react-intersection-observer

react-lazy-load-image-component

This package is designed specifically for lazy loading images in React applications. It provides components like `LazyLoadImage` for easy integration. While it's focused on images, react-intersection-observer offers a more general approach for observing any element's visibility.

react-waypoint

React Waypoint is another package for handling the visibility of elements during scroll. It triggers a function when you scroll to an element. Compared to react-intersection-observer, it's less flexible with the observer options but still a solid choice for simple use cases.

README

react-intersection-observer

React implementation of the Intersection Observer API to tell you when an element enters or leaves the viewport. Contains both a Hooks, render props and plain children implementation.

Storybook Demo: https://react-intersection-observer.vercel.app

Features

• 🎣 Hooks or Component API - With useInView it's easier than ever to monitor elements
• ⚡️ Optimized performance - Reuses Intersection Observer instances where possible
• ⚙️ Matches native API - Intuitive to use
• 🧪 Ready to test - Mocks the Intersection Observer for easy testing with Jest
• 🌳 Tree-shakeable - Only include the parts you use
• 💥 Tiny bundle ~1.8 kB gzipped

Installation

Install using Yarn:

yarn add react-intersection-observer

or NPM:

npm install react-intersection-observer --save

Usage

useInView hook

// Use object destructing, so you don't need to remember the exact order
const { ref, inView, entry } = useInView(options);

// Or array destructing, making it easy to customize the field names
const [ref, inView, entry] = useInView(options);

The useInView hook makes it easy to monitor the inView state of your components. Call the useInView hook with the (optional) options you need. It will return an array containing a ref, the inView status and the current entry. Assign the ref to the DOM element you want to monitor, and the hook will report the status.

import React from 'react';
import { useInView } from 'react-intersection-observer';

const Component = () => {
  const { ref, inView, entry } = useInView({
    /* Optional options */
    threshold: 0,
  });

  return (
    <div ref={ref}>
      <h2>{`Header inside viewport ${inView}.`}</h2>
    </div>
  );
};

Render props

To use the <InView> component, you pass it a function. It will be called whenever the state changes, with the new value of inView. In addition to the inView prop, children also receive a ref that should be set on the containing DOM element. This is the element that the IntersectionObserver will monitor.

If you need it, you can also access the IntersectionObserverEntry on entry, giving you access to all the details about the current intersection state.

import { InView } from 'react-intersection-observer';

const Component = () => (
  <InView>
    {({ inView, ref, entry }) => (
      <div ref={ref}>
        <h2>{`Header inside viewport ${inView}.`}</h2>
      </div>
    )}
  </InView>
);

export default Component;

Plain children

You can pass any element to the <InView />, and it will handle creating the wrapping DOM element. Add a handler to the onChange method, and control the state in your own component. Any extra props you add to <InView> will be passed to the HTML element, allowing you set the className, style, etc.

import { InView } from 'react-intersection-observer';

const Component = () => (
  <InView as="div" onChange={(inView, entry) => console.log('Inview:', inView)}>
    <h2>Plain children are always rendered. Use onChange to monitor state.</h2>
  </InView>
);

export default Component;

⚠️ When rendering a plain child, make sure you keep your HTML output semantic. Change the as to match the context, and add a className to style the <InView />. The component does not support Ref Forwarding, so if you need a ref to the HTML element, use the Render Props version instead.

API

Options

Provide these as the options argument in the useInView hook or as props on the <InView /> component.

Name	Type	Default	Required	Description
root	Element	document	false	The IntersectionObserver interface's read-only root property identifies the Element or Document whose bounds are treated as the bounding box of the viewport for the element which is the observer's target. If the root is null, then the bounds of the actual document viewport are used.
rootMargin	string	'0px'	false	Margin around the root. Can have values similar to the CSS margin property, e.g. "10px 20px 30px 40px" (top, right, bottom, left).
threshold	number | number[]	0	false	Number between 0 and 1 indicating the percentage that should be visible before triggering. Can also be an array of numbers, to create multiple trigger points.
trackVisibility 🧪	boolean	false	false	A boolean indicating whether this IntersectionObserver will track visibility changes on the target.
delay 🧪	number	undefined	false	A number indicating the minimum delay in milliseconds between notifications from this observer for a given target. This must be set to at least 100 if trackVisibility is true.
skip	boolean	false	false	Skip creating the IntersectionObserver. You can use this to enable and disable the observer as needed. If skip is set while inView, the current state will still be kept.
triggerOnce	boolean	false	false	Only trigger the observer once.
initialInView	boolean	false	false	Set the initial value of the inView boolean. This can be used if you expect the element to be in the viewport to start with, and you want to trigger something when it leaves.
fallbackInView	boolean	undefined	false	If the IntersectionObserver API isn't available in the client, the default behavior is to throw an Error. You can set a specific fallback behavior, and the inView value will be set to this instead of failing. To set a global default, you can set it with the defaultFallbackInView()

InView Props

The <InView /> component also accepts the following props:

Name	Type	Default	Required	Description
as	string	'div'	false	Render the wrapping element as this element. Defaults to div.
children	({ref, inView, entry}) => React.ReactNode, ReactNode		true	Children expects a function that receives an object containing the inView boolean and a ref that should be assigned to the element root. Alternatively pass a plain child, to have the <InView /> deal with the wrapping element. You will also get the IntersectionObserverEntry as `entry, giving you more details.
onChange	(inView, entry) => void		false	Call this function whenever the in view state changes. It will receive the inView boolean, alongside the current IntersectionObserverEntry.

IntersectionObserver v2 🧪

The new v2 implementation of IntersectionObserver extends the original API, so you can track if the element is covered by another element or has filters applied to it. Useful for blocking clickjacking attempts or tracking ad exposure.

To use it, you'll need to add the new trackVisibility and delay options. When you get the entry back, you can then monitor if isVisible is true.

const TrackVisible = () => {
  const { ref, entry } = useInView({ trackVisibility: true, delay: 100 });
  return <div ref={ref}>{entry?.isVisible}</div>;
};

This is still a very new addition, so check caniuse for current browser support. If trackVisibility has been set, and the current browser doesn't support it, a fallback has been added to always report isVisible as true.

It's not added to the TypeScript lib.d.ts file yet, so you will also have to extend the IntersectionObserverEntry with the isVisible boolean.

Recipes

The IntersectionObserver itself is just a simple but powerful tool. Here's a few ideas for how you can use it.

• Lazy image load
• Trigger animations
• Track impressions (Google Analytics, Tag Manager, etc)

FAQ

How can I assign multiple refs to a component?

You can wrap multiple ref assignments in a single useCallback:

import React, { useRef, useCallback } from 'react';
import { useInView } from 'react-intersection-observer';

function Component(props) {
  const ref = useRef();
  const [inViewRef, inView] = useInView();

  // Use `useCallback` so we don't recreate the function on each render - Could result in infinite loop
  const setRefs = useCallback(
    (node) => {
      // Ref's from useRef needs to have the node assigned to `current`
      ref.current = node;
      // Callback refs, like the one from `useInView`, is a function that takes the node as an argument
      inViewRef(node);
    },
    [inViewRef],
  );

  return <div ref={setRefs}>Shared ref is visible: {inView}</div>;
}

rootMargin isn't working as expected

When using rootMargin, the margin gets added to the current root - If your application is running inside a <iframe>, or you have defined a custom root this will not be the current viewport.

You can read more about this on these links:

• Intersection Observer API
• w3c/IntersectionObserver: IntersectionObserver rootMargin ignored within iframe
• w3c/IntersectionObserver: Cannot track intersection with an iframe's viewport
• w3c/Support iframe viewport tracking

Testing

In order to write meaningful tests, the IntersectionObserver needs to be mocked. If you are writing your tests in Jest, you can use the included test-utils.js. It mocks the IntersectionObserver, and includes a few methods to assist with faking the inView state. When setting the isIntersecting value you can pass either a boolean value or a threshold between 0 and 1.

test-utils.js

You can use these test utilities as imports in individual files OR you can globally mock Intersection Observer for all Jest tests. If you use a library or an application with a lot of Intersection Observer usage, you may wish to globally mock it; however, the official recommendation is to be purposeful about your mocking and do so on a per-usage basis.

Indvidual Methods

Import these from react-intersection-observer/test-utils.

mockAllIsIntersecting(isIntersecting:boolean | number)
Set isIntersecting on all current IntersectionObserver instances.

mockIsIntersecting(element:Element, isIntersecting:boolean | number)
Set isIntersecting for the IntersectionObserver of a specific element.

intersectionMockInstance(element:Element): IntersectionObserver
Call the intersectionMockInstance method with an element, to get the (mocked) IntersectionObserver instance. You can use this to spy on the observe and unobserve methods.

Global Intersection Observer Behavior

Use Fallback

You can create a Jest setup file that leverages the unsupported fallback option. In this case, you only mock the IntersectionObserver in test files were you actively import react-intersection-observer/test-utils:

import { defaultFallbackInView } from 'react-intersection-observer';

defaultFallbackInView(true); // or 'false' - whichever consistent behavior makes the most sense for your use case.

Mock Everywhere

In your Jest config, add 'react-intersection-observer/test-utils' to the array value for the setupFilesAfterEnv option.

module.exports = {
  // other config lines
  setupFilesAfterEnv: ['react-intersection-observer/test-utils'],
  // other config lines
};

Test Example

import React from 'react';
import { screen, render } from '@testing-library/react';
import { useInView } from 'react-intersection-observer';
import { mockAllIsIntersecting } from 'react-intersection-observer/test-utils';

const HookComponent = ({ options }) => {
  const [ref, inView] = useInView(options);
  return <div ref={ref}>{inView.toString()}</div>;
};

test('should create a hook inView', () => {
  render(<HookComponent />);

  // This causes all (existing) IntersectionObservers to be set as intersecting
  mockAllIsIntersecting(true);
  screen.getByText('true');
});

test('should create a hook inView with threshold', () => {
  render(<HookComponent options={{ threshold: 0.3 }} />);

  mockAllIsIntersecting(0.1);
  screen.getByText('false');

  // Once the threshold has been passed, it will trigger inView.
  mockAllIsIntersecting(0.3);
  screen.getByText('true');
});

Intersection Observer

Intersection Observer is the API used to determine if an element is inside the viewport or not. Browser support is really good - With Safari adding support in 12.1, all major browsers now support Intersection Observers natively. Add the polyfill, so it doesn't break on older versions of iOS and IE11.

Unsupported fallback

If the client doesn't have support for the IntersectionObserver, then the default behavior is to throw an error. This will crash the React application, unless you capture it with an Error Boundary.

If you prefer, you can set a fallback inView value to use if the IntersectionObserver doesn't exist. This will make react-intersection-observer fail gracefully, but you must ensure your application can correctly handle all your observers firing either true or false at the same time.

You can set the fallback globally:

import { defaultFallbackInView } from 'react-intersection-observer';
defaultFallbackInView(true); // or 'false'

You can also define the fallback locally on useInView or <InView> as an option. This will override the global fallback value.

import React from 'react';
import { useInView } from 'react-intersection-observer';

const Component = () => {
  const { ref, inView, entry } = useInView({
    fallbackInView: true,
  });

  return (
    <div ref={ref}>
      <h2>{`Header inside viewport ${inView}.`}</h2>
    </div>
  );
};

Polyfill

You can import the polyfill directly or use a service like polyfill.io to add it when needed.

yarn add intersection-observer

Then import it in your app:

import 'intersection-observer';

If you are using Webpack (or similar) you could use dynamic imports, to load the Polyfill only if needed. A basic implementation could look something like this:

/**
 * Do feature detection, to figure out which polyfills needs to be imported.
 **/
async function loadPolyfills() {
  if (typeof window.IntersectionObserver === 'undefined') {
    await import('intersection-observer');
  }
}

Low level API

You can access the observe method, that react-intersection-observer uses internally to create and destroy IntersectionObserver instances. This allows you to handle more advanced use cases, where you need full control over when and how observers are created.

import { observe } from 'react-intersection-observer';
const destroy = observe(element, callback, options);

Name	Type	Required	Description
element	Element	true	DOM element to observe
callback	ObserverInstanceCallback	true	The callback function that IntersectionObserver will call
options	IntersectionObserverInit	false	The options for the IntersectionObserver

The observe method returns an unobserve function, that you must call in order to destroy the observer again.

⚠️ You most likely won't need this, but it can be useful if you need to handle IntersectionObservers outside React, or need full control over how instances are created.