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Showing content from https://github.com/thebuilder/react-intersection-observer below:

thebuilder/react-intersection-observer: React implementation of the Intersection Observer API to tell you when an element enters or leaves the viewport.

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

• 🪝 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
• 🛠 Written in TypeScript - It'll fit right into your existing TypeScript project
• 🧪 Ready to test - Mocks the Intersection Observer for easy testing with Jest or Vitest
• 🌳 Tree-shakeable - Only include the parts you use
• 💥 Tiny bundle - Around ~1.15kB for useInView and ~1.6kB for <InView>

Install the package with your package manager of choice:

npm install react-intersection-observer --save

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

// Or array destructuring, 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>
  );
};

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 Intersection Observer 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;

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;

Note

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.

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

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

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.

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.)

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 { ref: inViewRef, inView } = useInView();

  // Use `useCallback` so we don't recreate the function on each render
  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>;
}

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

Tip

Consider using Vitest Browser Mode instead of jsdom or happy-dom. This option allows you to utilize the real browser implementation and triggers correctly when scrolling or adding elements to the viewport. You can skip the react-intersection-observer/test-utils, or use it as needed.

In order to write meaningful tests, the IntersectionObserver needs to be mocked. You can use the included react-intersection-observer/test-utils to help with this. 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. It will emulate the real IntersectionObserver, allowing you to validate that your components are behaving as expected.

This library comes with built-in support for writing tests in both Jest and Vitest

Testing with Jest should work out of the box. Just import the react-intersection-observer/test-utils in your test files, and you can use the mocking methods.

If you're running Vitest with globals, then it'll automatically mock the IntersectionObserver, just like running with Jest. Otherwise, you'll need to manually setup/reset the mocking in either the individual tests, or a setup file.

import { vi, beforeEach, afterEach } from "vitest";
import {
  setupIntersectionMocking,
  resetIntersectionMocking,
} from "react-intersection-observer/test-utils";

beforeEach(() => {
  setupIntersectionMocking(vi.fn);
});

afterEach(() => {
  resetIntersectionMocking();
});

You only need to do this if the test environment does not support beforeEach globally, alongside either jest.fn or vi.fn.

See the instructions for Vitest. You should be able to use a similar setup/reset code, adapted to the testing library you are using. Failing that, copy the code from test-utils.ts, and make your own version.

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

test-setup.js

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

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

Alternatively, you can mock the Intersection Observer in all tests with a global setup file. Add react-intersection-observer/test-utils to setupFilesAfterEnv in the Jest config, or setupFiles in Vitest.

module.exports = {
  setupFilesAfterEnv: ["react-intersection-observer/test-utils"],
};

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

const HookComponent = ({ options }) => {
  const { ref, inView } = useInView(options);
  return (
    <div ref={ref} data-testid="wrapper">
      {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");
});

test("should mock intersecing on specific hook", () => {
  render(<HookComponent />);
  const wrapper = screen.getByTestId("wrapper");

  // Set the intersection state on the wrapper.
  mockIsIntersecting(wrapper, 0.5);
  screen.getByText("true");
});

test("should create a hook and call observe", () => {
  const { getByTestId } = render(<HookComponent />);
  const wrapper = getByTestId("wrapper");
  // Access the `IntersectionObserver` instance for the wrapper Element.
  const instance = intersectionMockInstance(wrapper);

  expect(instance.observe).toHaveBeenCalledWith(wrapper);
});

Intersection Observer is the API used to determine if an element is inside the viewport or not. Browser support is excellent - 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.

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>
  );
};

You can import the polyfill directly or use a service like https://cdnjs.cloudflare.com/polyfill 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");
  }
}

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);

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

Important

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.

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