React Hook recipe from https://usehooks.com. Demo: https://codesandbox.io/s/y7kr0vll4v

use-on-screen-example.jsx

import { useState, useEffect, useRef } from 'react';

// Usage
function App() {
  // Ref for the element that we want to detect whether on screen
  const ref = useRef();
  // Call the hook passing in ref and root margin
  // In this case it would only be considered onScreen if more ...
  // ... than 300px of element is visible.
  const onScreen = useOnScreen(ref, '-300px');

  return (
    <div>
      <div style={{ height: '100vh' }}>
        <h1>Scroll down to next section 👇</h1>
      </div>
      <div
        ref={ref}
        style={{
          height: '100vh',
          backgroundColor: onScreen ? '#23cebd' : '#efefef'
        }}
      >
        {onScreen ? (
          <div>
            <h1>Hey I'm on the screen</h1>
            <img src="https://i.giphy.com/media/ASd0Ukj0y3qMM/giphy.gif" />
          </div>
        ) : (
          <h1>Scroll down 300px from the top of this section 👇</h1>
        )}
      </div>
    </div>
  );
}

// Hook
function useOnScreen(ref, rootMargin = '0px') {
  // State and setter for storing whether element is visible
  const [isIntersecting, setIntersecting] = useState(false);

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        // Update our state when observer callback fires
        setIntersecting(entry.isIntersecting);
      },
      {
        rootMargin
      }
    );
    if (ref.current) {
      observer.observe(ref.current);
    }
    return () => {
      observer.unobserve(ref.current);
    };
  }, []); // Empty array ensures that effect is only run on mount and unmount

  return isIntersecting;
}

This is great! thanks :) 👍

One question on if (ref.current) - given the empty array on line 59, the effect only runs once on mount. Is it possible that ref.current is falsy, and the effect misses its only chance to observer.observe(ref.current)?

If yes - should the effect be run again at some point in the future?
If no - do we need the if (ref.current) gatecheck?

I made a couple small changes:

https://gist.github.com/ianobermiller/1146d469e22561f88a9b4d81ea477e4c

1. You can remove root entirely, since it defaults to the viewport anyway (also document.querySelector('body').current is always undefined, could be document.body but isn't needed anyway)
2. Rename margin to rootMargin for clarity and conciseness

If you pass [] as inputs you should cache ref.current in a local variable in the effect - otherwise you are risking a leak because the ref is mutable and can change over time - therefore u might call unobserve with the wrong element

@ianobermiller Good call, updated!

@mattfysh and @Andarist: Good points! So I'm thinking it would make sense to pass ref.current in input array so that effect is recalled if it changes and then wrap entire effect function body in if (ref.current) { ... } so that it does nothing if ref.current is falsy. Thoughts?

I agree. I do believe ref.current should be passed as argument of useEffect line 58. Currently if the ref changes, nothing happens and the previous is leaked.

You may also consider returning the ref from the hook instead of passing it as argument, c.f. https://usehooks.com/useHover/.

observer.unobserve(ref.current);

In the useEffect, we should keep a reference to the ref.current like this: const currentRef = ref.current.

Then do: observer.unobserve(currentRef);

In doing so, we will ensure that ref.current doesn't end up changing before this cleanup is run.