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;
}

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.