React Hook recipe from https://usehooks.com. Demo: https://codesandbox.io/s/23jk7wlw4y

use-onclick-outside-example.jsx

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

// Usage
function App() {
  // Create a ref that we add to the element for which we want to detect outside clicks
  const ref = useRef();
  // State for our modal
  const [isModalOpen, setModalOpen] = useState(false);
  const openModal = useCallback(() => setModalOpen(true), []);
  const closeModal = useCallback(() => setModalOpen(false), []);
  
  // Call hook passing in the ref and a function to call on outside click
  useOnClickOutside(ref, closeModal);

  return (
    <div>
      {isModalOpen ? (
        <div ref={ref}>
          👋 Hey, I'm a modal. Click anywhere outside of me to close.
        </div>
      ) : (
        <button onClick={openModal}>Open Modal</button>
      )}
    </div>
  );
}

// Hook
function useOnClickOutside(ref, handler) {
  useEffect(() => {
    const listener = event => {
      // Do nothing if clicking ref's element or descendent elements
      if (!ref.current || ref.current.contains(event.target)) {
        return;
      }

      handler(event);
    };

    document.addEventListener('mousedown', listener);
    document.addEventListener('touchstart', listener);

    return () => {
      document.removeEventListener('mousedown', listener);
      document.removeEventListener('touchstart', listener);
    };
  }, [handler]); // Effect is only run on mount and when handler changes
}