React Hook recipe from https://usehooks.com. Demo: https://codesandbox.io/s/z64on3ypm

use-event-listener.js

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

// Usage
function App(){
  // State for storing mouse coordinates
  const [coords, setCoords] = useState({ x: 0, y: 0 });
  
  // Event handler utilizing useCallback ...
  // ... so that reference never changes.
  const handler = useCallback(
    ({ clientX, clientY }) => {
      // Update coordinates
      setCoords({ x: clientX, y: clientY });
    },
    [setCoords]
  );
  
  // Add event listener using our hook
  useEventListener('mousemove', handler);
  
  return (
    <h1>
      The mouse position is ({coords.x}, {coords.y})
    </h1>
  );
}

// Hook
function useEventListener(eventName, handler, element = window){
  // Create a ref that stores handler
  const savedHandler = useRef();
  
  // Update ref.current value if handler changes.
  // This allows our effect below to always get latest handler ...
  // ... without us needing to pass it in effect deps array ...
  // ... and potentially cause effect to re-run every render.
  useEffect(() => {
    savedHandler.current = handler;
  }, [handler]);

  useEffect(
    () => {
      // Make sure element supports addEventListener
      // On 
      const isSupported = element && element.addEventListener;
      if (!isSupported) return;
      
      // Create event listener that calls handler function stored in ref
      const eventListener = event => savedHandler.current(event);
      
      // Add event listener
      element.addEventListener(eventName, eventListener);
      
      // Remove event listener on cleanup
      return () => {
        element.removeEventListener(eventName, eventListener);
      };
    },
    [eventName, element] // Re-run if eventName or element changes
  );
};

I am wondering if the props like eventName,event and listener will not change then why run effects again. If useEventListener is run again why should we care about the new values, since the listener is already attached in the first place. What case am i missing?