eventEmitter.addEventListener returns a unsubscribe function to remove the event listeners. useEffect hook calls the passed function's return value if it is not

eventEmitter.addEventListener returns a unsubscribe function to remove the event listeners. useEffect hook calls the passed function's return value if it is not a void. https://reactjs.org/docs/hooks-effect.html#example-using-hooks-1 https://reactjs.org/docs/hooks-effect.html#example-using-hoo... So my eventEmitter.addEventListener method in the blog post returns an ubsubscriber when you call it :)