React, ComponentDidUpdate Hook logic

The useRef creates an "instance variable" in functional component. It acts as a flag to indicate whether it is in mount or update phase without updating state.

const mounted = useRef();

useEffect(() => {

  if (!mounted.current) {
    // do componentDidMount logic
    mounted.current = true;
  } else {
    // do componentDidUpdate logic
  }

	// written for cool kids
	(!canvasRef.current) 
      ? isMounted.current = true 
      : draw()
  
});

Credits