React useInterval hook

Implements setInterval() in a declarative manner.

• Create a custom hook that takes a callback and a delay.
• Use the useRef() hook to create a ref for the callback function.
• Use a useEffect() hook to remember the latest callback whenever it changes.
• Use a useEffect() hook dependent on delay to set up the interval and clean up.

const useInterval = (callback, delay) => {
  const savedCallback = React.useRef();

  React.useEffect(() => {
    savedCallback.current = callback;
  }, [callback]);

  React.useEffect(() => {
    const tick = () => {
      savedCallback.current();
    }
    if (delay !== null) {
      let id = setInterval(tick, delay);
      return () => clearInterval(id);
    }
  }, [delay]);
};

const Timer = props => {
  const [seconds, setSeconds] = React.useState(0);
  useInterval(() => {
    setSeconds(seconds + 1);
  }, 1000);

  return <p>{seconds}</p>;
};

ReactDOM.render(<Timer />, document.getElementById('root'));