React hooks – right way to clear timeouts and intervals

Defined return () => { /*code/* } function inside useEffect runs every time useEffect runs (except first render on component mount) and on component unmount (if you don’t display component any more).

This is a working way to use and clear timeouts or intervals:

Sandbox example.

import { useState, useEffect } from "react";

const delay = 5;

export default function App() {
  const [show, setShow] = useState(false);

  useEffect(
    () => {
      let timer1 = setTimeout(() => setShow(true), delay * 1000);

      // this will clear Timeout
      // when component unmount like in willComponentUnmount
      // and show will not change to true
      return () => {
        clearTimeout(timer1);
      };
    },
    // useEffect will run only one time with empty []
    // if you pass a value to array,
    // like this - [data]
    // than clearTimeout will run every time
    // this value changes (useEffect re-run)
    []
  );

  return show ? (
    <div>show is true, {delay}seconds passed</div>
  ) : (
    <div>show is false, wait {delay}seconds</div>
  );
}

If you need to clear timeouts or intervals in another component:

Sandbox example.

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

const delay = 1;

export default function App() {
  const [counter, setCounter] = useState(0);
  const timer = useRef(null); // we can save timer in useRef and pass it to child

  useEffect(() => {
    // useRef value stored in .current property
    timer.current = setInterval(() => setCounter((v) => v + 1), delay * 1000);

    // clear on component unmount
    return () => {
      clearInterval(timer.current);
    };
  }, []);

  return (
    <div>
      <div>Interval is working, counter is: {counter}</div>
      <Child counter={counter} currentTimer={timer.current} />
    </div>
  );
}

function Child({ counter, currentTimer }) {
  // this will clearInterval in parent component after counter gets to 5
  useEffect(() => {
    if (counter < 5) return;

    clearInterval(currentTimer);
  }, [counter, currentTimer]);

  return null;
}

Article from Dan Abramov.