Why is clearTimeout not clearing the timeout in this react component?

Question

I am attempting to clear a former timeout before initiating a new timeout, because I want messages to display for 4 seconds and disappear UNLESS a new message pops up before the 4 seconds is up. The Problem: Old timeouts are clearing the current message, so clearTimeout() is not working in this component, in this scenario:

  let t; // "t" for "timer"

  const [message, updateMessage] = useState('This message is to appear for 4 seconds. Unless a new message replaces it.');

  function clearLogger() {
    clearTimeout(t);
    t = setTimeout(() => {
      console.log('wiping message');
      updateMessage('');
    }, 4000);
  }

  function initMessage(msg) {
    updateMessage(msg);
    clearLogger();
  }

The funny thing is that this works:

  function clearLogger() {
    t = setTimeout(() => {
      console.log('wiping message');
      updateMessage('');
    }, 4000);
    clearTimeout(t);
  }

...but obviously defeats the purpose, since it just immediately obliterates the timeout. In practice, I should be able to trigger initMessage() every two seconds and never see, "wiping message' logged to the console.

Answer 1

The issue is that on every render the value of t is reset to null. Once you call updateMessage, it will trigger a re-render and will lose it's value. Any variables inside a functional react component get reset on every render (just like inside the render function of a class-based component). You need to save away the value of t using setState if you want to preserve the reference so you can call clearInterval.

However, another way to solve it is to promisify setTimeout. By making it a promise, you remove needing t because it won't resolve until setTimeout finishes. Once it's finished, you can updateMessage('') to reset message. This allows avoids the issue that you're having with your reference to t.

clearLogger = () => {
  return new Promise(resolve => setTimeout(() => updateMessage(''), resolve), 5000));
};

const initMessage = async (msg) => {
  updateMessage(msg);
  await clearLogger();
}

Answer 2

I solved this with useEffect. You want to clear the timeout in the return function

const [message, updateMessage] = useState(msg);

useEffect(() => {
  const t = setTimeout(() => {
    console.log('wiping message');
    updateMessage('');
  }, 4000);

  return () => {
    clearTimeout(t)
  }
}, [message])



function initMessage(msg) {
  updateMessage(msg);
}