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.

Have you tried calling `clearLogger()` before `updateMessage()`? — ChrisG, Sep 18 '19 at 15:23

score 8 · Accepted Answer · answered Sep 18 '19 at 15:27

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();
}

This answer reminded me that **t** was disappearing each render. I adjusted the functionality of my app slightly by eliminating some set-up I had in useEffect. useEffect indirectly ran **clearLogger**, so was running it ever re-render. — Gabriel Kunkel, Sep 18 '19 at 20:33

score 6 · Answer 2 · answered Mar 10 '21 at 13:37

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);
}

score 0 · Answer 3 · answered Sep 18 '19 at 15:29

Try execute set timeout after clearTimeout() completes

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

        });

function clearTimeout(param, callback) {
  //`enter code here`do stuff
}

Or you can use .then() as well.

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

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

3 Answers3

Linked