Argument of type '() => () => Promise<void>' is not assignable to parameter of type 'EffectCallback'

Question

My code is:

    useEffect(() => {
        document.querySelector('body').style['background-color'] = 'rgba(173, 232, 244,0.2)';
        window.$crisp.push(['do', 'chat:hide']);

        return async () => {
            window.$crisp.push(['do', 'chat:show']);
            document.querySelector('body').style['background-color'] = '#ffffff';
            if (router.query.id) {
                const resDoc = await firebase.firestore().doc(`documents/${router.query.id}`).get();
                if (resDoc.exists) {
                    await clearDocPrediction(router.query.id);
                }
            }
        };
    }, []);

Now the reason I'm returning is because I believe that it performs function cleanup on unload, so I need to keep that. This is some sort of build error that I'm not quite sure how to resolve. Any help would be greatly appreciated.

Answer 1

Effect's callback function should return void, e.g.:

useEffect(() => { // callback; it returns void
  console.log("Hi")
}, [])

Or

It should return a "cleanup" function of type () => void, e.g.:

useEffect(() => {
  console.log("Hi")
  return () => { // cleanup function of type : () => void
    console.log("Cleanup")
  }
}, [])

You are getting the error because the cleanup function in your code is not of type () => void, but of type () => Promise<void>, e.g.:

Argument of type '() => () => Promise' is not assignable to parameter of type 'EffectCallback'. Type '() => Promise' is not assignable to type 'void | Destructor'.

useEffect(() => {
  console.log("Hi")
  return async () => { // WRONG
    console.log("Cleanup")
  }
}, [])

Hence, here is how you can refactor your "cleanup" function to fix it:

return () => { // Remove async from here
  const clear = async () => { // Create a new async function: clear
    // write your cleanup code here
  };
  clear() // Call the clear() function
};

Answer 2

The useEffect callback should have return type void. So you can't return a Promise:

useEffect(() => {
        document.querySelector('body').style['background-color'] = 'rgba(173, 232, 244,0.2)';
        window.$crisp.push(['do', 'chat:hide']);

        window.$crisp.push(['do', 'chat:show']);
        document.querySelector('body').style['background-color'] = '#ffffff';
        if (router.query.id) {
            firebase.firestore()
                .doc(`documents/${router.query.id}`)
                .get()
                .then((resDoc) => {
                    if (resDoc.exists) {
                        clearDocPrediction(router.query.id);
                    }
                });
        }
}, []);