score:49

Accepted answer

Sharing the AbortController between the fetch() requests is the right approach.
When any of the Promises are aborted, Promise.all() will reject with AbortError:

function Component(props) {
  const [fetched, setFetched] = React.useState(false);
  React.useEffect(() => {
    const ac = new AbortController();
    Promise.all([
      fetch('http://placekitten.com/1000/1000', {signal: ac.signal}),
      fetch('http://placekitten.com/2000/2000', {signal: ac.signal})
    ]).then(() => setFetched(true))
      .catch(ex => console.error(ex));
    return () => ac.abort(); // Abort both fetches on unmount
  }, []);
  return fetched;
}
const main = document.querySelector('main');
ReactDOM.render(React.createElement(Component), main);
setTimeout(() => ReactDOM.unmountComponentAtNode(main), 1); // Unmount after 1ms
<script src="//cdnjs.cloudflare.com/ajax/libs/react/16.8.3/umd/react.development.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.3/umd/react-dom.development.js"></script>
<main></main>

score:-1

The easy way

    let fetchingFunction= async()=>{
      // fetching
    }

React.useEffect(() => {
    fetchingFunction();
    return () => {
        fetchingFunction= null
    }
}, [])

score:-1

options={{ filterType: "checkbox" , textLabels: { body: { noMatch: isLoading ? : 'Sorry, there is no matching data to display', }, }, }}

score:-1

useEffect(() => {
    const abortController = new AbortController();
MyFunction()
    return () => {
      abortController.abort();
    };
  }, []);

score:0

If the user navigates away, or something else causes the component to get destroyed before the async call comes back and tries to setState on it, it will cause the error. It's generally harmless if it is, indeed, a late-finish async call. There's a couple of ways to silence the error.

If you're implementing a hook like useAsync you can declare your useStates with let instead of const, and, in the destructor returned by useEffect, set the setState function(s) to a no-op function.


export function useAsync<T, F extends IUseAsyncGettor<T>>(gettor: F, ...rest: Parameters<F>): IUseAsync<T> {
  let [parameters, setParameters] = useState(rest);
  if (parameters !== rest && parameters.some((_, i) => parameters[i] !== rest[i]))
    setParameters(rest);

  const refresh: () => void = useCallback(() => {
    const promise: Promise<T | void> = gettor
      .apply(null, parameters)
      .then(value => setTuple([value, { isLoading: false, promise, refresh, error: undefined }]))
      .catch(error => setTuple([undefined, { isLoading: false, promise, refresh, error }]));
    setTuple([undefined, { isLoading: true, promise, refresh, error: undefined }]);
    return promise;
  }, [gettor, parameters]);

  useEffect(() => {
    refresh();
    // and for when async finishes after user navs away //////////
    return () => { setTuple = setParameters = (() => undefined) } 
  }, [refresh]);

  let [tuple, setTuple] = useState<IUseAsync<T>>([undefined, { isLoading: true, refresh, promise: Promise.resolve() }]);
  return tuple;
}

That won't work well in a component, though. There, you can wrap useState in a function which tracks mounted/unmounted, and wraps the returned setState function with the if-check.

export const MyComponent = () => {
  const [numPendingPromises, setNumPendingPromises] = useUnlessUnmounted(useState(0));
  // ..etc.

// imported from elsewhere ////

export function useUnlessUnmounted<T>(useStateTuple: [val: T, setVal: Dispatch<SetStateAction<T>>]): [T, Dispatch<SetStateAction<T>>] {
  const [val, setVal] = useStateTuple;
  const [isMounted, setIsMounted] = useState(true);
  useEffect(() => () => setIsMounted(false), []);
  return [val, newVal => (isMounted ? setVal(newVal) : () => void 0)];
}

You could then create a useStateAsync hook to streamline a bit.

export function useStateAsync<T>(initialState: T | (() => T)): [T, Dispatch<SetStateAction<T>>] {
  return useUnlessUnmounted(useState(initialState));
}

score:0

Try to add the dependencies in useEffect:

  useEffect(() => {
    fetchData()
    return () => { props.spotifyAPI.cancelRequest() }
  }, [fetchData, props.spotifyAPI])

score:0

Usually this problem occurs when you showing the component conditionally, for example:

showModal && <Modal onClose={toggleModal}/> 

You can try to do some little tricks in the Modal onClose function, like

setTimeout(onClose, 0)

score:0

This works for me :')

   const [state, setState] = useState({});
    useEffect( async ()=>{
          let data= await props.data; // data from API too
          setState(users);
        },[props.data]);

score:0

Why do I keep getting this warning?

The intention of this warning is to help you prevent memory leaks in your application. If the component updates it's state after it has been unmounted from the DOM, this is an indication that there could be a memory leak, but it is an indication with a lot of false positives.

How do I know if I have a memory leak?

You have a memory leak if an object that lives longer than your component holds a reference to it, either directly or indirectly. This usually happens when you subscribe to events or changes of some kind without unsubscribing when your component unmounts from the DOM.

It typically looks like this:

useEffect(() => {
  function handleChange() {
     setState(store.getState())
  }
  // "store" lives longer than the component, 
  // and will hold a reference to the handleChange function.
  // Preventing the component to be garbage collected after 
  // unmount.
  store.subscribe(handleChange)

  // Uncomment the line below to avoid memory leak in your component
  // return () => store.unsubscribe(handleChange)
}, [])

Where store is an object that lives further up the React tree (possibly in a context provider), or in global/module scope. Another example is subscribing to events:

useEffect(() => {
  function handleScroll() {
     setState(window.scrollY)
  }
  // document is an object in global scope, and will hold a reference
  // to the handleScroll function, preventing garbage collection
  document.addEventListener('scroll', handleScroll)
  // Uncomment the line below to avoid memory leak in your component
  // return () => document.removeEventListener(handleChange)
}, [])

Another example worth remembering is the web API setInterval, which can also cause memory leak if you forget to call clearInterval when unmounting.

But that is not what I am doing, why should I care about this warning?

React's strategy to warn whenever state updates happen after your component has unmounted creates a lot of false positives. The most common I've seen is by setting state after an asynchronous network request:

async function handleSubmit() {
  setPending(true)
  await post('/someapi') // component might unmount while we're waiting
  setPending(false)
}

You could technically argue that this also is a memory leak, since the component isn't released immediately after it is no longer needed. If your "post" takes a long time to complete, then it will take a long time to for the memory to be released. However, this is not something you should worry about, because it will be garbage collected eventually. In these cases, you could simply ignore the warning.

But it is so annoying to see the warning, how do I remove it?

There are a lot of blogs and answers on stackoverflow suggesting to keep track of the mounted state of your component and wrap your state updates in an if-statement:

let isMountedRef = useRef(false)
useEffect(() => {
  isMountedRef.current = true
  return () => {
    isMountedRef.current = false
  }
}, [])

async function handleSubmit() {
  setPending(true)
  await post('/someapi')
  if (!isMountedRef.current) {
    setPending(false)
  }
}

This is not an recommended approach! Not only does it make the code less readable and adds runtime overhead, but it might also might not work well with future features of React. It also does nothing at all about the "memory leak", the component will still live just as long as without that extra code.

The recommended way to deal with this is to either cancel the asynchronous function (with for instance the AbortController API), or to ignore it.

In fact, React dev team recognises the fact that avoiding false positives is too difficult, and has removed the warning for the next release of React. I tested the beta release of React 18, and it is no longer present there.

score:0

Similar problem with my app, I use a useEffect to fetch some data, and then update a state with that:

useEffect(() => {
  const fetchUser = async() => {
    const {
      data: {
        queryUser
      },
    } = await authFetch.get(`/auth/getUser?userId=${createdBy}`);

    setBlogUser(queryUser);
  };

  fetchUser();

  return () => {
    setBlogUser(null);
  };
}, [_id]);

This improves upon Carlos Vallejo's answer.

score:0

I had this problem in React Native iOS and fixed it by moving my setState call into a catch. See below:

Bad code (caused the error):

  const signupHandler = async (email, password) => {
    setLoading(true)
    try {
      const token = await createUser(email, password)
      authContext.authenticate(token) 
    } catch (error) {
      Alert.alert('Error', 'Could not create user.')
    }
    setLoading(false) // this line was OUTSIDE the catch call and triggered an error!
  }

Good code (no error):

  const signupHandler = async (email, password) => {
    setLoading(true)
    try {
      const token = await createUser(email, password)
      authContext.authenticate(token) 
    } catch (error) {
      Alert.alert('Error', 'Could not create user.')
      setLoading(false) // moving this line INTO the catch call resolved the error!
    }
  }

score:2

I have getting same warning, This solution Worked for me ->

useEffect(() => {
    const unsubscribe = fetchData(); //subscribe
    return unsubscribe; //unsubscribe
}, []);

if you have more then one fetch function then

const getData = () => {
    fetch1();
    fetch2();
    fetch3();
}

useEffect(() => {
    const unsubscribe = getData(); //subscribe
    return unsubscribe; //unsubscribe
}, []);

score:3

This error occurs when u perform state update on current component after navigating to other component:

for example

  axios
      .post(API.BASE_URI + API.LOGIN, { email: username, password: password })
      .then((res) => {
        if (res.status === 200) {
          dispatch(login(res.data.data)); // line#5 logging user in
          setSigningIn(false); // line#6 updating some state
        } else {
          setSigningIn(false);
          ToastAndroid.show(
            "Email or Password is not correct!",
            ToastAndroid.LONG
          );
        }
      })

In above case on line#5 I'm dispatching login action which in return navigates user to the dashboard and hence login screen now gets unmounted.
Now when React Native reaches as line#6 and see there is state being updated, it yells out loud that how do I do this, the login component is there no more.

Solution:

  axios
      .post(API.BASE_URI + API.LOGIN, { email: username, password: password })
      .then((res) => {
        if (res.status === 200) {
          setSigningIn(false); // line#6 updating some state -- moved this line up
          dispatch(login(res.data.data)); // line#5 logging user in
        } else {
          setSigningIn(false);
          ToastAndroid.show(
            "Email or Password is not correct!",
            ToastAndroid.LONG
          );
        }
      })

Just move react state update above, move line 6 up the line 5.
Now state is being updated before navigating the user away. WIN WIN

score:3

there are many answers but I thought I could demonstrate more simply how the abort works (at least how it fixed the issue for me):

useEffect(() => {
  // get abortion variables
  let abortController = new AbortController();
  let aborted = abortController.signal.aborted; // true || false
  async function fetchResults() {
    let response = await fetch(`[WEBSITE LINK]`);
    let data = await response.json();
    aborted = abortController.signal.aborted; // before 'if' statement check again if aborted
    if (aborted === false) {
      // All your 'set states' inside this kind of 'if' statement
      setState(data);
    }
  }
  fetchResults();
  return () => {
    abortController.abort();
  };
}, [])

Other Methods: https://medium.com/wesionary-team/how-to-fix-memory-leak-issue-in-react-js-using-hook-a5ecbf9becf8

score:5

I had a similar issue with a scroll to top and @CalosVallejo answer solved it :) Thank you so much!!

const ScrollToTop = () => { 

  const [showScroll, setShowScroll] = useState();

//------------------ solution
  useEffect(() => {
    checkScrollTop();
    return () => {
      setShowScroll({}); // This worked for me
    };
  }, []);
//-----------------  solution

  const checkScrollTop = () => {
    setShowScroll(true);
 
  };

  const scrollTop = () => {
    window.scrollTo({ top: 0, behavior: "smooth" });
 
  };

  window.addEventListener("scroll", checkScrollTop);

  return (
    <React.Fragment>
      <div className="back-to-top">
        <h1
          className="scrollTop"
          onClick={scrollTop}
          style={{ display: showScroll }}
        >
          {" "}
          Back to top <span>&#10230; </span>
        </h1>
      </div>
    </React.Fragment>
  );
};

score:9

You can try this set a state like this and check if your component mounted or not. This way you are sure that if your component is unmounted you are not trying to fetch something.

const [didMount, setDidMount] = useState(false); 

useEffect(() => {
   setDidMount(true);
   return () => setDidMount(false);
}, [])

if(!didMount) {
  return null;
}

return (
    <ArtistProfileContainer>
      <AlbumContainer>
        {artistData ? artistData.artistAlbums.items.map(album => {
          return (
            <AlbumTag
              image={album.images[0].url}
              name={album.name}
              artists={album.artists}
              key={album.id}
            />
          )
        })
        : null}
      </AlbumContainer>
    </ArtistProfileContainer>
  )

Hope this will help you.

score:44

For example, you have some component that does some asynchronous actions, then writes the result to state and displays the state content on a page:

export default function MyComponent() {
    const [loading, setLoading] = useState(false);
    const [someData, setSomeData] = useState({});
    // ...
    useEffect( async () => {
        setLoading(true);
        someResponse = await doVeryLongRequest(); // it takes some time
        // When request is finished:
        setSomeData(someResponse.data); // (1) write data to state
        setLoading(false); // (2) write some value to state
    }, []);

    return (
        <div className={loading ? "loading" : ""}>
            {someData}
            <Link to="SOME_LOCAL_LINK">Go away from here!</Link>
        </div>
    );
}

Let's say that user clicks some link when doVeryLongRequest() still executes. MyComponent is unmounted but the request is still alive and when it gets a response it tries to set state in lines (1) and (2) and tries to change the appropriate nodes in HTML. We'll get an error from subject.

We can fix it by checking whether compponent is still mounted or not. Let's create a componentMounted ref (line (3) below) and set it true. When component is unmounted we'll set it to false (line (4) below). And let's check the componentMounted variable every time we try to set state (line (5) below).

The code with fixes:

export default function MyComponent() {
    const [loading, setLoading] = useState(false);
    const [someData, setSomeData] = useState({});
    const componentMounted = useRef(true); // (3) component is mounted
    // ...
    useEffect( async () => {
        setLoading(true);
        someResponse = await doVeryLongRequest(); // it takes some time
        // When request is finished:
        if (componentMounted.current){ // (5) is component still mounted?
            setSomeData(someResponse.data); // (1) write data to state
            setLoading(false); // (2) write some value to state
        }
        return () => { // This code runs when component is unmounted
            componentMounted.current = false; // (4) set it to false when we leave the page
        }
    }, []);

    return (
        <div className={loading ? "loading" : ""}>
            {someData}
            <Link to="SOME_LOCAL_LINK">Go away from here!</Link>
        </div>
    );
}

score:102

For me, clean the state in the unmount of the component helped.

 const [state, setState] = useState({});

useEffect(() => {
    myFunction();
    return () => {
      setState({}); // This worked for me
    };
}, []);

const myFunction = () => {
    setState({
        name: 'Jhon',
        surname: 'Doe',
    })
}


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