Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.

You can declare let isMounted = true inside useEffect, which will be changed in the cleanup callback, as soon as the component is unmounted. Before state updates, you now check this variable conditionally: useEffect(() => { let isMounted = true; // note mutable flag someAsyncOperation().then(data => { if (isMounted) setState(data); // add conditional check }) return () => { isMounted = false }; // cleanup toggles value, if unmounted }, []);

## Description This issue usually happens when you're trying to apply state changes to a component that does not exists anymore. Often happens when the component gets removed from a tree while there is a promise running in one of it's lifecycles. The best way to fix it is to make sure that if there is any promises in your component lifecycle, it should abort those once the component gets unmounted from the tree. Here is an interesting article that gives you a reproduction of this error together with one type of solution https://www.debuggr.io/react-update-unmounted-component/

function useIsMountedRef(){  const isMountedRef = useRef(null);  useEffect(() => {    isMountedRef.current = true;    return () => isMountedRef.current = false;  });  return isMountedRef;}
function Pets() {
  const [pets, dispatch] = useReducer(petsReducer, initialState);
  const isMountedRef = useIsMountedRef();
  const onChange = ({ target }) => {
    dispatch({ type: "PET_SELECTED", payload: target.value });
  };

  useEffect(() => {
    if (pets.selectedPet) {
      dispatch({ type: "FETCH_PET" });
      getPet(pets.selectedPet).then(data => {
        if(isMountedRef.current){          dispatch({ type: "FETCH_PET_SUCCESS", payload: data });
        }
      });
    } else {
      dispatch({ type: "RESET" });
    }
  }, [pets.selectedPet, isMountedRef]);

  return (
    <div>
      <select value={pets.selectedPet} onChange={onChange}>
        <option value="">Select a pet</option>
        <option value="cats">Cats</option>
        <option value="dogs">Dogs</option>
      </select>
      {pets.loading && <div>Loading...</div>}
      {pets.petData && <Pet {...pets.petData} />}
    </div>
  );
}

Posted by: Guest on September-25-2020

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