React-hooks. Can't perform a React state update on an unmounted component

I get this error:

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.

when fetching of data is started and component was unmounted, but function is trying to update state of unmounted component.

What is the best way to solve this?

CodePen example.

default function Test() {
    const [notSeenAmount, setNotSeenAmount] = useState(false)

    useEffect(() => {
        let timer = setInterval(updateNotSeenAmount, 2000) 

        return () => clearInterval(timer)
    }, [])

    async function updateNotSeenAmount() {
        let data // here i fetch data

        setNotSeenAmount(data) // here is problem. If component was unmounted, i get error.
    }

    async function anotherFunction() {
       updateNotSeenAmount() //it can trigger update too
    }

    return <button onClick={updateNotSeenAmount}>Push me</button> //update can be triggered manually
}

The easiest solution is to use a local variable that keeps track of whether the component is mounted or not. This is a common pattern with the class based approach. Here is an example that implement it with hooks:

function Example() {
  const [text, setText] = React.useState("waiting...");

  React.useEffect(() => {
    let isCancelled = false;

    simulateSlowNetworkRequest().then(() => {
      if (!isCancelled) {
        setText("done!");
      }
    });

    return () => {
      isCancelled = true;
    };
  }, []);

  return <h2>{text}</h2>;
}

Here is an alternative with useRef (see below). Note that with a list of dependencies this solution won't work. The value of the ref will stay true after the first render. In that case the first solution is more appropriate.

function Example() {
  const isCancelled = React.useRef(false);
  const [text, setText] = React.useState("waiting...");

  React.useEffect(() => {
    fetch();

    return () => {
      isCancelled.current = true;
    };
  }, []);

  function fetch() {
    simulateSlowNetworkRequest().then(() => {
      if (!isCancelled.current) {
        setText("done!");
      }
    });
  }

  return <h2>{text}</h2>;
}

You can find more information about this pattern inside this article. Here is an issue inside the React project on GitHub that showcase this solution.

TL;DR

Here is a CodeSandBox example

The other answers work of course, I just wanted to share a solution I came up with. I built this hook that works just like React's useState, but will only setState if the component is mounted. I find it more elegant because you don't have to mess arround with an isMounted variable in your component !

Installation :

npm install use-state-if-mounted

Usage :

const [count, setCount] = useStateIfMounted(0);

You can find more advanced documentation on the npm page of the hook.