React Hook Warnings for async function in useEffect: useEffect function must return a cleanup function or nothing

I was trying the useEffect example something like below:

useEffect(async () => {
    try {
        const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
        const json = await response.json();
        setPosts(json.data.children.map(it => it.data));
    } catch (e) {
        console.error(e);
    }
}, []);

and I get this warning in my console. But the cleanup is optional for async calls I think. I am not sure why I get this warning. Linking sandbox for examples. https://codesandbox.io/s/24rj871r0p

I suggest to look at Dan Abramov (one of the React core maintainers) answer here:

> I think you're making it more complicated than it needs to be.

function Example() {
  const [data, dataSet] = useState<any>(null)

  useEffect(() => {
    async function fetchMyAPI() {
      let response = await fetch('api/data')
      response = await response.json()
      dataSet(response)
    }

    fetchMyAPI()
  }, [])

  return <div>{JSON.stringify(data)}</div>
}

> Longer term we'll discourage this pattern because it encourages race conditions. Such as — anything could happen between your call starts and ends, and you could have gotten new props. Instead, we'll recommend Suspense for data fetching which will look more like

const response = MyAPIResource.read();

> and no effects. But in the meantime you can move the async stuff to a separate function and call it.

You can read more about experimental suspense here.

---

If you want to use functions outside with eslint.

 function OutsideUsageExample({ userId }) {
  const [data, dataSet] = useState<any>(null)

  const fetchMyAPI = useCallback(async () => {
    let response = await fetch('api/data/' + userId)
    response = await response.json()
    dataSet(response)
  }, [userId]) // if userId changes, useEffect will run again

  useEffect(() => {
    fetchMyAPI()
  }, [fetchMyAPI])

  return (
    <div>
      <div>data: {JSON.stringify(data)}</div>
      <div>
        <button onClick={fetchMyAPI}>manual fetch</button>
      </div>
    </div>
  )
}

---

If you use useCallback, look at example of how it works useCallback. Sandbox.

import React, { useState, useEffect, useCallback } from "react";

export default function App() {
  const [counter, setCounter] = useState(1);

  // if counter is changed, than fn will be updated with new counter value
  const fn = useCallback(() => {
    setCounter(counter + 1);
  }, [counter]);

  // if counter is changed, than fn will not be updated and counter will be always 1 inside fn
  /*const fnBad = useCallback(() => {
      setCounter(counter + 1);
    }, []);*/

  // if fn or counter is changed, than useEffect will rerun
  useEffect(() => {
    if (!(counter % 2)) return; // this will stop the loop if counter is not even

    fn();
  }, [fn, counter]);

  // this will be infinite loop because fn is always changing with new counter value
  /*useEffect(() => {
    fn();
  }, [fn]);*/

  return (
    <div>
      <div>Counter is {counter}</div>
      <button onClick={fn}>add +1 count</button>
    </div>
  );
}

When you use an async function like

async () => {
    try {
        const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
        const json = await response.json();
        setPosts(json.data.children.map(it => it.data));
    } catch (e) {
        console.error(e);
    }
}

it returns a promise and useEffect doesn't expect the callback function to return Promise, rather it expects that nothing is returned or a function is returned.

As a workaround for the warning you can use a self invoking async function.

useEffect(() => {
    (async function() {
        try {
            const response = await fetch(
                `https://www.reddit.com/r/${subreddit}.json`
            );
            const json = await response.json();
            setPosts(json.data.children.map(it => it.data));
        } catch (e) {
            console.error(e);
        }
    })();
}, []);

or to make it more cleaner you could define a function and then call it

useEffect(() => {
    async function fetchData() {
        try {
            const response = await fetch(
                `https://www.reddit.com/r/${subreddit}.json`
            );
            const json = await response.json();
            setPosts(json.data.children.map(it => it.data));
        } catch (e) {
            console.error(e);
        }
    };
    fetchData();
}, []);

the second solution will make it easier to read and will help you write code to cancel previous requests if a new one is fired or save the latest request response in state

Working codesandbox

Until React provides a better way, you can create a helper, useEffectAsync.js:

import { useEffect } from 'react';


export default function useEffectAsync(effect, inputs) {
    useEffect(() => {
        effect();
    }, inputs);
}

Now you can pass an async function:

useEffectAsync(async () => {
    const items = await fetchSomeItems();
    console.log(items);
}, []);

Update

If you choose this approach, note that it's bad form. I resort to this when I know it's safe, but it's always bad form and haphazard.

Suspense for Data Fetching, which is still experimental, will solve some of the cases.

In other cases, you can model the async results as events so that you can add or remove a listener based on the component life cycle.

Or you can model the async results as an Observable so that you can subscribe and unsubscribe based on the component life cycle.

You can also use IIFE format as well to keep things short

function Example() {
    const [data, dataSet] = useState<any>(null)

    useEffect(() => {
        (async () => {
            let response = await fetch('api/data')
            response = await response.json()
            dataSet(response);
        })();
    }, [])

    return <div>{JSON.stringify(data)}</div>
}

void operator could be used here.
Instead of:

React.useEffect(() => {
    async function fetchData() {
    }
    fetchData();
}, []);

or

React.useEffect(() => {
    (async function fetchData() {
    })()
}, []);

you could write:

React.useEffect(() => {
    void async function fetchData() {
    }();
}, []);

It is a little bit cleaner and prettier.

---

Async effects could cause memory leaks so it is important to perform cleanup on component unmount. In case of fetch this could look like this:

function App() {
    const [ data, setData ] = React.useState([]);

    React.useEffect(() => {
        const abortController = new AbortController();
        void async function fetchData() {
            try {
                const url = 'https://jsonplaceholder.typicode.com/todos/1';
                const response = await fetch(url, { signal: abortController.signal });
                setData(await response.json());
            } catch (error) {
                console.log('error', error);
            }
        }();
        return () => {
            abortController.abort(); // cancel pending fetch request on component unmount
        };
    }, []);

    return <pre>{JSON.stringify(data, null, 2)}</pre>;
}

I read through this question, and feel the best way to implement useEffect is not mentioned in the answers. Let's say you have a network call, and would like to do something once you have the response. For the sake of simplicity, let's store the network response in a state variable. One might want to use action/reducer to update the store with the network response.

const [data, setData] = useState(null);

/* This would be called on initial page load */
useEffect(()=>{
	fetch(`https://www.reddit.com/r/${subreddit}.json`)
	.then(data => {
		setData(data);
	})
	.catch(err => {
		/* perform error handling if desired */
	});
}, [])

/* This would be called when store/state data is updated */
useEffect(()=>{
	if (data) {
		setPosts(data.children.map(it => {
			/* do what you want */
		}));
	}
}, [data]);

Reference => https://reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects

For other readers, the error can come from the fact that there is no brackets wrapping the async function:

Considering the async function initData

  async function initData() {
  }

This code will lead to your error:

  useEffect(() => initData(), []);

But this one, won't:

  useEffect(() => { initData(); }, []);

(Notice the brackets around initData()

For fetching from an external API using React Hooks, you should call a function that fetches from the API inside of the useEffect hook.

Like this:

async function fetchData() {
    const res = await fetch("https://swapi.co/api/planets/4/");
    res
      .json()
      .then(res => setPosts(res))
      .catch(err => setErrors(err));
  }

useEffect(() => {
    fetchData();
}, []);

I strongly recommend that you do not define your query inside the useEffect Hook, because it will be re-render infinite times. And since you cannot make the useEffect async, you can make the function inside of it to be async.

In the example shown above, the API call is in another separated async function so it makes sure that the call is async and that it only happens once. Also, the useEffect's dependency array (the []) is empty, which means that it will behave just like the componentDidMount from React Class Components, it will only be executed once when the component is mounted.

For the loading text, you can use React's conditional rendering to validate if your posts are null, if they are, render a loading text, else, show the posts. The else will be true when you finish fetching data from the API and the posts are not null.

{posts === null ? <p> Loading... </p> 
: posts.map((post) => (
    <Link key={post._id} to={`/blog/${post.slug.current}`}>
      <img src={post.mainImage.asset.url} alt={post.mainImage.alt} />
      <h2>{post.title}</h2>
   </Link>
))}

I see you already are using conditional rendering so I recommend you dive more into it, especially for validating if an object is null or not!

I recommend you read the following articles in case you need more information about consuming an API using Hooks.

https://betterprogramming.pub/how-to-fetch-data-from-an-api-with-react-hooks-9e7202b8afcd

https://reactjs.org/docs/conditional-rendering.html

try

const MyFunctionnalComponent: React.FC = props => {
  useEffect(() => {
    // Using an IIFE
    (async function anyNameFunction() {
      await loadContent();
    })();
  }, []);
  return <div></div>;
};

Other answers have been given by many examples and are clearly explained, so I will explain them from the point of view of TypeScript type definition.

The useEffect hook TypeScript signature:

function useEffect(effect: EffectCallback, deps?: DependencyList): void;

The type of effect:

// NOTE: callbacks are _only_ allowed to return either void, or a destructor.
type EffectCallback = () => (void | Destructor);

// Destructors are only allowed to return void.
type Destructor = () => void | { [UNDEFINED_VOID_ONLY]: never };

Now we should know why effect can't be an async function.

useEffect(async () => {
  //...
}, [])

The async function will return a JS promise with an implicit undefined value. This is not the expectation of useEffect.

Please try this

 useEffect(() => {
        (async () => {
          const products = await api.index()
          setFilteredProducts(products)
          setProducts(products)
        })()
      }, [])

With useAsyncEffect hook provided by a custom library, safely execution of async code and making requests inside effects become trivially since it makes your code auto-cancellable (this is just one thing from the feature list). Check out the Live Demo with JSON fetching

import React from "react";
import { useAsyncEffect } from "use-async-effect2";
import cpFetch from "cp-fetch";

/*
 Notice: the related network request will also be aborted
 Checkout your network console
 */

function TestComponent(props) {
  const [cancel, done, result, err] = useAsyncEffect(
    function* () {
      const response = yield cpFetch(props.url).timeout(props.timeout);
      return yield response.json();
    },
    { states: true, deps: [props.url] }
  );

  return (
    <div className="component">
      <div className="caption">useAsyncEffect demo:</div>
      <div>
        {done ? (err ? err.toString() : JSON.stringify(result)) : "loading..."}
      </div>
      <button className="btn btn-warning" onClick={cancel} disabled={done}>
        Cancel async effect
      </button>
    </div>
  );
}

export default TestComponent;

The same demo using axios

Just a note about HOW AWESOME the purescript language handles this problem of stale effects with Aff monad

WITHOUT PURESCRIPT

you have to use AbortController

function App() {
    const [ data, setData ] = React.useState([]);

    React.useEffect(() => {
        const abortController = new AbortController();
        void async function fetchData() {
            try {
                const url = 'https://jsonplaceholder.typicode.com/todos/1';
                const response = await fetch(url, { signal: abortController.signal });
                setData(await response.json());
            } catch (error) {
                console.log('error', error);
            }
        }();
        return () => {
            abortController.abort(); // cancel pending fetch request on component unmount
        };
    }, []);

    return <pre>{JSON.stringify(data, null, 2)}</pre>;
}

or stale (from NoahZinsmeister/web3-react example)

function Balance() {
  const { account, library, chainId } = useWeb3React()

  const [balance, setBalance] = React.useState()
  React.useEffect((): any => {
    if (!!account && !!library) {      
      let stale = false
      
      library 
        .getBalance(account)
        .then((balance: any) => {
          if (!stale) {
            setBalance(balance)
          }
        })
        .catch(() => {
          if (!stale) {
            setBalance(null)
          }
        })

      return () => { // NOTE: will be called every time deps changes
        stale = true
        setBalance(undefined)
      }
    }
  }, [account, library, chainId]) // ensures refresh if referential identity of library doesn't change across chainIds

  ...

WITH PURESCRIPT

check how useAff kills it's Aff in the cleanup function

the Aff is implemented as a state machine (without promises)

but what is relevant to us here is that:

• the Aff encodes how to stop the Aff - You can put your AbortController here

• it will STOP running Effects (not tested) and Affs (it will not run then from the second example, so it will NOT setBalance(balance)) IF the error was thrown TO the fiber OR INSIDE the fiber

Ignore the warning, and use the useEffect hook with an async function like this:

import { useEffect, useState } from "react";

function MyComponent({ objId }) {
  const [data, setData] = useState();

  useEffect(() => {
    if (objId === null || objId === undefined) {
      return;
    }

    async function retrieveObjectData() {
      const response = await fetch(`path/to/api/objects/${objId}/`);
      const jsonData = response.json();
      setData(jsonData);
    }
    retrieveObjectData();

  }, [objId]);

  if (objId === null || objId === undefined) {
    return (<span>Object ID needs to be set</span>);
  }

  if (data) {
    return (<span>Object ID is {objId}, data is {data}</span>);
  }

  return (<span>Loading...</span>);
}

I know it is late but just I had the same problem and I wanted to share that I solved it with a function like this!

useEffect(() => {
(async () => { 
  try {
   const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
   const json = await response.json();
    setPosts(json.data.children.map(it => it.data));
  } catch (e) {
console.error(e);
}
}) ()   
}, [])    

Wrap it into a useCallback and use it as a dependency of the useEffect hook.

const getPosts = useCallback(async () => {
    try {
        const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
        const json = await response.json();
        setPosts(json.data.children.map(it => it.data));
    } catch (e) {
        console.error(e);
    }
}, []);

useEffect(async () => {
    getPosts();
}, [getPosts]);