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

Question

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
}

Answer 1

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.

Answer 2

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.

Answer 3

If you are fetching data from axios(using hooks) and the error still occurs, just wrap the setter inside the condition

let isRendered = useRef(false);
useEffect(() => {
    isRendered = true;
    axios
        .get("/sample/api")
        .then(res => {
            if (isRendered) {
                setState(res.data);
            }
            return null;
        })
        .catch(err => console.log(err));
    return () => {
        isRendered = false;
    };
}, []);

Answer 4

Here is a simple solution for this. This warning is due to when we do some fetch request while that request is in the background (because some requests take some time.)and we navigate back from that screen then react cannot update the state. here is the example code for this. write this line before every state Update.

if(!isScreenMounted.current) return;

Here is Complete Example

import React , {useRef} from 'react'
import { Text,StatusBar,SafeAreaView,ScrollView, StyleSheet } from 'react-native'
import BASEURL from '../constants/BaseURL';
const SearchScreen = () => {
    const isScreenMounted = useRef(true)
    useEffect(() => {
        return () =>  isScreenMounted.current = false
    },[])

    const ConvertFileSubmit = () => {
        if(!isScreenMounted.current) return;
         setUpLoading(true)
 
         var formdata = new FormData();
         var file = {
             uri: `file://${route.params.selectedfiles[0].uri}`,
             type:`${route.params.selectedfiles[0].minetype}`,
             name:`${route.params.selectedfiles[0].displayname}`,
         };
         
         formdata.append("file",file);
         
         fetch(`${BASEURL}/UploadFile`, {
             method: 'POST',
             body: formdata,
             redirect: 'manual'
         }).then(response => response.json())
         .then(result => {
             if(!isScreenMounted.current) return;
             setUpLoading(false)    
         }).catch(error => {
             console.log('error', error)
         });
     }

    return(
    <>
        <StatusBar barStyle="dark-content" />
        <SafeAreaView>
            <ScrollView
            contentInsetAdjustmentBehavior="automatic"
            style={styles.scrollView}>
               <Text>Search Screen</Text>
            </ScrollView>
        </SafeAreaView>
    </>
    )
}

export default SearchScreen;


const styles = StyleSheet.create({
    scrollView: {
        backgroundColor:"red",
    },
    container:{
        flex:1,
        justifyContent:"center",
        alignItems:"center"
    }
})