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
For React version <=17
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>
)
}
For React version >=18
Starting with React 18 you can also use Suspense, but it's not yet recommended if you are not using frameworks that correctly implement it:
In React 18, you can start using Suspense for data fetching in opinionated frameworks like Relay, Next.js, Hydrogen, or Remix. Ad hoc data fetching with Suspense is technically possible, but still not recommended as a general strategy.
If not part of the framework, you can try some libs that implement it like swr.
Oversimplified example of how suspense works. You need to throw a promise for Suspense to catch it, show fallback component first and render Main component when promise it's resolved.
let fullfilled = false;
let promise;
const fetchData = () => {
if (!fullfilled) {
if (!promise) {
promise = new Promise(async (resolve) => {
const res = await fetch('api/data')
const data = await res.json()
fullfilled = true
resolve(data)
});
}
throw promise
}
};
const Main = () => {
fetchData();
return <div>Loaded</div>;
};
const App = () => (
<Suspense fallback={"Loading..."}>
<Main />
</Suspense>
);
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)
})()
}, [])
To do it properly and avoid errors: "Warning: Can't perform a React state update on an unmounted..."
useEffect(() => {
let mounted = true;
(async () => {
try {
const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
const json = await response.json();
const newPosts = json.data.children.map(it => it.data);
if (mounted) {
setPosts(newPosts);
}
} catch (e) {
console.error(e);
}
})();
return () => {
mounted = false;
};
}, []);
OR External functions and using an object
useEffect(() => {
let status = { mounted: true };
query(status);
return () => {
status.mounted = false;
};
}, []);
const query = async (status: { mounted: boolean }) => {
try {
const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
const json = await response.json();
const newPosts = json.data.children.map(it => it.data);
if (status.mounted) {
setPosts(newPosts);
}
} catch (e) {
console.error(e);
}
};
OR AbortController
useEffect(() => {
const abortController = new AbortController();
(async () => {
try {
const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`, { signal: abortController.signal });
const json = await response.json();
const newPosts = json.data.children.map(it => it.data);
setPosts(newPosts);
} catch (e) {
if(!abortController.signal.aborted){
console.error(e);
}
}
})();
return () => {
abortController.abort();
};
}, []);
I know it is late but just I had the same problem and I wanted to share that I solved it with a function like this!
useEffect(() => {
(async () => {
try {
const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
const json = await response.json();
setPosts(json.data.children.map(it => it.data));
} catch (e) {
console.error(e);
}
}) ()
}, [])
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>);
}
The most easy way is to use useAsyncEffect from 'use-async-effect'
You can find it on NPM.
const ProtectedRoute = ({ children }) => {
const [isAuth, setIsAuth] = useState(false);
useAsyncEffect(async () => {
try {
const data = await axios("auth");
console.log(data);
setIsAuth(true);
} catch (error) {
console.log(error);
}
}, []);
if (!isAuth)
return <Navigate to="/signin" />
return children;
}
I need every 5 seconds (actually every 2 minutes, but having 5 seconds while testing) to make a GET request to an API.
This should be done only when the user is logged in.
The data should be displayed by one component that is called "Header".
My first thought was to have an interval inside that component.
So with "useEffect" I had this in the Header.js:
const [data, setData] = useState([]);
useEffect(async () => {
const interval = setInterval(async () => {
if(localStorage.getItem("loggedInUser")){
console.log("Logged in user:");
console.log(new Date());
try{
const data = await getData();
if(data){
setData(data);
}
console.log("data",data);
}
catch(err){
console.log("err",err);
}
}
}, 5000);
return () => clearInterval(interval);
}, []);
Im getting the data. But looking at the console, it does not look so good.
First of all, sometimes I get this:
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.
It seems to happen when I go to other pages that have been called with a "href" (so without using for example history to change page).
Also, probably because of the thing above, after a while that I browse the pages, I see that the interval is not every 5 seconds anymore. It happens 2-3 times every 5 seconds. Maybe every seconds.
To me this does not look good at all.
Im wondering first of all if this component is the right place to put the interval in.
Maybe the interval should be in App.js? But then I would need a mechanism to pass data to children component (the Header.js, maybe could do this with Context).
As Sergio stated in his answer on async actions you should aways check if the component is currently mounted before doing side effects.
However using a variable that is initialised within the component won't work as it is being destroyed on unmount. You should rather use useRef hook as it will persist between component mounts.
const [data, setData] = useState([]);
const mountedRef = useRef(null);
useEffect(() => {
mountedRef.current = true;
return () => {
mountedRef.current = false;
};
}, []);
useEffect(async () => {
const interval = setInterval(async () => {
if (localStorage.getItem("loggedInUser")){
console.log("Logged in user:", new Date());
try {
const data = await getData();
if (data && mountedRef.current) {
setData(data);
console.log("data", data);
}
} catch(err) {
if (mountedRef.current) console.log("err", err);
}
}
}, 5000);
return () => clearInterval(interval);
}, []);
Or alternatively declare a boolean variable in the module scope so that its not freed by the garbage collection process
You can create a isMounted variable to control if the async task happens after the component is unmounted.
As the error said, you are trying to update component state after it's unmounted, so you should "protect" the setData call.
const [data, setData] = useState([]);
useEffect(async () => {
let isMounted = true;
const interval = setInterval(async () => {
if(localStorage.getItem("loggedInUser")){
console.log("Logged in user:");
console.log(new Date());
try{
const data = await getData();
if(data && isMounted){
setData(data);
}
console.log("data",data);
}
catch(err){
console.log("err",err);
}
}
}, 5000);
return () => {
clearInterval(interval);
isMounted = false;
}
}, []);
I have this useEffect that should only run once. Hence I have set router.query.something as a dependency. The request must wait for router.query.something to be hydrated, then it only makes the request if the value is truthy.
This just seems like an overly-complicated function to make a simple one-time request to the API, but I haven't figured out a cleaner way to do it.
const MyPage = () => {
const [user, setUser] = useState(null)
const router = useRouter()
useEffect(async () => {
try {
if (router.query.something) {
const res = await api.get(`/resetTokens/${router.query.something}`)
setUser(res.data)
}
} catch (err) {
setUser(null)
}
}, [router.query.something])
return (<div>...</div>)
}
Normally, I would use getServerSideProps, but in this situation, I can't, because the request is setting a cookie. If I use getServerSideProps the cookie doesn't pass through to the client.
You should note two things.
First, useEffect should not be using async functions. There should be a warning in the console regarding your use of async in useEffect.
Secondly, you should be cleaning up the useEffect since you are dealing with promises. If you do not clean up you may encounter memory leaks which can cause unexpected results.
Your code should look something like this instead
useEffect(() => {
let isMounted = true;
if (router.query.something) {
api.get(`/resetTokens/${router.query.resetToken}`)
.then(res=> {
if(!isMounted) return;
setUser(res.data)
})
.catch(e => {
if(!isMounted) return;
setUser(null)
});
}
return () => {
isMounted = false;
};
}, [router.query.something])
You could also try doing this with AbortController. Academind has a good blog explaining what is happening and how to fix. https://academind.com/tutorials/useeffect-abort-http-requests/
const MyPage = () => {
const [user, setUser] = useState(null)
const router = useRouter()
useEffect(() => {
const abortController = new AbortController();
const fetchData = async () => {
try {
const res = fetch(`/resetTokens/${router.query.resetToken}`, { signal: abortController.signal });
setUser(res.data)
} catch (e) {
setUser(null)
}
};
if (router.query.something) {
fetchData();
}
return () => {
abortController.abort();
};
}, [router.query.something]);
return (<div>...</div>)
}
I have this code in my ReactJS web application:
useEffect(() => {
const fetchInfo = async () => {
const res = await fetch(`${api}&page=${page}`);
setLoading(true);
try {
const x = await res.json();
if (page === 1) {
setItems(x);
setAutoplay(true);
} else {
setItems({
hasMore: x.hasMore,
vacancies: [...items.vacancies, ...x.vacancies],
});
}
} catch (err){
console.log(err);
}
setLoading(false);
};
fetchInfo();
}, [page]);
When this component unmounts while running asynchronous function, it throws an error in console.
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.
How can i cancel asynchronous tasks in cleanup.
I'm assuming here that setLoading is the function setting the state after your component has un-mounted, and therefore throwing this warning. If yes, then what you need is a clean-up function.
The function passed to useEffect can return a function, which will be called before the component unmounts (you can think of it as the equivalent of the old componentWillUnmount) - more details here:
https://reactjs.org/docs/hooks-effect.html#example-using-hooks
Now what you probably want is some sort of flag to check whether it is safe for you to call setLoading, i.e, set that flag to be true by default, then set it to false in the return function. Here's a good article that should help:
https://juliangaramendy.dev/use-promise-subscription/
Now I haven't tested this but essentially your code would look something like this:
useEffect(() => {
const fetchInfo = async () => {
let isSubscribed = true;
const res = await fetch(`${api}&page=${page}`);
if (isSubscribed) setLoading(true);
try {
const x = await res.json();
if (page === 1) {
setItems(x);
setAutoplay(true);
} else {
setItems({
hasMore: x.hasMore,
vacancies: [...items.vacancies, ...x.vacancies]
});
}
} catch (err) {
console.log(err);
}
if (isSubscribed) setLoading(false);
return () => (isSubscribed = false);
};
fetchInfo();
}, [page]);
Most of you have probably seen this React warning. Me too, and it is clear to my why React gives warns us about unmounting during a state update. But I'm trying to figure out what is the best React pattern prevent a possible memory leak.
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.
In my projects I'm using this boolean flag didCancel to let my data fetching logic know about the state of the component. If the component did unmount, the flag should be set to true which results in preventing to set the component state after the data fetching has been asynchronously resolved eventually.
useEffect(() => {
let didCancel = false;
const fetchEvents = () => {
fetch(url, settings)
.then(res => {
return res.json();
})
.then(({ data }) => {
if (!didCancel) {
setEvents(data.events);
}
})
.catch(err => {
console.log(err);
if (!didCancel) {
setLoading(false);
}
});
};
fetchEvents();
return () => {
didCancel = true;
};
}, []);
So eventhough this works, I just want to know what you guys are using as a pattern. What could be seen as best practice? Is there remarkable difference when fetching data, then when you're only updating state? Please let me know!
I prefer using defer from rxjs library for this:
useEffect(() => {
const subscription = defer(() =>
fetch(url, settings).then(response => response.json())
).subscribe({
next: ({ data }) => {
setEvents(data.events);
},
error: () => {
setLoading(false);
},
complete: () => {
setLoading(false);
}
});
return () => subscription.unsubscribe();
}, []);