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/${objId}/`);
const jsonData = response.json();
setData(jsonData);
}
retrieveObjectData();
}, [objId]);
if (objId === null || objId === undefined) {
return (<span>Object ID needs to be set</span>);
}
if (data) {
return (<span>Object ID is {objId}, data is {data}</span>);
}
return (<span>Loading...</span>);
}
The most easy way is to use useAsyncEffect from 'use-async-effect'
You can find it on NPM.
const ProtectedRoute = ({ children }) => {
const [isAuth, setIsAuth] = useState(false);
useAsyncEffect(async () => {
try {
const data = await axios("auth");
console.log(data);
setIsAuth(true);
} catch (error) {
console.log(error);
}
}, []);
if (!isAuth)
return <Navigate to="/signin" />
return children;
}
I'm trying to fetch some data with Axios and React, But I'm having a problem resolving the promise and setting it on the state, that's weird.
Here is the Base:
export const fetchUserById = (username) => client.get(`/${username}`);
Here is the Call:
export const getUserById = async (username) => {
try {
const response = await api.fetchUserById(username);
const data = await response.data;
return data;
} catch (error) {
return error;
}
};
Here is in React:
const [user, setUser] = useState();
useEffect(() => {
const data = getUserById(params.username); // this gets the username and its working
setUser(data)
}, [])
useEffect(() => {
console.log("this is user: ", user)
}, [user])
If I console log user, I get undefined, If I console log data i get a promise.
getUserById is declared async so it implicitly returns a Promise that callers should either await or use a Promise chain on.
useEffect(() => {
const data = getUserById(params.username);
setUser(data); // <-- logs only the returned Promise object!
}, [])
async/await
useEffect(() => {
const getUser = async () => {
try {
const data = await getUserById(params.username);
setUser(data);
} catch(error) {
// handle error, log, etc...
}
};
getUser();
}, []);
Promise chain
useEffect(() => {
getUserById(params.username)
.then(data => {
setUser(data);
})
.catch(error => {
// handle error, log, etc...
});
};
}, []);
Or you could as well do:
useEffect(() => {
// fetch data
(async () => {
try {
const data = await getUserById(params.username);
// set state
setUser(data)
} catch(error) {
// handle error, log, etc...
// set init state
setUser(null)
}
})();
}, []);
I have a useEffect(.., []) initialization sequence which involves 2 operations, each an await-blocked sync Axios call.
I verified that the 1st Axios call await call is not honored and the flow jumps to the 2nd useEffect operation, getFlag(), right after the await axios.get() line. Does anyone know why?
useEffect(() => {
getAgreements(); // Step 1
getFlag(); // Step 2
}, []);
const getAgreements = async () => {
const url = '/getAgreements';
try {
const response = await axios.get(url); // This 'await' is not honored, jumps to getFlag()
setAgreementsList(response.data);
}
catch (error) {
setErrorObj({message: error.message, location: 'Agreements.js: getAgreements'});
}
finally {
setIsLoading(false);
}
}
const getFlag = async () => {
const url = '/getNewAgreementIndicator';
try {
const response = await axios.get(url);
setNewAgreementFlag(response.data);
}
catch (error) {
setErrorObj({message: error.message, location: 'Agreements.js: getNewAgreementIndicator'});
}
finally {
setIsLoading(false);
}
}
When I rewrite the code below with .then() I see that in the 2nd fetch, the await is not honored, and it goes to console.log() before the result of the 2nd fetch is returned. Can anyone clarify?
useEffect(() => {
getAgreements()
.then(() => {
getFlag()
})
.then(() => {
console.log('test');
})
}, []);
Wouldnt it be better to just have 2 use effects rather than all of them in the one that happens on mount? Have another useEffect which gets triggered when the agreementsList gets update
useEffect(() => {
getAgreements(); // Step 1
}, []);
useEffect(() => {
if(agreementsList){
getFlag(); // Step 2
}
}, [agreementsList]);
I am getting data in useEffect and looping through it to add additional data.
I want to do some calculations on it after all data being added to results, I make the calculations inside if (response.data.next) but after that inside then when I try to access data it prints old data.
How can I make make sure all data added then be able to use it in then?
const [results, setResults] = useState([]);
useEffect(() => {
async function handleAPIRequest(url) {
return await axios
.get(url)
.then(async (response) => {
await setResults((results) => [...results, ...response.data.results]);
if (response.data.next) {
await handleAPIRequest(response.data.next);
}
return results;
})
.then(async () => {
// this is where I want to use results
console.log("resultss: ", results);
});
}
handleAPIRequest(url)
}, []);
I would suggest to keep consistency between async/await or chaining promises. your approach can cause multiple setResults, not sure if that's what you desire. Below, I offer a solution that might suit your needs:
useEffect(() => {
async function handleAPIRequest(url, currentResults = []) {
try {
const response = await axios.get(url)
const nextResults = [...currentResults, ...response.data.results];
if (response.data.next) {
return await handleAPIRequest(response.data.next, nextResults);
}
return nextResults;
} catch (error) {
throw error;
}
}
try {
const finalResults = await handleAPIRequest(url);
setResults(results => [...results, ...finalResults]);
} catch (error) {
// here you can handle error response
console.log(error);
}
}, []);
// to do something after results state is updated use another use effect to accomplish that
useEffect(() => {
// do something on updated results state
}, [JSON.stringify(results)]);
You should change your code in the following way:
const [results, setResults] = useState([]);
useEffect(() => {
const handleAPIRequest = async url => {
const lastResult = await axios.get(url);
setResults([...results, ...lastResult.data.results]);
const toLogResults = [...results, ...lastResult.data.results];
if(response.data.next) {
await handleAPIRequest(response.data.next);
} else {
console.log("The final results are", toLogResults);
}
}
handleAPIRequest(url)
}, []);
The main problem is that results in lines following setResults() are not updated immediately, so the value is the old one.
The toLogResults is used just to display the actual result, it is not needed.
I am using a promise based hook in a React app to fetch async data from an API.
I am also using a Axios, a promise based http client to call the API.
Is it an anti-pattern to use a promise based client inside another promise? The below code does not seem to work.
const getData = () => {
return new Promise((resolve, reject) => {
const url = "/getData";
axios.get(url)
.then(function(response) {
resolve(response);
})
.catch(function(error) {
reject(error);
});
});
const useAsync = (asyncFunction) => {
const [value, setValue] = useState(null);
const execute = useCallback(() => {
setPending(true);
setValue(null);
setError(null);
return asyncFunction()
.then(response => setValue(response))
.catch(error => setError(error))
.finally(() => setPending(false));
}, [asyncFunction]);
useEffect(() => {
execute();
}, [execute]);
return { execute, pending, value, error };
};
};
const RidesList = () => {
const {
pending,
value,
error,
} = useAsync(getData);
Oh man. I think you have a fundamental misunderstanding about how Promises work.
First, axios already returns a Promise by default. So your whole first function of getData can be reduced to:
const getData = () => {
const url = "/getData"
return axios.get(url)
}
But the meat of your code seems to indicate you want a querable Promise - so you can check the status of it for whatever reason. Here's an example of how you would do it, adapted from this snippet:
function statusPromiseMaker(promise) {
if (promise.isResolved) return promise
let status = {
pending: true,
rejected: false,
fulfilled: false
}
let result = promise.then(
resolvedValue => {
status.fulfilled = true
return resolvedValue
},
rejectedError => {
status.rejected = true
throw rejectedError
}
)
.finally(() => {
status.pending = false
})
result.status = () => status
return result
}
In this way, you can then do something like let thing = statusPromiseMaker(getData()) and if you look up thing.status.pending you'll get true or false etc...
I didn't actually run what's above, I may have forgotten a bracket or two, but hopefully this helps.
I have to admit - I haven't seen anything like this ever used in the wild. I am interested in knowing what you're actually trying to accomplish by this.
Axios itself returns a promise but if you want to make a custom class having your custom logic after each API call then you can use interceptors I was having the same requirement and this is how I am returning promises after applying my custom logic on each API call.
Interceptors will get executed separately after and before each request you made so we can simply use them if we want to modify our request or response.
here is my working solution have a look at it.
callApi = (method, endpoint, params) => {
this.apiHandler.interceptors.request.use((config) => {
config.method = method
config.url = config.baseURL + endpoint
config.params = params
return config
})
return new Promise((resolve, reject) => {
this.apiHandler.interceptors.response.use((config) => {
if (config.status == 200) {
resolve(config.data)
} else {
reject(config.status)
}
// return config
}, error => reject(error))
this.apiHandler()
})
}
Below is the code to call this function
helper.callApi("get", "wo/getAllWorkOrders").then(d => {
console.log(d)
})