Why do I get this error when I make my page an async function? everything works fine when it isn't an async function. The only difference is that it returns a pending object which isn't what I want.
this is how I fetch the data:
const getData = async (id) => {
const res = await fetch({
url: "http://localhost:3000/api/user/getOne",
method: "POST",
credentials: true,
body: {
userId: "637846eb8f36e20663a9c948",
},
});
return res;
};
and then this is the page function
export default async function Page() {
const profileData = await getData();
useEffect(() => {
console.log(profileData);
}, []);
return(<div><p>Hello</p></div>)
}
I think the problem has to do with the async function and the await. How do I fix this?
Could try the idea as the below code.
import { useState } from "react";
export default async function Page() {
const [profileData, setProfileData] = useState([]);
async function initDate() {
const response = await getData();
setProfileData(response);
}
useEffect(() => {
initDate();
console.log(profileData);
}, []);
return (<div><p>Hello</p></div>)
}
Move the async in useEffect
export default function Page() {
useEffect(() => {
const getAll= async () => {
const profileData = await getData();
console.log(profileData);
};
getAll(); // run it, run it
}, []);
return(<div><p>Hello</p></div>)
}
It's not good to call a function directly inside of the component body in client components because every time with re-rendering of the component, your function gets executed.
It's better to write them in useEffect with dependencies.
If you want to run the function only once, you should use an empty array as the dependency:
useEffect(() => {
// your code
}, []);
And if you want to run them whenever one or more states change, you should do this:
useEffect(() => {
// your code
}, [/* list of the dependencies */]);
So in your case, it would be like this:
useEffect(() => {
const getProfileData = async () => {
await getData();
}
getProfileData();
}, []);
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 made this store:
export class CommentStore {
comments = []
constructor() {
makeAutoObservable(this, {}, { autoBind: true });
}
async loadPostComments(postId: number): Promise<void> {
const res = await API.loadPostComments(postId);
runInAction(() => {
this.comments = res;
});
}
async sendComment(postId: number, comment: Comment): Promise<void> {
try {
await API.sendComment(postId, comment);
await this.loadPostComments(postId);
return true;
} catch (err) {
console.log('oops');
}
}
}
Do i need use await in react components? For example:
useEffect(() => {
(async function () {
await loadPostComments(postId);
})();
}, [loadPostComments, postId]);
But this also works fine:
useEffect(() => {
loadPostComments(postId);
}, [loadPostComments, postId]);
Same for sendComment onClick:
onClick={()=>{sendComment(postId, comment)}}
onClick={async ()=>{await sendComment(postId, comment)}}
So, is it necessary to use await in this situations?
You want to await something only if it is necessary, e.g. when the next line of code uses the data from Promise.
In the useEffect case that you provided it is not necessary and on onClick handlers as well
Yes it is unnecessary to write async/await on them.
you just have to write the async call on the functions and that is enough.
for example:
const [posts, setPosts] = useState([]);
useEffect(() => {
const loadPost = async () => {
// Await make wait until that
// promise settles and return its result
const response = await axios.get(
"https://jsonplaceholder.typicode.com/posts/");
// After fetching data stored it in some state.
setPosts(response.data);
}
// Call the function
loadPost();
}, []);
`
there is no need to write promise and async / await on everything, remember ;P
I am trying to call Async function and use its response inside of useRef in react like the below:
const myComponent = () => {
const getTitle = async () => {
const res = await fetch('title');
return res;
}
const myref = useRef(
Employee().retire({
id: 1,
title: await getTitle(),
}),
);
}
However, I cannot use await inside of useRef. What other way to accomplish this?
Set the ref
const myRef = useRef(null);
And then use useEffect to update it once when the component renders:
// `getTitle` doesn't need to be async because
// you're apparently already returning a promise
function getTitle() {
return fetch('title');
}
useEffect(() => {
// Call an async function in your `useEffect`
// to wait for the promise to resolve and use the data
// to update the ref
async function getData() {
const title = await getTitle();
myRef.current = Employee().retire({ id: 1, title });
}
getData();
}, []);
The only way I know is set myref after async function is solved. Something like:
const myComponent = () => {
const getTitle = async () => {
const res = await fetch('title');
return res;
}
const myref = useRef(null);
useEffect(() => {
(async () => {
let title = await getTitle();
myref.current = Employee().retire({ id: 1, title });
console.log(myref.current);
})();
}, []);
}
Here a working example.
await keyword using only inside of async function
I'm trying to call an endpoint in an api that returns a value, I have a dream to call this fetch function inside another function, then save it to a variable.
But this returns a promise or if I get it working the function calls other things before the returned value.
Is the only way to do a timeout?
Here is a code example
async function someFetch() {
const res = await fetch("someurl");
const data = res.json();
return data;
}
function useFetched(someInput1, someInput2) {
const fetchedData = someFetch(); // need this input before anything else is called
const some_var = fetchedData + someInput1 + someInput2;
return some_var;
}
I've also tried to make the second function async and called await in front of someFetch(), but this returns a promise.
You need need to await the result of someFetch
async function useFetched(someInput1, someInput2){
const fetchedData = await someFetch() // need this input before anything else is called
const some_var = fetchedData + someInput1 + someInput2
return some_var
}
You cant make async useFetch[hook]. You need to use a callback here. The rough idea, you can pass setter function to useFetch to set value. And then can use that value.
Sample:
async function fetchTodo() {
const res = await fetch("someurl");
const data = res.json();
return data;
}
function useFetched(id, callback) {
useEffect(() => {
fetchTodo(id).then(data => {
console.log(data);
callback(data);
});
}, [id, callback]);
}
Working sample:
import React, { useEffect, useState } from "react";
async function fetchTodo(id) {
const res = await fetch("https://jsonplaceholder.typicode.com/todos/" + id);
const data = res.json();
return data;
}
function useFetched(id, callback) {
useEffect(() => {
fetchTodo(id).then(data => {
console.log(data);
callback(data);
});
}, [id, callback]);
}
export default function App() {
const [data, setData] = useState({});
useFetched(1, setData);
if (!data) return <h1>Loading...</h1>;
return (
<div className="App">
<h1>{JSON.stringify(data)}</h1>
</div>
);
}
Sandbox: https://codesandbox.io/s/misty-smoke-ls2y4?file=/src/App.js:0-607