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 am trying to refactor my code to work solo with firebase the issue that I am facing is firebase methods for example onSnapshot, could this method totally replace the need for the classic first data fetch ? I am ruining in to bunch of run time errors since many data are not available at start and I am starting to doubt my onSnapShot, below is the code to my entry component where most of the data fetching using onSnapShot happens but still many values at start are undefined any idea how to tackle such issue ? the data are there but at start they are not,
useEffect(() => {
context.SetUser('trainer');
const fetchClient = () => {
const colReClients = collection(db, 'Clients');
const id: string = _authContext?.currentUser?.uid;
const unsub = onSnapshot(doc(colReClients, id), (doc) => {
console.log(doc.data(), 'snapShot triggered');
///////////// Refactor to foreach
context.SetExerciseStateBackEnd('dataCenter', doc.data()?.clients);
//context.SetExerciseStateBackEnd('focus', doc.data()?.focus);
});
return () => unsub();
};
if (_authContext?.currentUser) {
fetchClient();
}
}, [_authContext?.currentUser?.uid]);
useEffect(() => {
const fetchWeeks = async () => {
// ADD WEEKS
// Weeks Collection Reference
const colRefWeeks = collection(db, 'Weeks');
const docRefWeeks = doc(colRefWeeks, `${context.focus}`);
const unsub = onSnapshot(docRefWeeks, (doc) => {
console.log(doc.data(), 'snapShot triggered');
context.SetExerciseStateBackEnd('weeks', { [`${context.focus}`]: doc.data()?.clientWeeks });
const arr = doc.data()?.clientWeeks.find((el: any) => el.weekId === context.weekFocus).routines;
if (arr) {
context.SetExerciseStateBackEnd('routines', arr);
}
});
return () => unsub();
};
fetchWeeks();
}, [context.focus]);
I'm trying to make a Tweets Application with React and Firebase and I have been suffering when trying to get info from more than 1 collection.
So this is the story:
I get the tweets using onSnapshot. All fine here
I need more info from 2 other collections: user_preferences and user_photo, so I use .get() within the onSnapshot
For managing asynchronism, I resolve my 2 promises before returning the tweet data + details data object for my map function.
I made a console.log of my mappedTweet and the values are OKEY. Here I can see the tweet data + details data
But my STATE "tweets" just have an array of undefined objects =(. It shows the right number of rows accoroding to my Tweets collection but rows of undefined data, and not the rows of my mappedTweets objects. Why?
Can anyone shed some light?
useEffect(() => {
//------------getting the TWEETS with onSnapshot()-------------
const cancelSuscription = firestore
.collection('tweets')
.onSnapshot((snapshot) => {
const promises = [];
const tweetsMapped = snapshot.docs.map((doc) => {
let tweetAndAuthor;
const tweetMappped = {
text: doc.data().text,
likes: doc.data().likes,
email: doc.data().email,
created: doc.data().created,
uid: doc.data().uid,
id: doc.id,
};
let authorPreference, authorPhoto;
const userPreferencePromise = firestore
.collection('user_preferences')
.where('uid', '==', tweetMappped.uid)
.get();
const userPhotoPromise = firestore
.collection('user_photos')
.where('id', '==', tweetMappped.uid)
.get();
promises.push(userPreferencePromise);
promises.push(userPhotoPromise);
//------------getting the AUTHOR USER PREFERENCES with .get()-------------
userPreferencePromise.then((snapshot2) => {
authorPreference = snapshot2.docs.map((doc) => {
return {
username: doc.data().username,
color: doc.data().color,
};
});
});
//------------getting the AUTHOR PHOTO with .get()-------------
userPhotoPromise.then((snapshot3) => {
authorPhoto = snapshot3.docs.map((doc) => {
return {
photoURL: doc.data().photoURL,
};
});
});
Promise.all(promises).then((x) => {
return {
...tweetMappped,
author: authorPreference[0].username,
authorColor: authorPreference[0].color,
authorPhoto: authorPhoto[0].photoURL,
};
});
});
Promise.all(promises).then((x) => {
setTweets(tweetsMapped);
});
});
return () => cancelSuscription();
}, []);
Well, I made it work by changing the model I was using to retrieve the data from Firebase.
I was using an outer onSnapshot with nested promises (I think I was very near here), but now I'm using nested onSnapshots and now app is behaving as expected.
So this is the new useEffect
useEffect(() => {
let cancelUserPrefSuscription, cancelUserPhotoSuscription;
// First onSnapshot
const cancelTweetSuscription = firestore
.collection('tweets')
.onSnapshot((tweetSnapshot) => {
const list = [];
tweetSnapshot.docs.forEach((tweetDoc) => {
//Second onSnapshot
cancelUserPrefSuscription = firestore
.collection('user_preferences')
.where('uid', '==', tweetDoc.data().uid)
.onSnapshot((userPrefSnapshot) => {
userPrefSnapshot.docs.forEach((userPrefDoc) => {
//Third onSnapshot
cancelUserPhotoSuscription = firestore
.collection('user_photos')
.where('id', '==', tweetDoc.data().uid)
.onSnapshot((userPhotoSnapshot) => {
userPhotoSnapshot.docs.forEach((userPhotoDoc) => {
//Taking the whole data i need from all snapshots
const newData = {
id: tweetDoc.id,
...tweetDoc.data(),
author: userPrefDoc.data().username,
authorColor: userPrefDoc.data().color,
authorPhoto: userPhotoDoc.data().photoURL,
};
list.push(newData);
//Updating my state
if (tweetSnapshot.docs.length === list.length) {
setTweets(list);
}
});
});
});
});
});
});
return () => {
cancelTweetSuscription();
cancelUserPrefSuscription();
cancelUserPhotoSuscription();
};
}, []);
Edit: Fix from comments of above code
Author: #samthecodingman
For each call to onSnapshot, you should keep track of its unsubscribe function and keep an array filled with the unsubscribe functions of any nested listeners. When an update is received, unsubscribe each nested listener, clear the array of nested unsubscribe functions and then insert each new nested listener into the array. For each onSnapshot listener attached, a single unsubscribe function should be created that cleans up the listener itself along with any nested listeners.
Note: Instead of using this approach, create a Tweet component that pulls the author's name and photo inside it.
useEffect(() => {
// helper function
const callIt = (unsub) => unsub();
// First onSnapshot
const tweetsNestedCancelListenerCallbacks = [];
const tweetsCancelListenerCallback = firestore
.collection('tweets')
.onSnapshot((tweetSnapshot) => {
const newTweets = [];
const expectedTweetCount = tweetSnapshot.docs.length;
// cancel nested subscriptions
tweetsNestedCancelListenerCallbacks.forEach(callIt);
// clear the array, but don't lose the reference
tweetsNestedCancelListenerCallbacks.length = 0;
tweetsNestedCancelListenerCallbacks.push(
...tweetSnapshot.docs
.map((tweetDoc) => { // (tweetDoc) => Unsubscribe
const tweetId = tweetDoc.id;
//Second onSnapshot
const userPrefNestedCancelListenerCallbacks = [];
const userPrefCancelListenerCallback = firestore
.collection('user_preferences')
.where('uid', '==', tweetDoc.data().uid)
.limitToFirst(1)
.onSnapshot((userPrefSnapshot) => {
const userPrefDoc = userPrefSnapshot.docs[0];
// cancel nested subscriptions
userPrefNestedCancelListenerCallbacks.forEach(callIt);
// clear the array, but don't lose the reference
userPrefNestedCancelListenerCallbacks.length = 0;
//Third onSnapshot
const userPhotoCancelListenerCallback = firestore
.collection('user_photos')
.where('id', '==', tweetDoc.data().uid)
.limitToFirst(1)
.onSnapshot((userPhotoSnapshot) => {
const userPhotoDoc = userPhotoSnapshot.docs[0];
// Taking the whole data I need from all snapshots
const newData = {
id: tweetId,
...tweetDoc.data(),
author: userPrefDoc.data().username,
authorColor: userPrefDoc.data().color,
authorPhoto: userPhotoDoc.data().photoURL,
};
const existingTweetObject = tweets.find(t => t.id === tweetId);
if (existingTweetObject) {
// merge in changes to existing tweet
Object.assign(existingTweetObject, newData);
if (expectedTweetCount === newTweets.length) {
setTweets([...newTweets]); // force rerender with new info
}
} else {
// fresh tweet
tweets.push(newData);
if (expectedTweetCount === newTweets.length) {
setTweets(newTweets); // trigger initial render
}
}
});
userPrefNestedCancelListenerCallbacks.push(userPhotoCancelListenerCallback);
});
// return an Unsubscribe callback for this listener and its nested listeners.
return () => {
userPrefCancelListenerCallback();
userPrefNestedCancelListenerCallbacks.forEach(callIt);
}
})
);
});
// return an Unsubscribe callback for this listener and its nested listeners.
return () => {
tweetsCancelListenerCallback();
tweetsNestedCancelListenerCallbacks.forEach(callIt);
};
}, []);
Edit: Splitting the code in two components
Note: Changed limitToFirst(1) --> limit(1). Splitting the fetch logic in two components simplified the onSnapshot approach!
1.The Parent Component
useEffect(() => {
const tweetsUnsubscribeCallback = firestore
.collection('tweets')
.onSnapshot((tweetSnapshot) => {
const mappedtweets = tweetSnapshot.docs.map((tweetDoc) => {
return {
id: tweetDoc.id,
...tweetDoc.data(),
};
});
setTweets(mappedtweets);
});
return () => tweetsUnsubscribeCallback();
}, []);
2.The Child Component: Tweet
useEffect(() => {
// Helper Function
const unSubscribe = (unsub) => unsub();
//------------getting the AUTHOR USER PREFERENCE
const userPrefNestedUnsubscribeCallbacks = [];
const userPrefUnsubscribeCallback = firestore
.collection('user_preferences')
.where('uid', '==', tweet.uid)
.limit(1)
.onSnapshot((userPrefSnapshot) => {
userPrefNestedUnsubscribeCallbacks.forEach(unSubscribe); // cancel nested subscriptions
userPrefNestedUnsubscribeCallbacks.length = 0; // clear the array, but don't lose the reference
//------------getting the AUTHOR PHOTO
const userPhotoUnsubscribeCallback = firestore
.collection('user_photos')
.where('id', '==', tweet.uid)
.limit(1)
.onSnapshot((userPhotoSnapshot) => {
// Taking the whole data I need from all snapshots
setAuthor({
author: userPrefSnapshot.docs[0].data().username,
authorColor: userPrefSnapshot.docs[0].data().color,
authorPhoto: userPhotoSnapshot.docs[0].data().photoURL,
});
});
userPrefNestedUnsubscribeCallbacks.push(userPhotoUnsubscribeCallback);
});
return () => {
userPrefUnsubscribeCallback();
userPrefNestedUnsubscribeCallbacks.forEach(unSubscribe);
};
}, []);
Basically, you've pushed the promises to your promise array in the state they were before you you processed their data. You want to make use of the Promise.all(docs.map((doc) => Promise<Result>)) pattern here where each document should return a single Promise containing its final result. This then means that the Promise.all will resolve with Result[].
Note: If inside a Promise you are mutating a variable outside of the Promise (e.g. pushing to an array), that is generally a sign that you are doing something wrong and you should rearrange your code.
Here's a quick example of throwing this together:
useEffect(() => {
let unsubscribed = false;
//------------getting the TWEETS with onSnapshot()-------------
const cancelSuscription = firestore
.collection('tweets')
.onSnapshot((snapshot) => {
const tweetsMappedPromises = snapshot.docs.map((doc) => {
let tweetAndAuthor;
const tweetMappped = {
text: doc.data().text,
likes: doc.data().likes,
email: doc.data().email,
created: doc.data().created,
uid: doc.data().uid,
id: doc.id,
};
//------------getting the AUTHOR USER PREFERENCES with .get()-------------
const userPreferencePromise = firestore
.collection('user_preferences')
.where('uid', '==', tweetMappped.uid)
.limitToFirst(1)
.get()
.then((prefDocQuerySnapshot) => {
const firstPrefDoc = photoDocQuerySnapshot.docs[0];
const { username, color } = firstPrefDoc.data();
return { username, color };
});
//------------getting the AUTHOR PHOTO with .get()-------------
const userPhotoPromise = firestore
.collection('user_photos')
.where('id', '==', tweetMappped.uid)
.limitToFirst(1)
.get()
.then((photoDocQuerySnapshot) => {
const firstPhotoDoc = photoDocQuerySnapshot.docs[0];
return firstPhotoDoc.get("photoURL");
});
//--------------------assemble this result---------------------
return Promises.all([userPreferencePromise, userPhotoPromise])
.then(([authorPreference, authorPhoto]) => {
return {
...tweetMappped,
author: authorPreference.username,
authorColor: authorPreference.color,
authorPhoto: authorPhoto.photoURL,
};
});
});
Promise.all(tweetsMappedPromises)
.then((tweetsMapped) => {
if (unsubscribed) return; // ignore result, dealing with out of date data
setTweets(tweetsMapped);
})
.catch((err) => {
if (unsubscribed) return; // ignore result, dealing with out of date data
// important! handle errors
});
});
return () => {
unsubscribed = true;
cancelSuscription();
}
}, []);
Notes:
You may benefit from using async/await syntax here instead.
On new onSnapshot calls, snapshot.docChanges() can be used to make it more efficient and speed up rerenders by only updating the entries that have changed (e.g. added/removed/modified). You would use setTweets(previousTweetsMapped => /* newTweetsMapped */) for this.
I want to fetch 3 different things from firestore, hence have applied 3 useEffect in my code, out of which my react is not reading the map function inside 3rd useEffect. Here this map function in 3rd useEffect is not working
Code:
1st useEffect:
useEffect(() => {
var applications = [];
const hospitals = [];
const studentId = [];
const details = [];
firebaseConfig
.firestore()
.collection("counselor")
.doc(currentUser.uid)
.collection("studentDetails")
.get()
.then((snapshot) => {
snapshot.docs.forEach((detail) => {
let currentID = detail.id;
let appObj = { ...detail.data(), ["id"]: currentID };
details.push(appObj);
details.push(detail.data());
});
setDetails(details);
});
}, []);
2nd useffect:
useEffect(() => {
const item = [];
details.map((detail) => {
console.log(detail.id);
firebaseConfig
.firestore()
.collection("counselor")
.doc(currentUser.uid)
.collection("studentDetails")
.doc(detail.id)
.collection("studentApplications")
.get()
.then((snapshot) => {
snapshot.docs.forEach((detail) => {
let currentID = detail.id;
let appObj = { ...detail.data(), ["id"]: currentID };
item.push(detail.data().applicationStatus);
setStudentId(item);
});
});
});
setDemo(item);
}, [details]);
3rd useeffect:
useEffect(() => {
demo.map((hii) => {
if (hii === "PaymentFormFilled") {
complete = complete + 1;
setCom(complete);
}
});
}, [demo]);
Your error is not clear: "react is not reading the map function...".
There could be several issues:
Is setDemo assigning a value to a state variable called demo? If not, your 3rd useEffect won't be triggered
What is the value assigned to demo? You might not run .map on it
Because setCom is async. state complete only update value when component re-render. Please move setCom to the outside loop. And using forEach instead map in this case.
useEffect(() => {
let newCount = complete;
demo.forEach((hii) => {
if (hii === "PaymentFormFilled") {
newCount += 1;
}
});
setCom(newCount);
}, [demo]);
I'm getting data from Firebase and want to update state:
const [allProfile, setAllProfile] = useState([]);
.....
const displayProfileList = async () => {
try {
await profile
.get()
.then(querySnapshot => {
querySnapshot.docs.map(doc => {
const documentId = doc.id;
const nProfile = { id: documentId, doc: doc.data()}
console.log(nProfile);//nProfile contains data
setAllProfile([...allProfile, nProfile]);
console.log(allProfile); // is empty
}
);
})
} catch (error) {
console.log('xxx', error);
}
}
The setAllProfile will update the state when the iteration is done. So in order for your code to work, you will need to pass the callback function to the setAllProfile as shown in the docs
setAllProfile((prevState) => [...prevState, nProfile])
UPDATE
Example demonstrating this at work
Since setAllProfile is the asynchronous method, you can't get the updated value immediately after setAllProfile. You should get it inside useEffect with adding a allProfile dependency.
setAllProfile([...allProfile, nProfile]);
console.log(allProfile); // Old `allProfile` value will be printed, which is the initial empty array.
useEffect(() => {
console.log(allProfile);
}, [allProfile]);
UPDATE
const [allProfile, setAllProfile] = useState([]);
.....
const displayProfileList = async () => {
try {
await profile
.get()
.then(querySnapshot => {
const profiles = [];
querySnapshot.docs.map(doc => {
const documentId = doc.id;
const nProfile = { id: documentId, doc: doc.data()}
console.log(nProfile);//nProfile contains data
profiles.push(nProfile);
}
);
setAllProfile([...allProfile, ...profiles]);
})
} catch (error) {
console.log('xxx', error);
}
}
You are calling setState inside a map and therefore create few async calls, all referred to by current ..allProfile value call (and not prev => [...prev...)
Try
let arr=[]
querySnapshot.docs.map(doc => {
arr.push({ id: doc.id, doc: doc.data() })
}
setAllProfile(prev=>[...prev, ...arr])
I don't sure how the architecture of fetching the posts implemented (in terms of pagination and so on, so you might don't need to destruct ...prev