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 fetch records from dynamodb using graphql (AWS amplify reacjs setup). I have created a component CTCards and using this in App.js
The problem that we are facing is function fetchTodos is running in infinite loop though in Database there is only two records and I am getting exactly 2 records but this function is running in infinite loop
When I comment the fetching code then the program runs as expected
//== CTCards ( component which is giving issue) ===
function CTCards(props) {
const [todo, setTodos] = useState([]);
useEffect(() => {
fetchTodos();
}, []);
const fetchTodos = async () => {
try {
// == this-block ===
const todoData = await API.graphql(graphqlOperation(listTodos));
const todoList = todoData.data.listTodos.items;
console.log('To Do List', todoList);
setTodos(todoList);
// == /this-block ===
} catch (error) {
console.log('error on fetching to do list', error);
}
};
return (
<div style={{color: "red"}}>
{todo.map((todo, index) => {
<div style={{color: "red"}}>
<div>{todo.name}</div>
<div>{todo.description}</div>
</div>
})}
</div>
)
}
//== App.js code ===
function App() {
return (
<div>
<CTCards/>
</div>
);
}
The setTodos is remounting the component so the useEffect body runs again.
If you comment out the setTodos it should run normally.
You can use React.memo to compare props and not the virtual dom but it will still do a useless fetch. Consider restructuring your application so the fetch happens one layer above.
As #arti91 said it s cause you are updating the state and doing so react rerenders the component and runs the useEffect hook again.
A possible solution to this could be to check inside the hook if you already fetched the data.
useEffect(() => {
if (!todos) {
fetchTodos();
}
}, []);
Instead of this code:
try {
// == this-block ===
const todoData = await API.graphql(graphqlOperation(listTodos));
const todoList = todoData.data.listTodos.items;
console.log('To Do List', todoList);
setTodos(todoList);
// == /this-block ===
} catch (error) {
console.log('error on fetching to do list', error);
}
Try this code:
try {
const todoData = API.graphql({ query: queries.listTodos }).then(response => {
setTodos(response.data.listTodos.items);
}).catch(err => {
console.log(err);
});
} catch (err) {
console.log('error facing Todos:', err)
}
This issue occurs because the of the Promise that is still is prebuild phase so if we give it some time that it could complete the cycle and get the value to store it.
After Fetch /Vehicles from api I wanted to show them in component but inside useFetch function I can console.log res.data but inside the Trucks component I could not map trucks.
My Custom useFetch function:
import { useState, useEffect } from 'react';
import axios from 'axios';
const useFetch = (url) => {
const api = axios.create({
baseURL: 'api url',
headers: { Authorization: `Bearer ${localStorage.token}` },
});
const [data, setData] = useState(null);
const [isPending, setIsPending] = useState(true);
const [error, setError] = useState(null);
useEffect(() => {
const abortCont = new AbortController();
setTimeout(() => {
api
.get(url)
.then((res) => {
console.log(res.data);
if (!res.ok) {
throw Error('Could not with fetch data');
}
})
.then((data) => {
data.json();
console.log(data);
setIsPending(false);
setError(null);
})
.catch((err) => {
if (err.name === 'AbortError') {
console.log('fetch aborted');
} else {
setIsPending(false);
setError(err.message);
}
});
}, 1000);
return () => abortCont.abort();
}, [url]);
return { data, isPending, error };
};
export default useFetch;
I used this custom useFetch function and could get data (inside the function) its not usable to render
Some possible reasons I thought ;
useFetch function may not return data properly
I may need to use stringify or json function for API respond
Trucks Component:
const Trucks = () => {
const { data: trucks, error, isPending } = useFetch('/vehicles');//
console.log('they are ' + { trucks }); //console output ->[object,Object]
return (Some Jsx)};
export default Trucks;
Thanks
There are two issues with your Hook itself that I can see.
The first thing is that you never call setData. This means that although you are fetching the data inside your hook and consoling it out, you never set the data object to hold the retrieved data.
The second thing is that the second then isn't actually doing anything with the data you are retrieving. You can handle this all inside the first statement as such
.then((res) => {
setData(res.data);
setIsPending(false);
if (!res.ok) {
setIsPending(false);
throw Error("Could not with fetch data");
}
})
Once you do this you should be able to call your hook inside the related component, and render it using some JSX. Just make sure to add a check for isPending and display some sort of loading screen before actually rendering your data.
Another note, when you use axios, you do not need to call data.json(). That is something that is required for the fetch API but axios just requires you to call res.data when resolving the promise
Beginner with react and struggling with something that I am sure is probably very simple. I am just trying to make a simple component that will fetch data and display a part of it in a div. I am able to get the data and print it to console, but I am having trouble saving to a variable and displaying it. Here is my code (removed the actual url for privacy reasons):
let x = -1;
function getData(apiUrl){
fetch(apiUrl, {credentials: 'same-origin'})
.then((response) => {
if (!response.ok) {
Logging.error(`Did not get an ok. got: ${response.statusText}`);
}
return response.json();
})
.then(json => {x = json.value})
.catch((error) => {
Logging.error(`Error getting ad data: ${error.message}`);
})
}
const MyPage = () => {
getData('my endpoint')
return (
<div>{x}</div>
);
}
My issue is when I load the page it always displays my default value of "-1". So either x is never getting re-assigned, or the return is happening before it does.
Other commenters about setting state is not wrong.
However, you are also not exactly wrong, expecting a value for x.
Your getData function calls fetch, however you did not return anything from fetch. If you want to use x = getData(), you will need to ensure to add a return before the fetch function in order to return the data.
const getData = (apiUrl) => {
return fetch(apiUrl, {credentials: 'same-origin'})
.then((response) => {
if (!response.ok) {
Logging.error(`Did not get an ok. got: ${response.statusText}`);
}
return response.json();
})
.then(json => {x = json.value})
.catch((error) => {
Logging.error(`Error getting ad data: ${error.message}`);
})
}
let x = await getData(apiUrl)
However, fetch is asynchronous so it's you need to use x = await getData().
You cannot use await outside an async function, so you need to use effect, and useState to properly render the data you want.
import React, { useEffect, useState } from 'react';
const MyPage = () => {
const [ data, setData ] = useState();
useEffect(() => {
getData(apiUrl);
},[])
const getData = async (apiUrl) => {
fetch(apiUrl, {credentials: 'same-origin'})
.then((response) => {
if (!response.ok) {
Logging.error(`Did not get an ok. got: ${response.statusText}`);
}
return response.json();
})
.then(json => setData(json)) //setData here
.catch((error) => {
Logging.error(`Error getting ad data: ${error.message}`);
})
}
return (<pre>{ JSON.stringify(data, null, 4)}</pre>)
}
You need to use JSON.stringify to show your JSON results in your return statement.
You need to you use the state in react. Try something like:
import react, { useState, useEfect } from 'react';
const MyPage = () => {
const [data, setData] = useState(null);
const useEfect(() => {
const result = getData('my endpoint');
setData(result);
}, []);
return (
<div>{data}</div>
);
}
I'm trying the fetch data from a json API and setting it to a state. Currently using visual studio code with a pixel 4 emulator.
When I try to update my state inside of a useEffect method on the emulator's first launch or on reload, it doesn't change. If I save in vs code, the data in state updates as intended.
...
import React, {useState, useEffect} from 'react';
import {getJsonData} from './getJsonData';
const myApp = () => {
const [state, setState] = useState({
isLoading: true,
data: null,
});
const updateState = data => {
console.log(data); //Logs the correct Json data everytime
setState(state => ({...state, isLoading: false, data: data}));
console.log(state.isLoading); //Doesn't update on reload (updates only on save)
console.log(state.data); //Same as above
};
useEffect(() => {
getJsonData().then(data => updateState(data));
}, []);
return (
<View>
<Text>{state.data.title}</Text>
<Text>{data.data.completed}</Text>
</View>
);
}
And this is the getJsonData( ) function:
export async function getJsonData() {
try {
let response = await fetch('https://jsonplaceholder.typicode.com/todos/1');
let responseJson = await response.json();
return responseJson;
} catch (error) {
console.error(error);
}
}
I ultimately want the state to update on the application's first run, on reload, as well as each time I call a certain reloadApp( ) function.
If the above code is not the best practice, please don't hold back to correct me as I'm just learning about states.
setState function is asynchronous. So console.log immediately after setState will give old value but not the new value.
Also why don't you seperate the states like
const [isLoading,setIsLoading]=useState(true);
const [data,setData] = useState(null);
and set them separately so your code looks better.
in updateState(jsonData) you can do then
setIsloading(false);
setData(jsonData);