Infinite loop when pushing into array using UseState()? - node.js

I would like to store the data of a response into an array for reuse. I am using Axios for this. The issue I receive is that when I push into the array, it loops getBoroughAndId() and keeps pushing into the array. I can tell because I get a console.log() response where it keeps telling me I am making too many requests. Any advice? Thanks.
Edit: After taking another gander, I think the issue is that the id is always changing when running getBoroughAndId. I'm not sure how to stop this.
import React, { useEffect, useState } from 'react';
import { airtableApi } from '../services/api/airtable';
import { BoroughDay, BoroughGroup } from '../types/api';
const IndexPage = () => {
const [boroughs, setBoroughs] = useState<BoroughDay[]>([]);
const [boroughGroups, setBoroughGroups] = useState<BoroughGroup[]>([]);
const getBoroughsAndDays = () => {
airtableApi
.getBoroughsAndDays()
.then((response) => {
setBoroughs(response);
})
.catch(() => { });
};
const getBoroughAndId = (id: string) => {
airtableApi
.getBoroughAndId(id)
.then((response) => {
console.log(response);
setBoroughGroups(arr => [...arr, response])
return response;
})
.catch(() => { });
}
useEffect(() => {
getBoroughsAndDays()
}, [boroughGroups])
return (
<>
{boroughs.map((data) => {
getBoroughAndId(data.id);
})}
</>
)
}
export default IndexPage
Here is my corrected code. It works a lot better now, with less nonsense and everything being done in the first function.
import React, { useEffect, useState } from 'react';
import { airtableApi } from '../services/api/airtable';
import { BoroughDay, BoroughGroup } from '../types/api';
const IndexPage = () => {
const [boroughs, setBoroughs] = useState<BoroughDay[]>([]);
const [boroughGroups, setBoroughGroups] = useState<BoroughGroup[]>([]);
const getBoroughsDays = () => {
airtableApi
.getBoroughsAndDays()
.then((response) => {
setBoroughs(response.records);
response.records.map((data) => {
setBoroughGroups(arr => [...arr, {id: data.id, "Borough": data.fields["Borough"]}])
})
})
.catch(() => { })
}
useEffect(() => {
getBoroughsDays();
}, [])
return (
<>
{boroughGroups.map(data => <div>{data.id} {data.Borough}</div>)}
</>
)
}
export default IndexPage

In order to tell you the mistake you are committing, I will tell you the whole flow of your program.
First of all when component mounts, useEffect will be called, which will call the getBoroughsAndDays function.
Note: boroughGroups in dependency array in useEffect is causing an infinite loop
This function (getBoroughsAndDays()) will update the value of boroughs(using setBoroughs)
Now since the state updated the function will re render, hence output will be shown on the screen
Now observe, here you are calling "getBoroughAndId(data.id)" function (inside map function), which is updating the value of boroughGroups(using setBoroughGroups)
Now since the value of boroughGroups have changed, the useEffect method will be called, which will again trigger the "getBoroughsandDays()" function, repeating the whole process again, so that is the reason, it is creating infinite loop.
Note: When any value inside dependency array changes useEffect will be called.
Solution:
I don't know what functionality you want to achieve but remove "boroughGroups" dependency from useEffect (In this way it will behave like componentDidMount).

You have a useEffect hook that updates state: boroughs whenever value of state: boroughGroups changes.
In the return statement, you iterate through boroughs and update boroughGroups.
Back to the first statement.
To stop this infinite loop, stop updating boroughGroups, that triggers useEffect everytime.
useEffect(() => {getBoroughsAndDays()}, []);

Related

React JavaScript Axios requests problem appearing

I'm trying to get the data from Node.js, but it is making like 5000 requests and the server is getting very slow and don't even load some of the other images in other pages. Do I need to limit the requests?
Any solutions?
const [data, SetData] = useState([]);
useEffect(() => {
axios.get(`/api/data/${id}`).then((response) => {
SetData(response.data)
})
})
When I'm adding }, []) to useEffect the images from Public folder are not showing.
Your effect probably missing the id as dependency.
Change it to be like that:
const [data, setData] = useState([]);
useEffect(() => {
if (id) {
axios.get(`/api/data/${id}`).then((response) => {
setData(response.data)
});
}
}, [id]);
useEffect runs after every render without the dependencies array, causing infinite loop
useEffect(() => {
axios.get(`/api/data/${id}`).then((response) => {
SetData(response.data)
});
//Infinite loop
});
Provide an empty dependencies array, this will cause the effect function to only run once after the component has rendered the first time.
useEffect(() => {
axios.get(`/api/data/${id}`).then((response) => {
SetData(response.data)
});
// Correct! Runs once after render with empty array
}, []);

React and Easybase - Invalid hook call. Hooks can only be called inside of the body of a function component

I am trying to use React and Easybase (database). I'm having some issues however.
This is in the SolanaSignature.tsx file.
import { useWallet } from '#solana/wallet-adapter-react';
import bs58 from 'bs58';
import React, { FC, useCallback } from 'react';
import ReactDOM from 'react-dom';
import { sign } from 'tweetnacl';
import AddUser from './mainstorage';
export const SignMessageButton : FC = () => {
const { publicKey, signMessage } = useWallet();
const onClick = useCallback(async () => {
try {
if (!publicKey) throw new Error('Wallet not connected!');
if (!signMessage) throw new Error('Wallet does not support message signing! Please use a wallet such as Phantom or Solflare! NOTE: Some Ledgers wallets are not supported!');
const message = new TextEncoder().encode('Omega Protocol - Signature verification for Bold Badgers.');
const signature = await signMessage(message);
if (!sign.detached.verify(message, signature, publicKey.toBytes())) throw new Error('Invalid signature!');
//alert(`Message signature: ${bs58.encode(signature)}`);
AddUser();
} catch (error: any) {
alert(`Signing failed: ${error?.message}`);
}
}, [publicKey, signMessage]);
return signMessage ? (<button className="wallet-adapter-button wallet-adapter-button-trigger shine" onClick={onClick} disabled={!publicKey}>Verify</button>) : null;
};
and then the mainstorage file:
import { useEffect } from 'react';
import { useEasybase } from 'easybase-react';
const AddUser = () => {
const { db } = useEasybase();
useEffect(() => {
db('OMEGABB').insert({ walletid: "test", discordid: "test", signature: "test", valid: false, lastvalid: new Date() }).one()
.then(() => console.log("Success!"));
}, [])
return (
{/* ... */}
);
}
export default AddUser;
What is happening however when I click the button is that it comes up with a warning: Hooks can only be called inside the body of a function component.
This does work in the initial index file (aka the parent file) but does not work here. Right now this is only a dummy/test but trying to get it writing to the database.
Thanks!
As per React's documentation:
Don’t call Hooks inside loops, conditions, or nested functions. Instead, always use Hooks at the top level of your React function, before any early returns.
Currently, you're attempting to call a hook inside the onClick handler - AddUser is a custom hook since it also uses hooks and the better name for it should be useAddUser.
I suggest to make some improvements by returning a function from your custom hook that you can call to add a new user, e.g.:
export const useAddUser = () => {
const {db} = useEasybase()
const addUser = React.useCallback(() => {
db('OMEGABB')
.insert(/*...*/)
.then(/*...*/)
.catch(/*...*/)
}, [db])
return {
addUser,
/*...*/
}
}
Then, you can use useAddUser in the following way:
const {useAddUser} from './mainstorage'
const SignMessageButton: FC = () => {
const {publicKey, signMessage} = useWallet()
const {addUser} = useAddUser();
const onClick = React.useCallback(
async () => {
try {
// ...
addUser()
} catch (error) {/*...*/}
},
[publicKey, signMessage, addUser]
)
/*...*/
}

React 17.0.1 basic onChange is not updating values into state [duplicate]

I am trying to learn hooks and the useState method has made me confused. I am assigning an initial value to a state in the form of an array. The set method in useState is not working for me, both with and without the spread syntax.
I have made an API on another PC that I am calling and fetching the data which I want to set into the state.
Here is my code:
<div id="root"></div>
<script type="text/babel" defer>
// import React, { useState, useEffect } from "react";
// import ReactDOM from "react-dom";
const { useState, useEffect } = React; // web-browser variant
const StateSelector = () => {
const initialValue = [
{
category: "",
photo: "",
description: "",
id: 0,
name: "",
rating: 0
}
];
const [movies, setMovies] = useState(initialValue);
useEffect(() => {
(async function() {
try {
// const response = await fetch("http://192.168.1.164:5000/movies/display");
// const json = await response.json();
// const result = json.data.result;
const result = [
{
category: "cat1",
description: "desc1",
id: "1546514491119",
name: "randomname2",
photo: null,
rating: "3"
},
{
category: "cat2",
description: "desc1",
id: "1546837819818",
name: "randomname1",
rating: "5"
}
];
console.log("result =", result);
setMovies(result);
console.log("movies =", movies);
} catch (e) {
console.error(e);
}
})();
}, []);
return <p>hello</p>;
};
const rootElement = document.getElementById("root");
ReactDOM.render(<StateSelector />, rootElement);
</script>
<script src="https://unpkg.com/#babel/standalone#7/babel.min.js"></script>
<script src="https://unpkg.com/react#17/umd/react.production.min.js"></script>
<script src="https://unpkg.com/react-dom#17/umd/react-dom.production.min.js"></script>
Neither setMovies(result) nor setMovies(...result) works.
I expect the result variable to be pushed into the movies array.
Much like .setState() in class components created by extending React.Component or React.PureComponent, the state update using the updater provided by useState hook is also asynchronous, and will not be reflected immediately.
Also, the main issue here is not just the asynchronous nature but the fact that state values are used by functions based on their current closures, and state updates will reflect in the next re-render by which the existing closures are not affected, but new ones are created. Now in the current state, the values within hooks are obtained by existing closures, and when a re-render happens, the closures are updated based on whether the function is recreated again or not.
Even if you add a setTimeout the function, though the timeout will run after some time by which the re-render would have happened, the setTimeout will still use the value from its previous closure and not the updated one.
setMovies(result);
console.log(movies) // movies here will not be updated
If you want to perform an action on state update, you need to use the useEffect hook, much like using componentDidUpdate in class components since the setter returned by useState doesn't have a callback pattern
useEffect(() => {
// action on update of movies
}, [movies]);
As far as the syntax to update state is concerned, setMovies(result) will replace the previous movies value in the state with those available from the async request.
However, if you want to merge the response with the previously existing values, you must use the callback syntax of state updation along with the correct use of spread syntax like
setMovies(prevMovies => ([...prevMovies, ...result]));
Additional details to the previous answer:
While React's setState is asynchronous (both classes and hooks), and it's tempting to use that fact to explain the observed behavior, it is not the reason why it happens.
TLDR: The reason is a closure scope around an immutable const value.
Solutions:
read the value in render function (not inside nested functions):
useEffect(() => { setMovies(result) }, [])
console.log(movies)
add the variable into dependencies (and use the react-hooks/exhaustive-deps eslint rule):
useEffect(() => { setMovies(result) }, [])
useEffect(() => { console.log(movies) }, [movies])
use a temporary variable:
useEffect(() => {
const newMovies = result
console.log(newMovies)
setMovies(newMovies)
}, [])
use a mutable reference (if we don't need a state and only want to remember the value - updating a ref doesn't trigger re-render):
const moviesRef = useRef(initialValue)
useEffect(() => {
moviesRef.current = result
console.log(moviesRef.current)
}, [])
Explanation why it happens:
If async was the only reason, it would be possible to await setState().
However, both props and state are assumed to be unchanging during 1 render.
Treat this.state as if it were immutable.
With hooks, this assumption is enhanced by using constant values with the const keyword:
const [state, setState] = useState('initial')
The value might be different between 2 renders, but remains a constant inside the render itself and inside any closures (functions that live longer even after render is finished, e.g. useEffect, event handlers, inside any Promise or setTimeout).
Consider following fake, but synchronous, React-like implementation:
// sync implementation:
let internalState
let renderAgain
const setState = (updateFn) => {
internalState = updateFn(internalState)
renderAgain()
}
const useState = (defaultState) => {
if (!internalState) {
internalState = defaultState
}
return [internalState, setState]
}
const render = (component, node) => {
const {html, handleClick} = component()
node.innerHTML = html
renderAgain = () => render(component, node)
return handleClick
}
// test:
const MyComponent = () => {
const [x, setX] = useState(1)
console.log('in render:', x) // ✅
const handleClick = () => {
setX(current => current + 1)
console.log('in handler/effect/Promise/setTimeout:', x) // ❌ NOT updated
}
return {
html: `<button>${x}</button>`,
handleClick
}
}
const triggerClick = render(MyComponent, document.getElementById('root'))
triggerClick()
triggerClick()
triggerClick()
<div id="root"></div>
I know that there are already very good answers. But I want to give another idea how to solve the same issue, and access the latest 'movie' state, using my module react-useStateRef.
As you understand by using React state you can render the page every time the state change. But by using React ref, you can always get the latest values.
So the module react-useStateRef let you use state's and ref's together. It's backward compatible with React.useState, so you can just replace the import statement
const { useEffect } = React
import { useState } from 'react-usestateref'
const [movies, setMovies] = useState(initialValue);
useEffect(() => {
(async function() {
try {
const result = [
{
id: "1546514491119",
},
];
console.log("result =", result);
setMovies(result);
console.log("movies =", movies.current); // will give you the latest results
} catch (e) {
console.error(e);
}
})();
}, []);
More information:
react-usestsateref
I just finished a rewrite with useReducer, following #kentcdobs article (ref below) which really gave me a solid result that suffers not one bit from these closure problems.
See: https://kentcdodds.com/blog/how-to-use-react-context-effectively
I condensed his readable boilerplate to my preferred level of DRYness -- reading his sandbox implementation will show you how it actually works.
import React from 'react'
// ref: https://kentcdodds.com/blog/how-to-use-react-context-effectively
const ApplicationDispatch = React.createContext()
const ApplicationContext = React.createContext()
function stateReducer(state, action) {
if (state.hasOwnProperty(action.type)) {
return { ...state, [action.type]: state[action.type] = action.newValue };
}
throw new Error(`Unhandled action type: ${action.type}`);
}
const initialState = {
keyCode: '',
testCode: '',
testMode: false,
phoneNumber: '',
resultCode: null,
mobileInfo: '',
configName: '',
appConfig: {},
};
function DispatchProvider({ children }) {
const [state, dispatch] = React.useReducer(stateReducer, initialState);
return (
<ApplicationDispatch.Provider value={dispatch}>
<ApplicationContext.Provider value={state}>
{children}
</ApplicationContext.Provider>
</ApplicationDispatch.Provider>
)
}
function useDispatchable(stateName) {
const context = React.useContext(ApplicationContext);
const dispatch = React.useContext(ApplicationDispatch);
return [context[stateName], newValue => dispatch({ type: stateName, newValue })];
}
function useKeyCode() { return useDispatchable('keyCode'); }
function useTestCode() { return useDispatchable('testCode'); }
function useTestMode() { return useDispatchable('testMode'); }
function usePhoneNumber() { return useDispatchable('phoneNumber'); }
function useResultCode() { return useDispatchable('resultCode'); }
function useMobileInfo() { return useDispatchable('mobileInfo'); }
function useConfigName() { return useDispatchable('configName'); }
function useAppConfig() { return useDispatchable('appConfig'); }
export {
DispatchProvider,
useKeyCode,
useTestCode,
useTestMode,
usePhoneNumber,
useResultCode,
useMobileInfo,
useConfigName,
useAppConfig,
}
With a usage similar to this:
import { useHistory } from "react-router-dom";
// https://react-bootstrap.github.io/components/alerts
import { Container, Row } from 'react-bootstrap';
import { useAppConfig, useKeyCode, usePhoneNumber } from '../../ApplicationDispatchProvider';
import { ControlSet } from '../../components/control-set';
import { keypadClass } from '../../utils/style-utils';
import { MaskedEntry } from '../../components/masked-entry';
import { Messaging } from '../../components/messaging';
import { SimpleKeypad, HandleKeyPress, ALT_ID } from '../../components/simple-keypad';
export const AltIdPage = () => {
const history = useHistory();
const [keyCode, setKeyCode] = useKeyCode();
const [phoneNumber, setPhoneNumber] = usePhoneNumber();
const [appConfig, setAppConfig] = useAppConfig();
const keyPressed = btn => {
const maxLen = appConfig.phoneNumberEntry.entryLen;
const newValue = HandleKeyPress(btn, phoneNumber).slice(0, maxLen);
setPhoneNumber(newValue);
}
const doSubmit = () => {
history.push('s');
}
const disableBtns = phoneNumber.length < appConfig.phoneNumberEntry.entryLen;
return (
<Container fluid className="text-center">
<Row>
<Messaging {...{ msgColors: appConfig.pageColors, msgLines: appConfig.entryMsgs.altIdMsgs }} />
</Row>
<Row>
<MaskedEntry {...{ ...appConfig.phoneNumberEntry, entryColors: appConfig.pageColors, entryLine: phoneNumber }} />
</Row>
<Row>
<SimpleKeypad {...{ keyboardName: ALT_ID, themeName: appConfig.keyTheme, keyPressed, styleClass: keypadClass }} />
</Row>
<Row>
<ControlSet {...{ btnColors: appConfig.buttonColors, disabled: disableBtns, btns: [{ text: 'Submit', click: doSubmit }] }} />
</Row>
</Container>
);
};
AltIdPage.propTypes = {};
Now everything persists smoothly everywhere across all my pages
React's useEffect has its own state/lifecycle. It's related to mutation of state, and it will not update the state until the effect is destroyed.
Just pass a single argument in parameters state or leave it a black array and it will work perfectly.
React.useEffect(() => {
console.log("effect");
(async () => {
try {
let result = await fetch("/query/countries");
const res = await result.json();
let result1 = await fetch("/query/projects");
const res1 = await result1.json();
let result11 = await fetch("/query/regions");
const res11 = await result11.json();
setData({
countries: res,
projects: res1,
regions: res11
});
} catch {}
})(data)
}, [setData])
# or use this
useEffect(() => {
(async () => {
try {
await Promise.all([
fetch("/query/countries").then((response) => response.json()),
fetch("/query/projects").then((response) => response.json()),
fetch("/query/regions").then((response) => response.json())
]).then(([country, project, region]) => {
// console.log(country, project, region);
setData({
countries: country,
projects: project,
regions: region
});
})
} catch {
console.log("data fetch error")
}
})()
}, [setData]);
Alternatively, you can try React.useRef() for instant change in the React hook.
const movies = React.useRef(null);
useEffect(() => {
movies.current='values';
console.log(movies.current)
}, [])
The closure is not the only reason.
Based on the source code of useState (simplified below). Seems to me the value is never assigned right away.
What happens is that an update action is queued when you invoke setValue. And after the schedule kicks in and only when you get to the next render, these update action then is applied to that state.
Which means even we don't have closure issue, react version of useState is not going to give you the new value right away. The new value doesn't even exist until next render.
function useState(initialState) {
let hook;
...
let baseState = hook.memoizedState;
if (hook.queue.pending) {
let firstUpdate = hook.queue.pending.next;
do {
const action = firstUpdate.action;
baseState = action(baseState); // setValue HERE
firstUpdate = firstUpdate.next;
} while (firstUpdate !== hook.queue.pending);
hook.queue.pending = null;
}
hook.memoizedState = baseState;
return [baseState, dispatchAction.bind(null, hook.queue)];
}
function dispatchAction(queue, action) {
const update = {
action,
next: null
};
if (queue.pending === null) {
update.next = update;
} else {
update.next = queue.pending.next;
queue.pending.next = update;
}
queue.pending = update;
isMount = false;
workInProgressHook = fiber.memoizedState;
schedule();
}
There's also an article explaining the above in the similar way, https://dev.to/adamklein/we-don-t-know-how-react-state-hook-works-1lp8
I too was stuck with the same problem. As other answers above have clarified the error here, which is that useState is asynchronous and you are trying to use the value just after setState. It is not updating on the console.log() part because of the asynchronous nature of setState, it lets your further code to execute, while the value updating happens on the background. Thus you are getting the previous value. When the setState is completed on the background it will update the value and you will have access to that value on the next render.
If anyone is interested to understand this in detail. Here is a really good Conference talk on the topic.
https://www.youtube.com/watch?v=8aGhZQkoFbQ
I found this to be good. Instead of defining state (approach 1) as, example,
const initialValue = 1;
const [state,setState] = useState(initialValue)
Try this approach (approach 2),
const [state = initialValue,setState] = useState()
This resolved the rerender issue without using useEffect since we are not concerned with its internal closure approach with this case.
P.S.: If you are concerned with using old state for any use case then useState with useEffect needs to be used since it will need to have that state, so approach 1 shall be used in this situation.
If we have to update state only, then a better way can be if we use the push method to do so.
Here is my code. I want to store URLs from Firebase in state.
const [imageUrl, setImageUrl] = useState([]);
const [reload, setReload] = useState(0);
useEffect(() => {
if (reload === 4) {
downloadUrl1();
}
}, [reload]);
const downloadUrl = async () => {
setImages([]);
try {
for (let i = 0; i < images.length; i++) {
let url = await storage().ref(urls[i].path).getDownloadURL();
imageUrl.push(url);
setImageUrl([...imageUrl]);
console.log(url, 'check', urls.length, 'length', imageUrl.length);
}
}
catch (e) {
console.log(e);
}
};
const handleSubmit = async () => {
setReload(4);
await downloadUrl();
console.log(imageUrl);
console.log('post submitted');
};
This code works to put URLs in state as an array. This might also work for you.
With custom hooks from my library, you can wait for the state values to update:
useAsyncWatcher(...values):watcherFn(peekPrevValue: boolean)=>Promise - is a promise wrapper around useEffect that can wait for updates and return a new value and possibly a previous one if the optional peekPrevValue argument is set to true.
(Live Demo)
import React, { useState, useEffect, useCallback } from "react";
import { useAsyncWatcher } from "use-async-effect2";
function TestComponent(props) {
const [counter, setCounter] = useState(0);
const [text, setText] = useState("");
const textWatcher = useAsyncWatcher(text);
useEffect(() => {
setText(`Counter: ${counter}`);
}, [counter]);
const inc = useCallback(() => {
(async () => {
await new Promise((resolve) => setTimeout(resolve, 1000));
setCounter((counter) => counter + 1);
const updatedText = await textWatcher();
console.log(updatedText);
})();
}, []);
return (
<div className="component">
<div className="caption">useAsyncEffect demo</div>
<div>{counter}</div>
<button onClick={inc}>Inc counter</button>
</div>
);
}
export default TestComponent;
useAsyncDeepState is a deep state implementation (similar to this.setState (patchObject)) whose setter can return a promise synchronized with the internal effect. If the setter is called with no arguments, it does not change the state values, but simply subscribes to state updates. In this case, you can get the state value from anywhere inside your component, since function closures are no longer a hindrance.
(Live Demo)
import React, { useCallback, useEffect } from "react";
import { useAsyncDeepState } from "use-async-effect2";
function TestComponent(props) {
const [state, setState] = useAsyncDeepState({
counter: 0,
computedCounter: 0
});
useEffect(() => {
setState(({ counter }) => ({
computedCounter: counter * 2
}));
}, [state.counter]);
const inc = useCallback(() => {
(async () => {
await new Promise((resolve) => setTimeout(resolve, 1000));
await setState(({ counter }) => ({ counter: counter + 1 }));
console.log("computedCounter=", state.computedCounter);
})();
});
return (
<div className="component">
<div className="caption">useAsyncDeepState demo</div>
<div>state.counter : {state.counter}</div>
<div>state.computedCounter : {state.computedCounter}</div>
<button onClick={() => inc()}>Inc counter</button>
</div>
);
}
var [state,setState]=useState(defaultValue)
useEffect(()=>{
var updatedState
setState(currentState=>{ // Do not change the state by get the updated state
updateState=currentState
return currentState
})
alert(updateState) // the current state.
})
Without any addtional NPM package
//...
const BackendPageListing = () => {
const [ myData, setMyData] = useState( {
id: 1,
content: "abc"
})
const myFunction = ( x ) => {
setPagenateInfo({
...myData,
content: x
})
console.log(myData) // not reflecting change immediately
let myDataNew = {...myData, content: x };
console.log(myDataNew) // Reflecting change immediately
}
return (
<>
<button onClick={()=>{ myFunction("New Content")} }>Update MyData</button>
</>
)
Not saying to do this, but it isn't hard to do what the OP asked without useEffect.
Use a promise to resolve the new state in the body of the setter function:
const getState = <T>(
setState: React.Dispatch<React.SetStateAction<T>>
): Promise<T> => {
return new Promise((resolve) => {
setState((currentState: T) => {
resolve(currentState);
return currentState;
});
});
};
And this is how you use it (example shows the comparison between count and outOfSyncCount/syncCount in the UI rendering):
const App: React.FC = () => {
const [count, setCount] = useState(0);
const [outOfSyncCount, setOutOfSyncCount] = useState(0);
const [syncCount, setSyncCount] = useState(0);
const handleOnClick = async () => {
setCount(count + 1);
// Doesn't work
setOutOfSyncCount(count);
// Works
const newCount = await getState(setCount);
setSyncCount(newCount);
};
return (
<>
<h2>Count = {count}</h2>
<h2>Synced count = {syncCount}</h2>
<h2>Out of sync count = {outOfSyncCount}</h2>
<button onClick={handleOnClick}>Increment</button>
</>
);
};
Use the Background Timer library. It solved my problem.
const timeoutId = BackgroundTimer.setTimeout(() => {
// This will be executed once after 1 seconds
// even when the application is the background
console.log('tac');
}, 1000);
// replace
return <p>hello</p>;
// with
return <p>{JSON.stringify(movies)}</p>;
Now you should see, that your code actually does work. What does not work is the console.log(movies). This is because movies points to the old state. If you move your console.log(movies) outside of useEffect, right above the return, you will see the updated movies object.

Converting React Functional Component to Hook

While trying to learn full stack development I was trying out this tutorial ( https://www.freecodecamp.org/news/create-a-react-frontend-a-node-express-backend-and-connect-them-together-c5798926047c/ ) on a React-Express-Node basic app. However, it was written using functional components instead of hooks. I'm trying to convert this section to a hook:
constructor(props) {
super(props);
this.state = { apiResponse: "" };
}
callAPI() {
fetch("http://localhost:9000/testAPI")
.then(res => res.text())
.then(res => this.setState({ apiResponse: res }));
}
componentWillMount() {
this.callAPI();
}
with this in the render section:
<p className="App-intro">;{this.state.apiResponse}</p>
I tried this:
const [apiResponse, setApiResponse] = useState();
useEffect(() => {
const fetchApiResponse = async () => {
const result = await (
'http://localhost:9000/testAPI'
);
setApiResponse(result);
console.log("apiResponse " + apiResponse);
};
fetchApiResponse();
});
but the console.log of the apiResponse always shows as undefined. I know I must be doing something wrong but I can't figure it out.
You aren't far off in your attempt.
There are two problems:
Problem 1.
In order to get the same effect as componentWillMount (side note - this is a deprecated method, use componentDidMount or the constructor) you need to tell the useEffect to only run once on mount. To do this you give it an empty array of dependencies.
useEffect(() => {
// do stuff
}, []); // empty array as second argument
By not giving a second argument, the effect will run every single render.
Problem 2.
State updates are asynchronous. This means you cannot console log apiResponse immediately after updating it and expect it to contain the new value.
To get around this, just console.log inside the function body outside of the hook.
Here is a simplified example:
const {useState, useEffect} = React;
const Example = () => {
const [apiResponse, setApiResponse] = useState();
useEffect(() => {
const fetchApiResponse = () => {
const result = 'test';
setApiResponse(result);
// Will not be updated
console.log("wrong: apiResponse ", apiResponse);
}
fetchApiResponse();
}, []);
// Will be updated
console.log("right: apiResponse ", apiResponse);
return <span />
}
ReactDOM.render(<Example />, document.getElementById('root'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="root"></div>

Jest: How to test a component that dispatch an API action

I am new to testing,
I am trying to understand if it's possible to test an API call that dispatch from the component.
Is it possible to wait for a response from the server?
import React from 'react';
import { render, fireEvent, cleanup, waitForElement } from 'react-testing-library';
import 'jest-dom/extend-expect'
import App from 'src/app';
import { store } from 'stories/index.stories.js';
import { Provider, connect } from 'react-redux';
const renderComponent = () => (<App />);
it('renders correctly', () => {
const { container, getByText, queryAllByText, getByPlaceholderText } = renderComponent();
expect(container).not.toBeEmpty();
expect(getByText(/Discover/i)).toBeTruthy();
const discoverBtn = getByText(/Discover/i);
fireEvent.click(discoverBtn); // this will dispatch an action from the component
//what should i do next ?
});
This is how I do it.
First I put all my fetch requests in a separate file, say /api/index.js. In this way I can easily mock them in my tests.
Then I perform the actions that a user would do. Finally I check that the API got called and that the DOM was updated correctly.
import { aFetchMethod } from './api'
// With jest.mock our API method does nothing
// we don't want to hit the server in our tests
jest.mock('./api')
it('renders correctly', async () => {
const { getByText } = render(<App />)
aFetchMethod.mockResolvedValueOnce('some data that makes sense for you')
fireEvent.click(getByText('Discover'))
expect(aFetchMethod).toHaveBeenCalledTimes(1)
expect(aFetchMethod).toHaveBeenCalledWith('whatever you call it with')
// Let's assume you now render the returned data
await wait(() => getByText('some data that makes sense for you'))
})

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