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UseEffect runs on first render without being called using Context [duplicate]

According to the docs:
componentDidUpdate() is invoked immediately after updating occurs. This method is not called for the initial render.
We can use the new useEffect() hook to simulate componentDidUpdate(), but it seems like useEffect() is being ran after every render, even the first time. How do I get it to not run on initial render?
As you can see in the example below, componentDidUpdateFunction is printed during the initial render but componentDidUpdateClass was not printed during the initial render.
function ComponentDidUpdateFunction() {
const [count, setCount] = React.useState(0);
React.useEffect(() => {
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
class ComponentDidUpdateClass extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0,
};
}
componentDidUpdate() {
console.log("componentDidUpdateClass");
}
render() {
return (
<div>
<p>componentDidUpdateClass: {this.state.count} times</p>
<button
onClick={() => {
this.setState({ count: this.state.count + 1 });
}}
>
Click Me
</button>
</div>
);
}
}
ReactDOM.render(
<div>
<ComponentDidUpdateFunction />
<ComponentDidUpdateClass />
</div>,
document.querySelector("#app")
);
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>

We can use the useRef hook to store any mutable value we like, so we could use that to keep track of if it's the first time the useEffect function is being run.
If we want the effect to run in the same phase that componentDidUpdate does, we can use useLayoutEffect instead.
Example
const { useState, useRef, useLayoutEffect } = React;
function ComponentDidUpdateFunction() {
const [count, setCount] = useState(0);
const firstUpdate = useRef(true);
useLayoutEffect(() => {
if (firstUpdate.current) {
firstUpdate.current = false;
return;
}
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
ReactDOM.render(
<ComponentDidUpdateFunction />,
document.getElementById("app")
);
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>

You can turn it into custom hooks, like so:
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
if (didMount.current) func();
else didMount.current = true;
}, deps);
}
export default useDidMountEffect;
Usage example:
import React, { useState, useEffect } from 'react';
import useDidMountEffect from '../path/to/useDidMountEffect';
const MyComponent = (props) => {
const [state, setState] = useState({
key: false
});
useEffect(() => {
// you know what is this, don't you?
}, []);
useDidMountEffect(() => {
// react please run me if 'key' changes, but not on initial render
}, [state.key]);
return (
<div>
...
</div>
);
}
// ...

I made a simple useFirstRender hook to handle cases like focussing a form input:
import { useRef, useEffect } from 'react';
export function useFirstRender() {
const firstRender = useRef(true);
useEffect(() => {
firstRender.current = false;
}, []);
return firstRender.current;
}
It starts out as true, then switches to false in the useEffect, which only runs once, and never again.
In your component, use it:
const firstRender = useFirstRender();
const phoneNumberRef = useRef(null);
useEffect(() => {
if (firstRender || errors.phoneNumber) {
phoneNumberRef.current.focus();
}
}, [firstRender, errors.phoneNumber]);
For your case, you would just use if (!firstRender) { ....

Same approach as Tholle's answer, but using useState instead of useRef.
const [skipCount, setSkipCount] = useState(true);
...
useEffect(() => {
if (skipCount) setSkipCount(false);
if (!skipCount) runYourFunction();
}, [dependencies])
EDIT
While this also works, it involves updating state which will cause your component to re-render. If all your component's useEffect calls (and also all of its children's) have a dependency array, this doesn't matter. But keep in mind that any useEffect without a dependency array (useEffect(() => {...}) will be run again.
Using and updating useRef will not cause any re-renders.

#ravi, yours doesn't call the passed-in unmount function. Here's a version that's a little more complete:
/**
* Identical to React.useEffect, except that it never runs on mount. This is
* the equivalent of the componentDidUpdate lifecycle function.
*
* #param {function:function} effect - A useEffect effect.
* #param {array} [dependencies] - useEffect dependency list.
*/
export const useEffectExceptOnMount = (effect, dependencies) => {
const mounted = React.useRef(false);
React.useEffect(() => {
if (mounted.current) {
const unmount = effect();
return () => unmount && unmount();
} else {
mounted.current = true;
}
}, dependencies);
// Reset on unmount for the next mount.
React.useEffect(() => {
return () => mounted.current = false;
}, []);
};

a simple way is to create a let, out of your component and set in to true.
then say if its true set it to false then return (stop) the useEffect function
like that:
import { useEffect} from 'react';
//your let must be out of component to avoid re-evaluation
let isFirst = true
function App() {
useEffect(() => {
if(isFirst){
isFirst = false
return
}
//your code that don't want to execute at first time
},[])
return (
<div>
<p>its simple huh...</p>
</div>
);
}
its Similar to #Carmine Tambasciabs solution but without using state :)
‍‍‍‍‍‍
‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

function useEffectAfterMount(effect, deps) {
const isMounted = useRef(false);
useEffect(() => {
if (isMounted.current) return effect();
else isMounted.current = true;
}, deps);
// reset on unmount; in React 18, components can mount again
useEffect(() => {
isMounted.current = false;
});
}
We need to return what comes back from effect(), because it might be a cleanup function. But we don't need to determine if it is or not. Just pass it on and let useEffect figure it out.
In an earlier version of this post I said resetting the ref (isMounted.current = false) wasn't necessary. But in React 18 it is, because components can remount with their previous state (thanks #Whatabrain).

I thought creating a custom hook would be overkill and I didn't want to muddle my component's readability by using the useLayoutEffect hook for something unrelated to layouts, so, in my case, I simply checked to see if the value of my stateful variable selectedItem that triggers the useEffect callback is its original value in order to determine if it's the initial render:
export default function MyComponent(props) {
const [selectedItem, setSelectedItem] = useState(null);
useEffect(() => {
if(!selectedItem) return; // If selected item is its initial value (null), don't continue
//... This will not happen on initial render
}, [selectedItem]);
// ...
}

This is the best implementation I've created so far using typescript. Basically, the idea is the same, using the Ref but I'm also considering the callback returned by useEffect to perform cleanup on component unmount.
import {
useRef,
EffectCallback,
DependencyList,
useEffect
} from 'react';
/**
* #param effect
* #param dependencies
*
*/
export default function useNoInitialEffect(
effect: EffectCallback,
dependencies?: DependencyList
) {
//Preserving the true by default as initial render cycle
const initialRender = useRef(true);
useEffect(() => {
let effectReturns: void | (() => void) = () => {};
// Updating the ref to false on the first render, causing
// subsequent render to execute the effect
if (initialRender.current) {
initialRender.current = false;
} else {
effectReturns = effect();
}
// Preserving and allowing the Destructor returned by the effect
// to execute on component unmount and perform cleanup if
// required.
if (effectReturns && typeof effectReturns === 'function') {
return effectReturns;
}
return undefined;
}, dependencies);
}
You can simply use it, as usual as you use the useEffect hook but this time, it won't run on the initial render. Here is how you can use this hook.
useNoInitialEffect(() => {
// perform something, returning callback is supported
}, [a, b]);
If you use ESLint and want to use the react-hooks/exhaustive-deps rule for this custom hook:
{
"rules": {
// ...
"react-hooks/exhaustive-deps": ["warn", {
"additionalHooks": "useNoInitialEffect"
}]
}
}

#MehdiDehghani, your solution work perfectly fine, one addition you have to do is on unmount, reset the didMount.current value to false. When to try to use this custom hook somewhere else, you don't get cache value.
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
let unmount;
if (didMount.current) unmount = func();
else didMount.current = true;
return () => {
didMount.current = false;
unmount && unmount();
}
}, deps);
}
export default useDidMountEffect;

Simplified implementation
import { useRef, useEffect } from 'react';
function MyComp(props) {
const firstRender = useRef(true);
useEffect(() => {
if (firstRender.current) {
firstRender.current = false;
} else {
myProp = 'some val';
};
}, [props.myProp])
return (
<div>
...
</div>
)
}

You can use custom hook to run use effect after mount.
const useEffectAfterMount = (cb, dependencies) => {
const mounted = useRef(true);
useEffect(() => {
if (!mounted.current) {
return cb();
}
mounted.current = false;
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};
Here is the typescript version:
const useEffectAfterMount = (cb: EffectCallback, dependencies: DependencyList | undefined) => {
const mounted = useRef(true);
useEffect(() => {
if (!mounted.current) {
return cb();
}
mounted.current = false;
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};

For people who are having trouble with React 18 strict mode calling the useeffect on the initial render twice, try this:
// The init variable is necessary if your state is an object/array, because the == operator compares the references, not the actual values.
const init = [];
const [state, setState] = useState(init);
const dummyState = useRef(init);
useEffect(() => {
// Compare the old state with the new state
if (dummyState.current == state) {
// This means that the component is mounting
} else {
// This means that the component updated.
dummyState.current = state;
}
}, [state]);
Works in development mode...
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.development.js"></script>
<div id="app"></div>
And in production.
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.production.min.js"></script>
<div id="app"></div>

If you want to skip the first render, you can create a state "firstRenderDone" and set it to true in the useEffect with empty dependecy list (that works like a didMount). Then, in your other useEffect, you can check if the first render was already done before doing something.
const [firstRenderDone, setFirstRenderDone] = useState(false);
//useEffect with empty dependecy list (that works like a componentDidMount)
useEffect(() => {
setFirstRenderDone(true);
}, []);
// your other useEffect (that works as componetDidUpdate)
useEffect(() => {
if(firstRenderDone){
console.log("componentDidUpdateFunction");
}
}, [firstRenderDone]);

All previous are good, but this can be achieved in a simplier way considering that the action in useEffect can be "skipped" placing an if condition(or any other ) that is basically not run first time, and still with the dependency.
For example I had the case of :
Load data from an API but my title has to be "Loading" till the date were not there, so I have an array, tours that is empty at beginning and show the text "Showing"
Have a component rendered with different information from those API.
The user can delete one by one those info, even all making the tour array empty again as the beginning but this time the API fetch is been already done
Once the tour list is empty by deleting then show another title.
so my "solution" was to create another useState to create a boolean value that change only after the data fetch making another condition in useEffect true in order to run another function that also depend on the tour length.
useEffect(() => {
if (isTitle) {
changeTitle(newTitle)
}else{
isSetTitle(true)
}
}, [tours])
here my App.js
import React, { useState, useEffect } from 'react'
import Loading from './Loading'
import Tours from './Tours'
const url = 'API url'
let newTours
function App() {
const [loading, setLoading ] = useState(true)
const [tours, setTours] = useState([])
const [isTitle, isSetTitle] = useState(false)
const [title, setTitle] = useState("Our Tours")
const newTitle = "Tours are empty"
const removeTours = (id) => {
newTours = tours.filter(tour => ( tour.id !== id))
return setTours(newTours)
}
const changeTitle = (title) =>{
if(tours.length === 0 && loading === false){
setTitle(title)
}
}
const fetchTours = async () => {
setLoading(true)
try {
const response = await fetch(url)
const tours = await response.json()
setLoading(false)
setTours(tours)
}catch(error) {
setLoading(false)
console.log(error)
}
}
useEffect(()=>{
fetchTours()
},[])
useEffect(() => {
if (isTitle) {
changeTitle(newTitle)
}else{
isSetTitle(true)
}
}, [tours])
if(loading){
return (
<main>
<Loading />
</main>
)
}else{
return (
<main>
<Tours tours={tours} title={title} changeTitle={changeTitle}
removeTours={removeTours} />
</main>
)
}
}
export default App

const [dojob, setDojob] = useState(false);
yourfunction(){
setDojob(true);
}
useEffect(()=>{
if(dojob){
yourfunction();
setDojob(false);
}
},[dojob]);

TypeError: Cannot read properties of undefined (reading 'lat') using React version-18 #react-google-maps/api with Rapid Api [duplicate]

If we want to restrict useEffect to run only when the component mounts, we can add second parameter of useEffect with [].
useEffect(() => {
// ...
}, []);
But how can we make useEffect to run only when the moment when the component is updated except initial mount?

If you want the useEffect to run only on updates except initial mount, you can make use of useRef to keep track of initialMount with useEffect without the second parameter.
const isInitialMount = useRef(true);
useEffect(() => {
if (isInitialMount.current) {
isInitialMount.current = false;
} else {
// Your useEffect code here to be run on update
}
});

I really like Shubham's response, so I made it a custom Hook
/**
* A custom useEffect hook that only triggers on updates, not on initial mount
* #param {Function} effect
* #param {Array<any>} dependencies
*/
export default function useUpdateEffect(effect, dependencies = []) {
const isInitialMount = useRef(true);
useEffect(() => {
if (isInitialMount.current) {
isInitialMount.current = false;
} else {
return effect();
}
}, dependencies);
}

Both Shubham and Mario suggest the right approach, however the code is still incomplete and does not consider following cases.
If the component unmounts, it should reset it's flag
The passing effect function may have a cleanup function returned from it, that would never get called
Sharing below a more complete code which covers above two missing cases:
import React from 'react';
const useIsMounted = function useIsMounted() {
const isMounted = React.useRef(false);
React.useEffect(function setIsMounted() {
isMounted.current = true;
return function cleanupSetIsMounted() {
isMounted.current = false;
};
}, []);
return isMounted;
};
const useUpdateEffect = function useUpdateEffect(effect, dependencies) {
const isMounted = useIsMounted();
const isInitialMount = React.useRef(true);
React.useEffect(() => {
let effectCleanupFunc = function noop() {};
if (isInitialMount.current) {
isInitialMount.current = false;
} else {
effectCleanupFunc = effect() || effectCleanupFunc;
}
return () => {
effectCleanupFunc();
if (!isMounted.current) {
isInitialMount.current = true;
}
};
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};

Shorter One
const [mounted, setMounted] = useRef(false)
useEffect(() => {
if(!mounted) return setMounted(true)
...
})
React Hook Solution
Hook
export const useMounted = () => {
const mounted = useRef(false)
useEffect(() => {
mounted.current = true
return () => {
mounted.current = false
}
}, [])
return () => mounted.current
}
Usage
const Component = () => {
const mounted = useMounted()
useEffect(() => {
if(!mounted()) return
...
})
}

You can get around it by setting the state to a non-boolean initial value (like a null value) :
const [isCartOpen,setCartOpen] = useState(null);
const [checkout,setCheckout] = useState({});
useEffect(() => {
// check to see if its the initial state
if( isCartOpen === null ){
// first load, set cart to real initial state, after load
setCartOpen( false );
}else if(isCartOpen === false){
// normal on update callback logic
setCartOpen( true );
}
}, [checkout]);

Took help from Subham's answer This code will only run for particular item update not on every update and not on component initial mounting.
const isInitialMount = useRef(true); //useEffect to run only on updates except initial mount
//useEffect to run only on updates except initial mount
useEffect(() => {
if (isInitialMount.current) {
isInitialMount.current = false;
} else {
if(fromScreen!='ht1' && appStatus && timeStamp){
// let timeSpentBG = moment().diff(timeStamp, "seconds");
// let newHeatingTimer = ((bottomTab1Timer > timeSpentBG) ? (bottomTab1Timer - timeSpentBG) : 0);
// dispatch({
// type: types.FT_BOTTOM_TAB_1,
// payload: newHeatingTimer,
// })
// console.log('Appstaatus', appStatus, timeSpentBG, newHeatingTimer)
}
}
}, [appStatus])

To make a custom hook compliant with the rules of hooks you don't need to actually pass dependencies, just wrap your effect function with useCallback
function useEffectOnUpdate(callback) {
const mounted = useRef();
useEffect(() => {
if (!mounted.current) {
mounted.current = true;
} else {
callback();
}
}, [callback]);
};
function SomeComponent({ someProp }) {
useEffectOnUpdate(useCallback(() => {
console.log(someProp);
}, [someProp]));
return <div>sample text</div>;
}

If you tried Shubham's answer, and the useeffect is still being called on the initial mount, you can easily fix this by disabling React strictmode. But if you don't want to disable strictmode, use this.
// The init variable is necessary if your state is an object/array, because the == operator compares the references, not the actual values.
const init = [];
const [state, setState] = useState(init);
const dummyState = useRef(init);
useEffect(() => {
// Compare the old state with the new state
if (dummyState.current == state) {
// This means that the component is mounting
} else {
// This means that the component updated.
dummyState.current = state;
}
}, [state]);
Works in development mode...
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.development.js"></script>
<div id="app"></div>
And in production.
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.production.min.js"></script>
<div id="app"></div>

If you tried Shubham's answer, and the useeffect is still being called on the initial mount, and you are using React 18, you can easily fix this by disabling React strictmode. But if you don't want to disable strictmode, use this.
// The init variable is necessary if your state is an object/array, because the == operator compares the references, not the actual values.
const init = [];
const [state, setState] = useState(init);
const dummyState = useRef(init);
useEffect(() => {
// Compare the old state with the new state
if (dummyState.current == state) {
// This means that the component is mounting
} else {
// This means that the component updated.
dummyState.current = state;
}
}, [state]);
Works in development mode...
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.development.js"></script>
<div id="app"></div>
And in production.
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.production.min.js"></script>
<div id="app"></div>

Use the Cleanup function of the useEffect without using an empty array as a second parameter:
useEffect(() => {
return () => {
// your code to be run on update only.
}
});
You can use another useEffect (with an empty array as a second parameter) for initial mount, where you place your code in its main function.

Fix Can't perform a react state update on an unmounted component React native

how can I fix this error in my react native, see the code below.
Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.
const getData = () => {
setIsLoading(true)
axios.get(hosturl.loaduser + userlist.user_id + '&page=' +currentPage)
.then((response) => {
setEvent([...userData, ...response.data.data])
setIsLoading(false)
})
}
useEffect(() => {
getData();
}, [currentPage]);
I did something like this see below but error keep showing.
useEffect(() => {
let isMounted = true;
if (isMounted) getData();
return () => { isMounted = false };
}, [currentPage]);

You could use axios Cancellation:
const controller = new AbortController();
const getData = () => {
setIsLoading(true)
axios.get(hosturl.loaduser + userlist.user_id + '&page=' +currentPage, {
signal: controller.signal
}).then((response) => {
setEvent([...userData, ...response.data.data])
setIsLoading(false)
})
}
useEffect(() => {
getData();
return () => {
controller.abort();
};
}, [currentPage]);
In this way you are aborting axios get on component unmount, avoiding setEvent and setIsLoading (and then the warning).

function MyComponent() {
const mounted = useRef(false);
const getData = () => {
setIsLoading(true)
axios.get(hosturl.loaduser + userlist.user_id + '&page=' +currentPage)
.then((response) => {
if (mounted.current) {
setEvent([...userData, ...response.data.data])
setIsLoading(false)
}
})
}
useEffect(() => {
mounted.current = true;
return () => {
mounted.current = false;
};
}, []);
useEffect(() => {
if (mounted.current) {
getData();
}
}, [currentPage]);
return (
...
);
}
The useEffect() hook, which should handle the mounted ref, is called when the component is mounted and sets the mutable mounted.current value to true. The return function from the useEffect() hook is called when the component is unmounted and sets the mounted.current value to false.
After that you can use the mounted ref (it's like a member variable in old style class components). I.e to get your data, only if your component is mounted, or set state in callback, only if component is mounted.

transform Class based component to functional based component

guys i wanna convert this code:
export default class App extends Component {
constructor(props) {
super(props);
this.state = { isLoading: true };
}
performTimeConsumingTask = async () => {
return new Promise((resolve) =>
setTimeout(() => {
resolve('result');
}, 2000)
);
};
async componentDidMount() {
const data = await this.performTimeConsumingTask();
if (data !== null) this.setState({ isLoading: false });
}
render() {
if (this.state.isLoading) return <SplashScreen />;
const { state, navigate } = this.props.navigation;
return (something)
i wrote this code but it doesn`t work :
const App = () => {
const [fontLoaded, setFontLoaded] = useState(false);
const [isTimerOn, setIsTimerOn] = useState(true);
if (!fontLoaded) {
return (
<AppLoading
startAsync={fetchFonts}
onFinish={() => setFontLoaded(true)}
/>
);
}
useEffect(async () => {
const data = await performTimeConsumingTask();
if (data !== null) setIsTimerOn(false);
});
if (isTimerOn) return <SplashScreen />;
else {
return (something)
This will show an error :
Invariant Violation: Rendered More Hooks than during the previous render.
If I comment the useEffect hook it will run the splashScreen. Can any one help me in converting it?

Pass [] as an argument if you wanted to use this hook as componentDidMount
useEffect(async () => {
const data = await performTimeConsumingTask();
if (data !== null) setIsTimerOn(false);
}, []);
Here is a list of hooks how you can use hooks to replace lifecycle methods
https://medium.com/javascript-in-plain-english/lifecycle-methods-substitute-with-react-hooks-b173073052a
The Reason for getting an error is your component is rendering too many times and useEffect is also running on each render by passing [] will run the useEffect on first render as it will behave like componentDidMount.
Also follow this to make network calls inside useEffect
https://medium.com/javascript-in-plain-english/handling-api-calls-using-async-await-in-useeffect-hook-990fb4ae423

There must be no conditional return before using all the hooks, in your case you return before using useEffect.
Also useEffect must not run on every render since it sets state in your case. Since you only want it to run on initial render pass an empty array as the second argument.
Also useEffect callback function cannot be async.
Read more about useEffect hook in the documentation.
Check updated code below
const App = () => {
const [fontLoaded, setFontLoaded] = useState(false);
const [isTimerOn, setIsTimerOn] = useState(true);
const performTimeConsumingTask = async () => {
return new Promise((resolve) =>
setTimeout(() => {
resolve('result');
}, 2000)
);
};
useEffect(() => {
async function myFunction() {
const data = await performTimeConsumingTask();
if (data !== null) setIsTimerOn(false);
}
myFunction();
}, []); // With empty dependency it runs on initial render only like componentDidMount
if (!fontLoaded) {
return (
<AppLoading
startAsync={fetchFonts}
onFinish={() => setFontLoaded(true)}
/>
);
}
if (isTimerOn) return <SplashScreen />;
else {
return (something)

React programmatically recall a hook

https://codesandbox.io/s/react-hooks-usefetch-cniul
Please see above url for a very simplified version of my code.
I want to be able to refetch data from an API with my hook, within an interval (basically poll an endpoint for data).
What I want is to be able to just call something like refetch (as I've shown in the code as a comment), which would essentially just call fetchData again and update state with the response accordingly.
What's the best way to go about this? The only way I can think of is to add a checker variable in the hook which would be some sort of uuid (Math.random() maybe), return setChecker as what is refetch and just add checker to the array as 2nd useEffect argument to control rerendering. So whenever you call refetch it calls setChecker which updates the random number (checker) and then the function runs again.
Obviously this sounds "hacky", there must be a nicer way of doing it - any ideas?

If you want to have a constant poll going, I think you can move the setInterval() into the hook like so:
function useFetch() {
const [data, setDataState] = useState(null);
const [loading, setLoadingState] = useState(true);
useEffect(() => {
function fetchData() {
setLoadingState(true);
fetch(url)
.then(j => j.json())
.then(data => {
setDataState(data);
setLoadingState(false);
});
}
const interval = setInterval(() => {
fetchData();
}, 5000);
fetchData();
return () => clearInterval(interval);
}, []);
return [
{
data,
loading
}
];
}
Remember to include the return () => clearInterval(interval); so the hook is cleaned up correctly.

import React, { useEffect, useState, useCallback } from "react";
import ReactDOM from "react-dom";
const url = "https://api.etilbudsavis.dk/v2/dealerfront?country_id=DK";
function useFetch() {
const [data, setDataState] = useState(null);
const [loading, setLoadingState] = useState(true);
const refetch = useCallback(() => {
function fetchData() {
console.log("fetch");
setLoadingState(true);
fetch(url)
.then(j => j.json())
.then(data => {
setDataState(data);
setLoadingState(false);
});
}
fetchData();
}, []);
return [
{
data,
loading
},
refetch
// fetchData <- somehow return ability to call fetchData function...
];
}
function App() {
const [
{ data, loading },
refetch
// refetch
] = useFetch();
useEffect(() => {
const id = setInterval(() => {
// Use the refetch here...
refetch();
}, 5000);
return () => {
clearInterval(id);
};
}, [refetch]);
if (loading) return <h1>Loading</h1>;
return (
<>
<button onClick={refetch}>Refetch</button>
<code style={{ display: "block" }}>
<pre>{JSON.stringify(data[0], null, 2)}</pre>
</code>
</>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

Maybe the following will work, it needs some adjustments to useFetch but you can still call it normally in other places.
//maybe you can pass url as well so you can use
// it with other components and urls
function useFetch(refresh) {
//code removed
useEffect(() => {
//code removed
}, [refresh]);
//code removed
}
const [refresh, setRefresh] = useState({});
const [{ data, loading }] = useFetch(refresh);
useEffect(() => {
const interval = setInterval(
() => setRefresh({}), //forces re render
5000
);
return () => clearInterval(interval); //clean up
});

Simple answer to question:
export default function App() {
const [entities, setEntities] = useState();
const [loading, setLoadingState] = useState(true);
const getEntities = () => {
setLoadingState(true);
//Changet the URL with your own
fetch("http://google.com", {
method: "GET",
})
.then((data) => data.json())
.then((resp) => {
setEntities(resp);
setLoadingState(false);
});
};
useEffect(() => {
const interval = setInterval(() => {
getEntities();
}, 5000);
return () => clearInterval(interval);
}, []);
}