The useEffect React hook will run the passed-in function on every change. This can be optimized to let it call only when the desired properties change.
What if I want to call an initialization function from componentDidMount and not call it again on changes? Let's say I want to load an entity, but the loading function doesn't need any data from the component. How can we make this using the useEffect hook?
class MyComponent extends React.PureComponent {
componentDidMount() {
loadDataOnlyOnce();
}
render() { ... }
}
With hooks this could look like this:
function MyComponent() {
useEffect(() => {
loadDataOnlyOnce(); // this will fire on every change :(
}, [...???]);
return (...);
}
If you only want to run the function given to useEffect after the initial render, you can give it an empty array as second argument.
function MyComponent() {
useEffect(() => {
loadDataOnlyOnce();
}, []);
return <div> {/* ... */} </div>;
}
TL;DR
useEffect(yourCallback, []) - will trigger the callback only after the first render.
Detailed explanation
useEffect runs by default after every render of the component (thus causing an effect).
When placing useEffect in your component you tell React you want to run the callback as an effect. React will run the effect after rendering and after performing the DOM updates.
If you pass only a callback - the callback will run after each render.
If passing a second argument (array), React will run the callback after the first render and every time one of the elements in the array is changed. for example when placing useEffect(() => console.log('hello'), [someVar, someOtherVar]) - the callback will run after the first render and after any render that one of someVar or someOtherVar are changed.
By passing the second argument an empty array, React will compare after each render the array and will see nothing was changed, thus calling the callback only after the first render.
useMountEffect hook
Running a function only once after component mounts is such a common pattern that it justifies a hook of its own that hides implementation details.
const useMountEffect = (fun) => useEffect(fun, [])
Use it in any functional component.
function MyComponent() {
useMountEffect(function) // function will run only once after it has mounted.
return <div>...</div>;
}
About the useMountEffect hook
When using useEffect with a second array argument, React will run the callback after mounting (initial render) and after values in the array have changed. Since we pass an empty array, it will run only after mounting.
We have to stop thinking in component-life-cycle-methods (i.e. componentDidMount). We have to start thinking in effects. React effects are different from old-style class-life-cycle-methods.
By default effects run after every render cycle, but there are options to opt out from this behaviour. To opt out, you can define dependencies that mean that an effect is only carried out when a change to one of the dependencies is made.
If you explicitly define, that an effect has no dependecy, the effect runs only once, after the first render-cycle.
1st solution (with ESLint-complaint)
So, the first solution for your example would be the following:
function MyComponent() {
const loadDataOnlyOnce = () => {
console.log("loadDataOnlyOnce");
};
useEffect(() => {
loadDataOnlyOnce(); // this will fire only on first render
}, []);
return (...);
}
But then the React Hooks ESLint plugin will complain with something like that:
React Hook useEffect has missing dependency: loadDataOnlyOnce. Either include it or remove the dependency array.
At first this warning seems annoying, but please don't ignore it. It helps you code better and saves you from "stale closures". If you don't know what "stale closures" are, please read this great post.
2nd solution (the right way, if dependency is not dependent on component)
If we add loadDataOnlyOnce to the dependency array, our effect will run after every render-cycle, because the reference of loadDataOnlyOnce changes on every render, because the function is destroyed(garbarge-collected) and a new function is created, but that's exactly what we don't want.
We have to keep the same reference of loadDataOnlyOnce during render-cycles.
So just move the function-definition above:
const loadDataOnlyOnce = () => {
console.log("loadDataOnlyOnce");
};
function MyComponent() {
useEffect(() => {
loadDataOnlyOnce(); // this will fire only on first render
}, [loadDataOnlyOnce]);
return (...);
}
With this change you ensure that the reference of loadDataOnlyOnce will never change. Therefore you can also safely add the reference to the dependency array.
3rd solution (the right way, if dependency is dependent on component)
If the dependency of the effect (loadDataOnlyOnce), is dependent on the component (need props or state), there's React's builtin useCallback-Hook.
An elementary sense of the useCallback-Hook is to keep the reference of a function identical during render-cycles.
function MyComponent() {
const [state, setState] = useState("state");
const loadDataOnlyOnce = useCallback(() => {
console.log(`I need ${state}!!`);
}, [state]);
useEffect(() => {
loadDataOnlyOnce(); // this will fire only when loadDataOnlyOnce-reference changes
}, [loadDataOnlyOnce]);
return (...);
}
function useOnceCall(cb, condition = true) {
const isCalledRef = React.useRef(false);
React.useEffect(() => {
if (condition && !isCalledRef.current) {
isCalledRef.current = true;
cb();
}
}, [cb, condition]);
}
and use it.
useOnceCall(() => {
console.log('called');
})
or
useOnceCall(()=>{
console.log('Fetched Data');
}, isFetched);
Pass an empty array as the second argument to useEffect. This effectively tells React, quoting the docs:
This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run.
Here's a snippet which you can run to show that it works:
function App() {
const [user, setUser] = React.useState(null);
React.useEffect(() => {
fetch('https://randomuser.me/api/')
.then(results => results.json())
.then(data => {
setUser(data.results[0]);
});
}, []); // Pass empty array to only run once on mount.
return <div>
{user ? user.name.first : 'Loading...'}
</div>;
}
ReactDOM.render(<App/>, 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>
I like to define a mount function, it tricks EsLint in the same way useMount does and I find it more self-explanatory.
const mount = () => {
console.log('mounted')
// ...
const unmount = () => {
console.log('unmounted')
// ...
}
return unmount
}
useEffect(mount, [])
leave the dependency array blank . hope this will help you understand better.
useEffect(() => {
doSomething()
}, [])
empty dependency array runs Only Once, on Mount
useEffect(() => {
doSomething(value)
}, [value])
pass value as a dependency. if dependencies has changed since the last time, the effect will run again.
useEffect(() => {
doSomething(value)
})
no dependency. This gets called after every render.
I had this issue with React 18. This is how I handled it:
import { useEffect, useRef } from "react";
export default function Component() {
const isRunned = useRef(false);
useEffect(() => {
if(isRunned.current) return;
isRunned.current = true;
/* CODE THAT SHOULD RUN ONCE */
}, []);
return <div> content </div>;
}
Check here how they explain why useEffect is called more than once.
Here is my version of Yasin's answer.
import {useEffect, useRef} from 'react';
const useOnceEffect = (effect: () => void) => {
const initialRef = useRef(true);
useEffect(() => {
if (!initialRef.current) {
return;
}
initialRef.current = false;
effect();
}, [effect]);
};
export default useOnceEffect;
Usage:
useOnceEffect(
useCallback(() => {
nonHookFunc(deps1, deps2);
}, [deps1, deps2])
);
This does not answer your question exactly but may have the same intended affect of only running a function once and after the first render. Very similar to the componentDidMount function. This uses useState instead of useEffect to avoid dependency lint errors. You simply pass a self-executing anonymous function as the first argument to useState. As an aside, I'm not sure why React doesn't simply provide a hook that does this.
import React, { useState } from "react"
const Component = () => {
useState((() => {
console.log('componentDidMountHook...')
}))
return (
<div className='component'>Component</div>
)
}
export default Component
I found out after some time spend on the internet. useEffect fires once on component mount, then componennt unmounts and mounts again, useEffect fires again. You have to check more on React docs, why they do that.
So, I used custom hook for that. On unmount you have to change your useRef state. In this case do not forget a return statement: when component unmounts, useEffect runs cleanup function after return.
import React, { useEffect, useRef } from "react"
const useDidMountEffect = (
func: () => void,
deps: React.DependencyList | undefined
) => {
const didMount = useRef(false)
useEffect(() => {
if (didMount.current) {
func()
}
return () => {
didMount.current = true
}
// eslint-disable-next-line react-hooks/exhaustive-deps
}, deps)
}
export default useDidMountEffect
Use it like normal useEffect:
useDidMountEffect(() => {
// your action
}, [])
window.onpageshow works even if the user presses the back button to navigate to the page, unlike passing an empty array as second argument of the use-effect hook which does not fire when returning to the page via the back button (thus not on every form of initial page load).
useEffect(() => {
window.onpageshow = async function() {
setSomeState(false)
let results = await AsyncFunction()
console.log(results, 'Fires on on first load,
refresh, or coming to the page via the back button')
};
};
I found that with the once function from lodash the problem may be solved concisely and elegantly.
import { once } from "lodash";
import { useEffect, useRef } from "react";
export const useEffectOnce = (cb: () => void) => {
const callBackOnce = useRef(once(cb)).current;
useEffect(() => callBackOnce(), [callBackOnce]);
};
Incase you just call the function in useeffect after render you add an empty array as the second argument for the useeffect
useEffect=(()=>{
functionName(firstName,lastName);
},[firstName,lastName])
React throw an error when we try to update the state on an unmounted component.So When I test react component for that I am getting errors on the first render only.
I made a component that enable child component based on click. And child component have button which updates state after some settimeout which throw react warning
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.
Which is perfectly valid. But to overcome this I am passing enable props from the parent component based on that there is a condition just before setTimeout in the child component. So why does it throw an error the first time only?
To reproduce
Click on the child component button which is false and click on parent component button enable which unmount the child component.
**My question is why react throw an error on the first time only ? And why it is working fine on second time **
Parent component
import { useState } from "react";
import "./styles.css";
import { Test } from "./Test";
export default function App() {
const [state, setstate] = useState(true);
const changeState = () => {
setstate(!state);
};
return (
<div className="App">
<button onClick={changeState}>enable </button>
{state && <Test enable={state} />}
</div>
);
}
Child Component
import React, { useState } from "react";
export const Test = (props) => {
const [state, setstate] = useState(false);
const fetchData = () => {
setstate(!state);
if (props.enable) {
setTimeout(() => {
setstate(false);
}, 1000);
}
};
return (
<>
<button onClick={fetchData}> {`${state}`}</button>
</>
);
};
Codesandbox link to test
Nice track, Just you are missing a minor point, when you write a state thats needed some time to execute and the same time we can visit the flow again and again base on any action, then we need to clear old subscription before go to new one...
For example, in your code here, you update state flow, but the state flow is register a new subscription every time we visit a component with valid props and click on button, so that, prev execution may still work when you trigger new event, so simply, what we need to do unmounted old subscription and we can do that by this for your case:
import React, { useState, useEffect } from "react";
export const Test = (props) => {
const [state, setstate] = useState(false);
useEffect(() => {
if (props.enable) {
const timer = setTimeout(() => {
setstate(false);
}, 1000);
return () => clearTimeout(timer);
}
}, [state, props.enable]);
const fetchData = () => {
setstate((prev) => !prev);
};
return (
<>
<button onClick={fetchData}> {`${state}`}</button>
</>
);
};
Look at code above, simply we add code need to cleanup in effect which its look to my state and prop, now when I click on button, the effect will trigger, if we do that again, the clearTime will work for prev subscription and then add new one and so on...
Notes:
In your case we can remove function and use setState direct on your button.
Prefer to use useCallBack in your function like const fetchData = useCAllback...
You can use setstate((prev) => !prev); its will be work as snapshot, and its usefull when you depends on old value..but may it not needed in some cases too, but just to know about this feature.
Update 1:
What is Subscription:
You can say the subscription represents a disposable resource, such as the execution of an Observable. A Subscription has one important method, unsubscribe, that takes no argument and just disposes the resource held by the subscription, in another word, you can say yes, any async task or any job will be invoke to react life-cycle state and its needed to observe changes, then you talk about subscribe, like API or time out or time interval and so on, any of these action thats need to clear prev subscribe (stop observer - unsubscribe) to prevent any memory leek and clear memory to keep state flow safe and prevent unneeded reredner.
I have this type of situation in my current React (v17.0.2) project that gives me 'React Hook useEffect has a missing dependency onTogglePopup' warning.
I need to pass a shared function from parent component to child component and there add an event listener to window when component 'mounts'. It currently does what I want but I believe there must be a better way to accomplish the same and make the linter happy.
I can neither include the function into dependency array nor remove the empty array since I'd loose the functionality.
How do I solve this situation properly without any anti-patterns, hacks or just plain hiding of linter warnings?
const App = ()=>{
const [showPopup, setShowPopup] = useState(false);
const onTogglePopup = ()=>{
setShowPopup(prev => !prev);
// do more App-related stuff here
}
return(
<main>
{/* other elements */}
{showPopup && <Popup onTogglePopup={onTogglePopup}/> }
</main>
)
}
const Popup = ({onTogglePopup})=>{
useEffect(()=>{
const onKeyDown = ev =>{
if (ev.key === 'Escape') onTogglePopup(); // the problem spot
}
window.addEventListener('keydown', onKeyDown);
// remove kbd listener on unmount
return ()=>{
window.removeEventListener('keydown', onKeyDown);
}
},[]);
return createPortal(
<aside id="popup">...
)
}
You can memoize onTogglePopup function before passing it to the child component as a prop.
To memoize the function, use the useCallback hook as shown below:
const onTogglePopup = useCallback(() => {
setShowPopup(prev => !prev);
// do more App-related stuff here
}, []);
Doing this will allow you to safely specify this function in the dependency array of the useEffect hook without causing unnecessary execution of the useEffect hook's callback function every time the parent component re-renders.
Note: useCallback hook also has a dependency array, so you need to be careful not to omit any of its dependencies to avoid the problems because of stale closure.
In your case, if setShowPopup is the only dependency of the useCallback hook, it can be included or excluded without any problem because setShowPopup is a state setter function whose reference is guaranteed to never change.
In App, instead of this:
const onTogglePopup = ()=>{
setShowPopup(prev => !prev);
// do more App-related stuff here
}
Use this:
const onTogglePopup = useCallback(()=>{
setShowPopup(prev => !prev);
// do more App-related stuff here
}, [setShowPopup]);
useCallback preserves the reference to the function unless dependencies change, so you don't trigger useEffect every render.
hoping someone can help lead me in the right direction with this or point to me to any docs/learnings that might be helpful.
I have two React components:
<Widget/>
<Modal/>
When <Widget/> mounted, <Modal/> is hidden (and vice-versa) using conditional rendering and the same boolean in my ShowModal state value (this boolean gets switched by a button). I made a simplified CodeSandbox for this example here
Where I am getting stuck is I have an async function takeScreenShot() which needs to run after <Widget/> is unmounted completely and before <Modal/> is mounted. Neither <Widget/> (just unmounted) nor <Modal/> (about to mount) should be visible in the DOM when takeScreenShot() is called. I need to make sure this is the case because I am taking a screenshot of the underlying page where I do not want to include either of the components in this. In my CodeSandbox example the output of my screenshot function would render a the gray background without showing the blue or red box
What I have tried
I have tried running a cleanup function in the useEffect hook in the <Widget/> component like so.
useEffect(() => {
return () => takeScreenShot();
}, []);
However it doesn't work because the cleanup function, similar to componentWillUnmount() runs right as the component is about to unmount, not fully unmounted from the DOM. This causes my screenshot to capture the un-mounting component in the image. Does anyone have an ideas to point me in the right direction on this?
Move the code from your App into the Modal so that it gets called when the Modal Loads for the first time. I removed all references to useEffect or useLayoutEffect, although the useLayoutEffect may still be required in the modal.
Code Sample Here ...
You could try running it in a useLayoutEffect hook like ...
useLayoutEffect(() => {
if (!showModal) takeScreenshot();
}, [showModal]);
See ...
https://reactjs.org/docs/hooks-reference.html#uselayouteffect
According to the Docs ...
The signature is identical to useEffect, but it fires synchronously
after all DOM mutations. Use this to read layout from the DOM and
synchronously re-render. Updates scheduled inside useLayoutEffect will
be flushed synchronously, before the browser has a chance to paint.
You will probably need to keep a separate state of whether or not the modal has been loaded to ensure that the effect hook doesn't execute on initial load of your component.
Code Sample Here ...
You could have another state bool that indicates if a screenshot is being taken, if thats true render neither of the two elements.
so something like this:
() => this.setState({screenshot: true}, () => this.takeScreenshot())
then within your takeScreenshot function, after you're done with its execution it sets it back to false
takeScreenshot = () => {
// ...
// ...
this.setState({screenshot: false})
}
now while rendering
{!screenshot && (showModal ? <Modal /> : <Widget />)}
Here's how you can (and should) do it.
async function takeScreenshot() {
console.log("Taking screenshot...");
return new Promise(resolve => {
window.setTimeout(() => {
resolve();
console.log("Screenshot captured!");
}, 3000);
});
}
function Widget({ onUnmount }) {
React.useEffect(() => {
return () => onUnmount();
// eslint-disable-next-line react-hooks/exhaustive-deps
}, []);
return <p>Widget</p>;
}
function Modal() {
return <p>Modal</p>;
}
function App() {
const [open, setOpen] = React.useState('widget');
const handleUnmount = async () => {
await takeScreenshot();
setOpen('modal');
}
return (
<React.Fragment>
{open === 'widget' && <Widget onUnmount={handleUnmount} />}
{open === 'modal' && <Modal />}
<button
onClick={() => {
if (open !== "widget") return;
setOpen("");
}}
>
Show modal
</button>
</React.Fragment>
)
}
You can run the demo here.
React memo isn't capturing the props neither the prevProps nor the nextProps and the component render well. The react docs say
If your function component renders the same result given the same props, you can wrap it in a call to React.memo for a performance boost.
my problem is to stop twice rendering using react memo, but memo seems to be not working and the component renders twice with the same props.
The component renders when the Create New Event is clicked on /events
here is the live sandbox.
Child Component located at /components/Event/CreateEvent/CreateEvent.js
the parent component is located at /Pages/Event/Event.js line number 999' from where the child component is being triggered
Here is the Code:
import React from "react";
import AuthContext from "../../context/global-context";
import CreateEvent from "../../components/Event/CreateEvent/CreateEvent";
function Events({ location }) {
// Sate Managing
const [allEvents, setAllEvents] = React.useState([]);
const [creating, setCreating] = React.useState(false);
// Context As State
const { token, email } = React.useContext(AuthContext);
// Creating Event Showing
const modelBoxHandler = () => {
// works on when the ViewEvent is open
if (eventSelected) {
setEventSelected(null);
return;
}
setCreating(!creating);
};
return (
<div className="events">
{/* New Event Creating */}
{creating && (
<CreateEvent onHidder={modelBoxHandler} allEvents={allEvents} />
)}
{console.log("Event Rendered.js =>")}
</div>
);
}
export default React.memo(Events, () => true);
Child Component where the Rect memo doesn't have props:
import React from "react";
import AuthContext from "../../../context/global-context";
function CreateEvent({ onHidder, allEvents }) {
// Context
const { token } = React.useContext(AuthContext);
console.log("CreatedEvent.js REnder");
return (
... Some code here
);
}
export default React.memo(CreateEvent, (prevProps, nextProps) => {
console.log("Hello", prevProps, nextProps);
});
Thanks in advance for your valuable answer and times!
The problem is that on basis of creating variable you are actually remounting and not rendering the CreateEvent component. What it means is that if creating variable changes, the component is unmounted and re-mounted when creating is true, so its not a re-render
Also you must note that modelBoxHandler function reference also changes on each re-render so even if your CreateEvent component is in rendered state and the parent re-rendered due to some reason , the CreateEvent component too will re-render
There are 2 changes that you need to make to make it work better
Define modelBoxHandler with a useCallback hook
perform conditional rendering in createEvent based on creating prop
// Creating Event Showing
const modelBoxHandler = useCallback(() => {
// works on when the ViewEvent is open
if (eventSelected) {
setEventSelected(null);
return;
}
setCreating(prevCreating => !prevCreating);
}, [eventSelected]);
...
return (
<div className="events">
{/* New Event Creating */}
<CreateEvent creating={creating} onHidder={modelBoxHandler} allEvents={allEvents} />
{console.log("Event Rendered.js =>")}
</div>
);
and in createEvent
function CreateEvent({ onHidder, allEvents, creating }) {
// Context
const { token } = React.useContext(AuthContext);
console.log("CreatedEvent.js REnder");
if(!creating) {
return null;
}
return (
... Some code here
);
}
export default React.memo(CreateEvent);
In your example, you don't have an additional render for React.memo to work.
According to your render logic, there aren't any nextProps, you unmount the component with conditional rendering (creating).
// You toggle with `creating` value, there is only single render each time
creating && <CreateEvent onHidder={modelBoxHandler} allEvents={allEvents}/>
// Works, because there will be multiple renders (nextProps)
true && <CreateEvent onHidder={modelBoxHandler} allEvents={allEvents} />
In this case, you might not need React.memo.