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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])
I have gone through a couple of articles on useCallback and useMemo on when to use and when not to use but I have mostly seen very contrived code. I was looking at a code at my company where I have noticed someone have done this:
const takePhoto = useCallback(() => {
launchCamera({ mediaType: "photo", cameraType: "front" }, onPickImage);
}, []);
const pickPhotoFromLibrary = async () => {
launchImageLibrary({ mediaType: "photo" }, onPickImage);
}
const onUploadPress = useCallback(() => {
Alert.alert(
"Upload Photo",
"From where would you like to take your photo?",
[
{ text: "Camera", onPress: () => takePhoto() },
{ text: "Library", onPress: () => pickPhotoFromLibrary() },
]
);
}, [pickPhotoFromLibrary, takePhoto]);
This is how onUploadPress is called:
<TouchableOpacity
style={styles.retakeButton}
onPress={onUploadPress}
>
Do you think this is the correct way of calling it? Based on my understanding from those articles, this looks in-correct. Can someone tell me when to use useCallback and also maybe explain useCallback in more human terms?
Article I read: When to useMemo and useCallback.
useCallback returns a normal JavaScript function regarding how to use it. It is the same as the one it gets as first parameter regarding what it does. The difference is that this function doesn't get recreated on a new memory reference every time the component re-renders, while a normal function does. It gets recreated on a new reference if one of the variables inside useCalback's dependency array changes.
Now, why would you wanna bother with this? Well, It's worth it whenever the normal behavior of a function is problematic for you. For example, if you have that function in the dependency array of an useEffect, or if you pass it down to a component that is memoized with memo.
The callback of an useEffect gets called on the first render and every time one of the variables inside the dependency array changes. And since normally a new version of that function is created on every render, the callback might get called infinitely. So useCallback is used to memoize it.
A memoized component with memo re-renders only if its state or props changes, not because its parent re-renders. And since normally a new version of that passed function as props is created, when the parent re-renders, the child component gets a new reference, hence it re-renders. So useCallback is used to memoize it.
To illustrate, I created the below working React application. Click on that button to trigger re-renders of the parent and watch the console. Hope it clears things up!
const MemoizedChildWithMemoizedFunctionInProps = React.memo(
({ memoizedDummyFunction }) => {
console.log("MemoizedChildWithMemoizedFunctionInProps renders");
return <div></div>;
}
);
const MemoizedChildWithNonMemoizedFunctionInProps = React.memo(
({ nonMemoizedDummyFunction }) => {
console.log("MemoizedChildWithNonMemoizedFunctionInProps renders");
return <div></div>;
}
);
const NonMemoizedChild = () => {
console.log("Non memoized child renders");
return <div></div>;
};
const Parent = () => {
const [state, setState] = React.useState(true);
const nonMemoizedFunction = () => {};
const memoizedFunction = React.useCallback(() => {}, []);
React.useEffect(() => {
console.log("useEffect callback with nonMemoizedFunction runs");
}, [nonMemoizedFunction]);
React.useEffect(() => {
console.log("useEffect callback with memoizedFunction runs");
}, [memoizedFunction]);
console.clear();
console.log("Parent renders");
return (
<div>
<button onClick={() => setState((prev) => !prev)}>Toggle state</button>
<MemoizedChildWithMemoizedFunctionInProps
memoizedFunction={memoizedFunction}
/>
<MemoizedChildWithNonMemoizedFunctionInProps
nonMemoizedFunction={nonMemoizedFunction}
/>
<NonMemoizedChild />
</div>
);
}
ReactDOM.render(
<Parent />,
document.getElementById("root")
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.0/umd/react-dom.production.min.js"></script>
<div id="root"></div>
It's to know that memoizing is not free, doing it wrong is worse than not having it. In your case, using useCallback for onUploadPress is a waste because a non memoized function, pickPhotoFromLibrary, is in the dependency array. Also, it's a waste if TouchableOpacity is not memoized with memo, which I'm not sure it's.
As a side note, there is useMemo, which behaves and is used like useCallback to memoize non-function but referenced values such as objects and arrays for the same reasons, or to memoize any result of a heavy calculation that you don't wanna repeat between renders.
A great resource to understand React render process in depth to know when to memorize and how to do it well: React Render.
In simple words, useCallback is used to save the function reference somewhere outside the component render so we could use the same reference again. That reference will be changed whenever one of the variables in the dependencies array changes.
As you know React tries to minimize the re-rendering process by watching some variables' value changes, then it decides to re-render not depending on the old-value and new-value of those variables.
So, the basic usage of useCallback is to hold old-value and the new-value equally.
I will try to demonstrate it more by giving some examples in situations we must use useCalback in.
Example 1: When the function is one of the dependencies array of the useEffect.
function Component(){
const [state, setState] = useState()
// Should use `useCallback`
function handleChange(input){
setState(...)
}
useEffect(()=>{
handleChange(...)
},[handleChange])
return ...
}
Example 2: When the function is being passed to one of the children components. Especially when it is being called on their useEffect hook, it leads to an infinite loop.
function Parent(){
const [state, setState] = useState()
function handleChange(input){
setState(...)
}
return <Child onChange={handleChange} />
}
function Child({onChange}){
const [state, setState] = useState()
useEffect(()=>{
onChange(...)
},[onChange])
return "Child"
}
Example 3: When you use React Context that holds a state and returns only the state setters functions, you need the consumer of that context to not rerender every time the state update as it may harm the performance.
const Context = React.createContext();
function ContextProvider({children}){
const [state, setState] = useState([]);
// Should use `useCallback`
const addToState = (input) => {
setState(prev => [...prev, input]);
}
// Should use `useCallback`
const removeFromState = (input) => {
setState(prev => prev.filter(elem => elem.id !== input.id));
}
// Should use `useCallback` with empty []
const getState = () => {
return state;
}
const contextValue= React.useMemo(
() => ({ addToState , removeFromState , getState}),
[addToState , removeFromState , getState]
);
// if we used `useCallback`, our contextValue will never change, and all the subscribers will not re-render
<Context.Provider value={contextValue}>
{children}
</Context.Provider>
}
Example 4: If you are subscribed to the observer, timer, document events, and need to unsubscribe when the component unmounts or for any other reason. So we need to access the same reference to unsubscribe from it.
function Component(){
// should use `useCallback`
const handler = () => {...}
useEffect(() => {
element.addEventListener(eventType, handler)
return () => element.removeEventListener(eventType, handler)
}, [eventType, element])
return ...
}
That's it, there are multiple situations you can use it too, but I hope these examples demonstrated the main idea behind useCallback. And always remember you don't need to use it if the cost of the re-render is negligible.
The issue I'm having is that when I set up an event listener, the value the event listener sees doesn't update with the state. It's as if it's bound to the initial state.
What is the correct way to do this?
Simple example:
import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";
const App = () => {
const [name, setName] = useState("Colin");
const [nameFromEventHandler, setNameFromEventHandler] = useState("");
useEffect(() => {
document.getElementById("name").addEventListener("click", handleClick);
}, []);
const handleButton = () => {
setName("Ricardo");
};
const handleClick = () => {
setNameFromEventHandler(name);
};
return (
<React.Fragment>
<h1 id="name">name: {name}</h1>
<h2>name when clicked: {nameFromEventHandler}</h2>
<button onClick={handleButton}>change name</button>
</React.Fragment>
);
};
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
Gif below, since SO code snippet doesn't work for some reason.
So your problem is that you pass an empty array as the second argument to your effect so the effect will never be cleaned up and fired again. This means that handleClick will only ever be closed over the default state. You've essentially written: setNameFromEventHandler("Colin"); for the entire life of this component.
Try removing the second argument all together so the effect will be cleaned up and fired whenever the state changes. When the effect refires, the function that will be handling the click event that will be closed over the most recent version of your state. Also, return a function from your useEffect that will remove your event listener.
E.g.
useEffect(() => {
document.getElementById("name").addEventListener("click", handleClick);
return () => {
document.getElementById("name").removeEventListener("click", handleClick);
}
});
I think correct solution should be this: codesanbox. We are telling to the effect to take care about its dependency, which is the callback. Whenever it is changed we should do another binding with correct value in closure.
I believe the correct solution would be something like this:
useEffect(() => {
document.getElementById("name").addEventListener("click", handleClick);
}, [handleClick]);
const handleButton = () => {
setName("Ricardo");
};
const handleClick = useCallback(() => {
setNameFromEventHandler(name)
}, [name])
The useEffect should have handleClick as part of its dependency array otherwise it will suffer from what is known as a 'stale closure' i.e. having stale state.
To ensure the useEffect is not running on every render, move the handleClick inside a useCallback. This will return a memoized version of the callback that only changes if one of the dependencies has changed which in this case is 'name'.
I was experimenting with the new Hook feature in React. Considering I have the following two components (using React Hooks) -
const HookComponent = () => {
const [username, setUsername] = useState('Abrar');
const [count, setState] = useState();
const handleChange = (e) => {
setUsername(e.target.value);
}
return (
<div>
<input name="userName" value={username} onChange={handleChange}/>
<p>{username}</p>
<p>From HookComponent: {count}</p>
</div>
)
}
const HookComponent2 = () => {
const [count, setCount] = useState(999);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
Hooks claim to solve the problem of sharing stateful logic between components but I found that the states between HookComponent and HookComponent2 are not sharable. For example the change of count in HookComponent2 does not render a change in the HookComponent.
Is it possible to share states between components using the useState() hook?
If you are referring to component state, then hooks will not help you share it between components. Component state is local to the component. If your state lives in context, then useContext hook would be helpful.
Fundamentally, I think you misunderstood the line "sharing stateful logic between components". Stateful logic is different from state. Stateful logic is stuff that you do that modifies state. For e.g., a component subscribing to a store in componentDidMount() and unsubscribing in componentWillUnmount(). This subscribing/unsubscribing behavior can be implemented in a hook and components which need this behavior can just use the hook.
If you want to share state between components, there are various ways to do so, each with its own merits:
1. Lift State Up
Lift state up to a common ancestor component of the two components.
function Ancestor() {
const [count, setCount] = useState(999);
return <>
<DescendantA count={count} onCountChange={setCount} />
<DescendantB count={count} onCountChange={setCount} />
</>;
}
This state sharing approach is not fundamentally different from the traditional way of using state, hooks just give us a different way to declare component state.
2. Context
If the descendants are too deep down in the component hierarchy and you don't want to pass the state down too many layers, you could use the Context API.
There's a useContext hook which you can leverage on within the child components.
3. External State Management Solution
State management libraries like Redux or Mobx. Your state will then live in a store outside of React and components can connect/subscribe to the store to receive updates.
It is possible without any external state management library. Just use a simple observable implementation:
function makeObservable(target) {
let listeners = []; // initial listeners can be passed an an argument aswell
let value = target;
function get() {
return value;
}
function set(newValue) {
if (value === newValue) return;
value = newValue;
listeners.forEach((l) => l(value));
}
function subscribe(listenerFunc) {
listeners.push(listenerFunc);
return () => unsubscribe(listenerFunc); // will be used inside React.useEffect
}
function unsubscribe(listenerFunc) {
listeners = listeners.filter((l) => l !== listenerFunc);
}
return {
get,
set,
subscribe,
};
}
And then create a store and hook it to react by using subscribe in useEffect:
const userStore = makeObservable({ name: "user", count: 0 });
const useUser = () => {
const [user, setUser] = React.useState(userStore.get());
React.useEffect(() => {
return userStore.subscribe(setUser);
}, []);
const actions = React.useMemo(() => {
return {
setName: (name) => userStore.set({ ...user, name }),
incrementCount: () => userStore.set({ ...user, count: user.count + 1 }),
decrementCount: () => userStore.set({ ...user, count: user.count - 1 }),
}
}, [user])
return {
state: user,
actions
}
}
And that should work. No need for React.Context or lifting state up
This is possible using the useBetween hook.
See in codesandbox
import React, { useState } from 'react';
import { useBetween } from 'use-between';
const useShareableState = () => {
const [username, setUsername] = useState('Abrar');
const [count, setCount] = useState(0);
return {
username,
setUsername,
count,
setCount
}
}
const HookComponent = () => {
const { username, setUsername, count } = useBetween(useShareableState);
const handleChange = (e) => {
setUsername(e.target.value);
}
return (
<div>
<input name="userName" value={username} onChange={handleChange}/>
<p>{username}</p>
<p>From HookComponent: {count}</p>
</div>
)
}
const HookComponent2 = () => {
const { count, setCount } = useBetween(useShareableState);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
We move React hooks stateful logic from HookComponent to useShareableState.
We call useShareableState using useBetween in each component.
useBetween is a way to call any hook. But so that the state will not be stored in the React component.
For the same hook, the result of the call will be the same. So we can call one hook in different components and work together on one state. When updating the shared state, each component using it will be updated too.
Disclaimer: I'm the author of the use-between package.
the doc states:
We import the useState Hook from React. It lets us keep local state in a function component.
it is not mentioned that the state could be shared across components, useState hook just give you a quicker way to declare a state field and its correspondent setter in one single instruction.
I've created hooksy that allows you to do exactly this - https://github.com/pie6k/hooksy
import { createStore } from 'hooksy';
interface UserData {
username: string;
}
const defaultUser: UserData = { username: 'Foo' };
export const [useUserStore] = createStore(defaultUser); // we've created store with initial value.
// useUserStore has the same signature like react useState hook, but the state will be shared across all components using it
And later in any component
import React from 'react';
import { useUserStore } from './userStore';
export function UserInfo() {
const [user, setUser] = useUserStore(); // use it the same way like useState, but have state shared across any component using it (eg. if any of them will call setUser - all other components using it will get re-rendered with new state)
function login() {
setUser({ username: 'Foo' })
}
return (
<div>
{!user && <strong>You're logged out<button onPress={login}>Login</button></strong>}
{user && <strong>Logged as <strong>{user.username}</strong></strong>}
</div>
);
}
With hooks its not directly possible.
I recommend you to take a look at react-easy-state.
https://github.com/solkimicreb/react-easy-state
I use it in big Apps and it works like a charm.
I'm going to hell for this:
// src/hooks/useMessagePipe.ts
import { useReducer } from 'react'
let message = undefined
export default function useMessagePipe() {
const triggerRender = useReducer((bool) => !bool, true)[1]
function update(term: string) {
message = term.length > 0 ? term : undefined
triggerRender()
}
return {message: message, sendMessage: update}
}
Full explanation over at: https://stackoverflow.com/a/72917627/1246547
Yes, this is the dirtiest and most concise way i could come up with for solving that specific use case. And yes, for a clean way, you probably want to learn how to useContext, or alternatively take a look at react-easy-state or useBetween for low-footprint solutions, and flux or redux for the real thing.
You will still need to lift your state up to an ancestor component of HookComponent1 and HookComponent2. That's how you share state before and the latest hook api doesnt change anything about it.
Considering below hooks example
import { useState } from 'react';
function Example() {
const [count, setCount] = useState(0);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
Basically we use this.forceUpdate() method to force the component to re-render immediately in React class components like below example
class Test extends Component{
constructor(props){
super(props);
this.state = {
count:0,
count2: 100
}
this.setCount = this.setCount.bind(this);//how can I do this with hooks in functional component
}
setCount(){
let count = this.state.count;
count = count+1;
let count2 = this.state.count2;
count2 = count2+1;
this.setState({count});
this.forceUpdate();
//before below setState the component will re-render immediately when this.forceUpdate() is called
this.setState({count2: count
}
render(){
return (<div>
<span>Count: {this.state.count}></span>.
<button onClick={this.setCount}></button>
</div>
}
}
But my query is How can I force above functional component to re-render immediately with hooks?
This is possible with useState or useReducer, since useState uses useReducer internally:
const [, updateState] = React.useState();
const forceUpdate = React.useCallback(() => updateState({}), []);
forceUpdate isn't intended to be used under normal circumstances, only in testing or other outstanding cases. This situation may be addressed in a more conventional way.
setCount is an example of improperly used forceUpdate, setState is asynchronous for performance reasons and shouldn't be forced to be synchronous just because state updates weren't performed correctly. If a state relies on previously set state, this should be done with updater function,
If you need to set the state based on the previous state, read about the updater argument below.
<...>
Both state and props received by the updater function are guaranteed
to be up-to-date. The output of the updater is shallowly merged with
state.
setCount may not be an illustrative example because its purpose is unclear but this is the case for updater function:
setCount(){
this.setState(({count}) => ({ count: count + 1 }));
this.setState(({count2}) => ({ count2: count + 1 }));
this.setState(({count}) => ({ count2: count + 1 }));
}
This is translated 1:1 to hooks, with the exception that functions that are used as callbacks should better be memoized:
const [state, setState] = useState({ count: 0, count2: 100 });
const setCount = useCallback(() => {
setState(({count}) => ({ count: count + 1 }));
setState(({count2}) => ({ count2: count + 1 }));
setState(({count}) => ({ count2: count + 1 }));
}, []);
React Hooks FAQ official solution for forceUpdate:
const [_, forceUpdate] = useReducer((x) => x + 1, 0);
// usage
<button onClick={forceUpdate}>Force update</button>
Working example
const App = () => {
const [_, forceUpdate] = useReducer((x) => x + 1, 0);
return (
<div>
<button onClick={forceUpdate}>Force update</button>
<p>Forced update {_} times</p>
</div>
);
};
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.10.1/umd/react.production.min.js" integrity="sha256-vMEjoeSlzpWvres5mDlxmSKxx6jAmDNY4zCt712YCI0=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.10.1/umd/react-dom.production.min.js" integrity="sha256-QQt6MpTdAD0DiPLhqhzVyPs1flIdstR4/R7x4GqCvZ4=" crossorigin="anonymous"></script>
<script>var useReducer = React.useReducer</script>
<div id="root"></div>
Generally, you can use any state handling approach you want to trigger an update.
With TypeScript
codesandbox example
useState
const forceUpdate: () => void = React.useState({})[1].bind(null, {}) // see NOTE below
useReducer (recommended)
const forceUpdate = React.useReducer(() => ({}), {})[1] as () => void
as custom hook
Just wrap whatever approach you prefer like this
function useForceUpdate(): () => void {
return React.useReducer(() => ({}), {})[1] as () => void // <- paste here
}
How this works?
"To trigger an update" means to tell React engine that some value has changed and that it should rerender your component.
[, setState] from useState() requires a parameter. We get rid of it by binding a fresh object {}.
() => ({}) in useReducer is a dummy reducer that returns a fresh object each time an action is dispatched.
{} (fresh object) is required so that it triggers an update by changing a reference in the state.
PS: useState just wraps useReducer internally, so use reducer to reduce complexity. source
NOTE: Referential instability
Using .bind with useState causes a change in function reference between renders.
It is possible to wrap it inside useCallback as already explained in this answer here, but then it wouldn't be a sexy one-liner™. The Reducer version already keeps reference equality (stability) between renders. This is important if you want to pass the forceUpdate function in props to another component.
plain JS
const forceUpdate = React.useState({})[1].bind(null, {}) // see NOTE above
const forceUpdate = React.useReducer(() => ({}))[1]
As the others have mentioned, useState works - here is how mobx-react-lite implements updates - you could do something similar.
Define a new hook, useForceUpdate -
import { useState, useCallback } from 'react'
export function useForceUpdate() {
const [, setTick] = useState(0);
const update = useCallback(() => {
setTick(tick => tick + 1);
}, [])
return update;
}
and use it in a component -
const forceUpdate = useForceUpdate();
if (...) {
forceUpdate(); // force re-render
}
See https://github.com/mobxjs/mobx-react-lite/blob/master/src/utils.ts and https://github.com/mobxjs/mobx-react-lite/blob/master/src/useObserver.ts
Alternative to #MinhKha's answer:
It can be much cleaner with useReducer:
const [, forceUpdate] = useReducer(x => x + 1, 0);
Usage:
forceUpdate() - cleaner without params
You can simply define the useState like that:
const [, forceUpdate] = React.useState(0);
And usage: forceUpdate(n => !n)
Hope this help !
You should preferably only have your component depend on state and props and it will work as expected, but if you really need a function to force the component to re-render, you could use the useState hook and call the function when needed.
Example
const { useState, useEffect } = React;
function Foo() {
const [, forceUpdate] = useState();
useEffect(() => {
setTimeout(forceUpdate, 2000);
}, []);
return <div>{Date.now()}</div>;
}
ReactDOM.render(<Foo />, document.getElementById("root"));
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.production.min.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.production.min.js"></script>
<div id="root"></div>
Simple code
const forceUpdate = React.useReducer(bool => !bool)[1];
Use:
forceUpdate();
Potential option is to force update only on specific component using key. Updating the key trigger a rendering of the component (which failed to update before)
For example:
const [tableKey, setTableKey] = useState(1);
...
useEffect(() => {
...
setTableKey(tableKey + 1);
}, [tableData]);
...
<DataTable
key={tableKey}
data={tableData}/>
You can (ab)use normal hooks to force a rerender by taking advantage of the fact that React doesn't print booleans in JSX code
// create a hook
const [forceRerender, setForceRerender] = React.useState(true);
// ...put this line where you want to force a rerender
setForceRerender(!forceRerender);
// ...make sure that {forceRerender} is "visible" in your js code
// ({forceRerender} will not actually be visible since booleans are
// not printed, but updating its value will nonetheless force a
// rerender)
return (
<div>{forceRerender}</div>
)
One line solution:
const useForceUpdate = () => useState()[1];
useState returns a pair of values: the current state and a function that updates it - state and setter, here we are using only the setter in order to force re-render.
react-tidy has a custom hook just for doing that called useRefresh:
import React from 'react'
import {useRefresh} from 'react-tidy'
function App() {
const refresh = useRefresh()
return (
<p>
The time is {new Date()} <button onClick={refresh}>Refresh</button>
</p>
)
}
Learn more about this hook
Disclaimer I am the writer of this library.
My variation of forceUpdate is not via a counter but rather via an object:
// Emulates `forceUpdate()`
const [unusedState, setUnusedState] = useState()
const forceUpdate = useCallback(() => setUnusedState({}), [])
Because {} !== {} every time.
Solution in one single line:
const [,forceRender] = useReducer((s) => s+1, 0)
You can learn about useReducer here.
https://reactjs.org/docs/hooks-reference.html#usereducer
This will render depending components 3 times (arrays with equal elements aren't equal):
const [msg, setMsg] = useState([""])
setMsg(["test"])
setMsg(["test"])
setMsg(["test"])
const useForceRender = () => {
const [, forceRender] = useReducer(x => !x, true)
return forceRender
}
Usage
function Component () {
const forceRender = useForceRender()
useEffect(() => {
// ...
forceRender()
}, [])
For regular React Class based components, refer to React Docs for the forceUpdate api at this URL. The docs mention that:
Normally you should try to avoid all uses of forceUpdate() and only
read from this.props and this.state in render()
However, it is also mentioned in the docs that:
If your render() method depends on some other data, you can tell React
that the component needs re-rendering by calling forceUpdate().
So, although use cases for using forceUpdate might be rare, and I have not used it ever, however I have seen it used by other developers in some legacy corporate projects that I have worked on.
So, for the equivalent functionality for Functional Components, refer to the React Docs for HOOKS at this URL. Per the above URL, one can use the "useReducer" hook to provide a forceUpdate functionality for Functional Components.
A working code sample that does not use state or props is provided below, which is also available on CodeSandbox at this URL
import React, { useReducer, useRef } from "react";
import ReactDOM from "react-dom";
import "./styles.css";
function App() {
// Use the useRef hook to store a mutable value inside a functional component for the counter
let countref = useRef(0);
const [, forceUpdate] = useReducer(x => x + 1, 0);
function handleClick() {
countref.current++;
console.log("Count = ", countref.current);
forceUpdate(); // If you comment this out, the date and count in the screen will not be updated
}
return (
<div className="App">
<h1> {new Date().toLocaleString()} </h1>
<h2>You clicked {countref.current} times</h2>
<button
onClick={() => {
handleClick();
}}
>
ClickToUpdateDateAndCount
</button>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
NOTE: An alternate approach using the useState hook (instead of useReducer) is also available at this URL.
There are many ways to force re-render in Hook.
For me simple way with useState() and tip of reference object values.
const [, forceRender] = useState({});
// Anywhre
forceRender({});
Codesandbox Example
A bit late to the party but I notice that most (all) of the answers have missed the part where you can pass a callback to forceUpdate lifecycle method.
As per the react source code, this callback has the same behavior as the one in the setState method - it is executed after the update.
Hence, the most correct implementation would be like this:
/**
* Increments the state which causes a rerender and executes a callback
* #param {function} callback - callback to execute after state update
* #returns {function}
*/
export const useForceUpdate = (callback) => {
const [state, updater] = useReducer((x) => x + 1, 0);
useEffect(() => {
callback && callback();
}, [state]);
return useCallback(() => {
updater();
}, []);
};
I was working with an array and spotted this issue. However, instead of explicit forceUpdate I found another approach - to deconstruct an array and set a new value for it using this code:
setRoutes(arr => [...arr, newRoute]); // add new elements to the array
setRouteErrors(routeErrs => [...routeErrs]); // the array elements were changed
I found it very interesting that setting even a copy of an array will not trigger the hook. I assume React does the shallow comparison