I’m playing around with React Hooks and am facing a problem. It shows the wrong state when I’m trying to console log it using a button handled by event listener.
CodeSandbox: https://codesandbox.io/s/lrxw1wr97m
- Click on ‘Add card’ button 2 times
- In first card, click on Button1 and see in console that there are 2 cards in state (correct behaviour)
- In first card, click on Button2 (handled by event listener) and see in console that there is only 1 card in state (wrong behaviour)
Why does it show the wrong state?
In first card, Button2
should display 2
cards in the console. Any ideas?
const { useState, useContext, useRef, useEffect } = React; const CardsContext = React.createContext(); const CardsProvider = props => { const [cards, setCards] = useState([]); const addCard = () => { const id = cards.length; setCards([...cards, { id: id, json: {} }]); }; const handleCardClick = id => console.log(cards); const handleButtonClick = id => console.log(cards); return ( <CardsContext.Provider value={{ cards, addCard, handleCardClick, handleButtonClick }} > {props.children} </CardsContext.Provider> ); }; function App() { const { cards, addCard, handleCardClick, handleButtonClick } = useContext( CardsContext ); return ( <div className="App"> <button onClick={addCard}>Add card</button> {cards.map((card, index) => ( <Card key={card.id} id={card.id} handleCardClick={() => handleCardClick(card.id)} handleButtonClick={() => handleButtonClick(card.id)} /> ))} </div> ); } function Card(props) { const ref = useRef(); useEffect(() => { ref.current.addEventListener("click", props.handleCardClick); return () => { ref.current.removeEventListener("click", props.handleCardClick); }; }, []); return ( <div className="card"> Card {props.id} <div> <button onClick={props.handleButtonClick}>Button1</button> <button ref={node => (ref.current = node)}>Button2</button> </div> </div> ); } ReactDOM.render( <CardsProvider> <App /> </CardsProvider>, document.getElementById("root") );
<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script> <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script> <div id='root'></div>
I am using React 16.7.0-alpha.0 and Chrome 70.0.3538.110
BTW, if I rewrite the CardsProvider using a сlass, the problem is gone. CodeSandbox using class: https://codesandbox.io/s/w2nn3mq9vl
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Answer
This is a common problem for functional components that use the useState
hook. The same concerns are applicable to any callback functions where useState
state is used, e.g. setTimeout
or setInterval
timer functions.
Event handlers are treated differently in CardsProvider
and Card
components.
handleCardClick
and handleButtonClick
used in the CardsProvider
functional component are defined in its scope. There are new functions each time it runs, they refer to cards
state that was obtained at the moment when they were defined. Event handlers are re-registered each time the CardsProvider
component is rendered.
handleCardClick
used in the Card
functional component is received as a prop and registered once on component mount with useEffect
. It’s the same function during the entire component lifespan and refers to stale state that was fresh at the time when the handleCardClick
function was defined the first time. handleButtonClick
is received as a prop and re-registered on each Card
render, it’s a new function each time and refers to fresh state.
Mutable state
A common approach that addresses this problem is to use useRef
instead of useState
. A ref is basically a recipe that provides a mutable object that can be passed by reference:
const ref = useRef(0); function eventListener() { ref.current++; }
In this case a component should be re-rendered on a state update like it’s expected from useState
, refs aren’t applicable.
It’s possible to keep state updates and mutable state separately but forceUpdate
is considered an anti-pattern in both class and function components (listed for reference only):
const useForceUpdate = () => { const [, setState] = useState(); return () => setState({}); } const ref = useRef(0); const forceUpdate = useForceUpdate(); function eventListener() { ref.current++; forceUpdate(); }
State updater function
One solution is to use a state updater function that receives fresh state instead of stale state from the enclosing scope:
function eventListener() { // doesn't matter how often the listener is registered setState(freshState => freshState + 1); }
In this case a state is needed for synchronous side effects like console.log
, a workaround is to return the same state to prevent an update.
function eventListener() { setState(freshState => { console.log(freshState); return freshState; }); } useEffect(() => { // register eventListener once return () => { // unregister eventListener once }; }, []);
This doesn’t work well with asynchronous side effects, notably async
functions.
Manual event listener re-registration
Another solution is to re-register the event listener every time, so a callback always gets fresh state from the enclosing scope:
function eventListener() { console.log(state); } useEffect(() => { // register eventListener on each state update return () => { // unregister eventListener }; }, [state]);
Built-in event handling
Unless the event listener is registered on document
, window
or other event targets that are outside of the scope of the current component, React’s own DOM event handling has to be used where possible, this eliminates the need for useEffect
:
<button onClick={eventListener} />
In the last case the event listener can be additionally memoized with useMemo
or useCallback
to prevent unnecessary re-renders when it’s passed as a prop:
const eventListener = useCallback(() => { console.log(state); }, [state]);
- Previous edition of this answer suggested to use mutable state that was applicable to initial
useState
hook implementation in React 16.7.0-alpha version but isn’t workable in final React 16.8 implementation.useState
currently supports only immutable state.*