Wrong React hooks behaviour with event listener

Question

I'm playing around with React hooks and faced a problem. It shows the wrong state when I'm trying to console log it using button handled by event listener.

CodeSandbox: https://codesandbox.io/s/lrxw1wr97m

1. Click on 'Add card' button 2 times
2. In first card, click on Button1 and see in console that there are 2 cards in state (correct behaviour)
3. In first card, click on Button2 (handled by event listener) and see in console that there are only 1 card in state (wrong behaviour)

Why does it show the wrong state?
In first card, Button2 should display 2 cards in console. Any ideas?

I use React 16.7.0-alpha.0 and Chrome 70.0.3538.110

BTW, if I rewrite the CardsProvider using сlass, the problem is gone. CodeSandbox using class: https://codesandbox.io/s/w2nn3mq9vl

Answer 1

This is common problem for functional components that use 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 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 CardsProvider component is rendered.

handleCardClick used in Card functional component is received as a prop and registered once on component mount with useEffect. It's the same function during entire component lifespan and refers to stale state that was fresh at the time when 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 a basically a recipe that provides a mutable object that can be passed by reference:

const ref = useRef(0);

function eventListener() {
  ref.current++;
}

In case a component should be re-rendered on 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 antipattern 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 state updater function that receives fresh state instead of stale state from enclosing scope:

function eventListener() {
  // doesn't matter how often the listener is registered
  setState(freshState => freshState + 1);
}

In case a state is needed for synchronous side effect 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 event listener every time, so a callback always gets fresh state from enclosing scope:

function eventListener() {
  console.log(state);
}

useEffect(() => {
  // register eventListener on each state update

  return () => {
    // unregister eventListener
  };
}, [state]);

Built-in event handling

Unless event listener is registered on document, window or other event targets are outside of the scope of 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 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 the answer suggested to use mutable state that is 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.