React hooks: accessing up-to-date state from within a callback

For your scenario (where you cannot keep creating new callbacks and passing them to your 3rd party library), you can use useRef to keep a mutable object with the current state. Like so:

function Card(title) {
  const [count, setCount] = React.useState(0)
  const [callbackSetup, setCallbackSetup] = React.useState(false)
  const stateRef = useRef();

  // make stateRef always have the current count
  // your "fixed" callbacks can refer to this object whenever
  // they need the current value.  Note: the callbacks will not
  // be reactive - they will not re-run the instant state changes,
  // but they *will* see the current value whenever they do run
  stateRef.current = count;

  function setupConsoleCallback(callback) {
    console.log("Setting up callback")
    setInterval(callback, 3000)
  }

  function clickHandler() {
    setCount(count+1);
    if (!callbackSetup) {
      setupConsoleCallback(() => {console.log(`Count is: ${stateRef.current}`)})
      setCallbackSetup(true)
    }
  }


  return (<div>
      Active count {count} <br/>
      <button onClick={clickHandler}>Increment</button>
    </div>);

}

Your callback can refer to the mutable object to "read" the current state. It will capture the mutable object in its closure, and every render the mutable object will be updated with the current state value.

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Update June 2021:

Use the NPM module react-usestateref to get always the latest state value. It's fully backward compatible with React useState API.

Example code how to use it:

import useState from 'react-usestateref';

const [count, setCount, counterRef] = useState(0);

console.log(couterRef.current); // it will always have the latest state value
setCount(20);
console.log(counterRef.current);

The NPM package react-useStateRef lets you access the latest state (like ref), by using useState.

Update Dec 2020:

To solve exactly this issue I have created a react module for that. react-usestateref (React useStateRef). E.g. of use:

var [state, setState, ref] = useState(0);

It's works exaclty like useState but in addition, it gives you the current state under ref.current

Learn more:

https://www.npmjs.com/package/react-usestateref

Original Answer

You can get the latest value by using the setState

For example:

var [state, setState] = useState(defaultValue);

useEffect(() => {
   var updatedState;
   setState(currentState => { // Do not change the state by getting the updated state
      updateState = currentState;
      return currentState;
   })
   alert(updateState); // the current state.
})

---

I encountered a similar bug trying to do exactly the same thing you're doing in your example - using a setInterval on a callback that references props or state from a React component.

Hopefully I can add to the good answers already here by coming at the problem from a slightly different direction - the realisation that it's not even a React problem, but a plain old Javascript problem.

I think what catches one out here is thinking in terms of the React hooks model, where the state variable, just a local variable after all, can be treated as though it's stateful within the context of the React component. You can be sure that at runtime, the value of the variable will always be whatever React is holding under the hood for that particular piece of state.

However, as soon as you break out of the React component context - using the variable in a function inside a setInterval for instance, the abstraction breaks and you're back to the truth that that state variable really is just a local variable holding a value.

The abstraction allows you to write code as if the value at runtime will always reflect what's in state. In the context of React, this is the case, because what happens is whenever you set the state the entire function runs again and the value of the variable is set by React to whatever the updated state value is. Inside the callback, however, no such thing happens - that variable doesn't magically update to reflect the underlying React state value at call time. It just is what it is when the callback was defined (in this case 0), and never changes.

Here's where we get to the solution: if the value pointed to by that local variable is in fact a reference to a mutable object, then things change. The value (which is the reference) remains constant on the stack, but the mutable value(s) referenced by it on the heap can be changed.

This is why the technique in the accepted answer works - a React ref provides exactly such a reference to a mutable object. But I think it's really important to emphasise that the 'React' part of this is just a coincidence. The solution, like the problem, has nothing to do with React per-se, it's just that a React ref happens to be one way to get a reference to a mutable object.

You can also use, for instance, a plain Javascript class, holding its reference in React state. To be clear, I'm not suggesting this is a better solution or even advisable (it probably isn't!) but just using it to illustrate the point that there is no 'React' aspect to this solution - it's just Javascript:

class Count {
  constructor (val) { this.val = val }
  get () { return this.val }
  
  update (val) {
    this.val += val
    return this
  }
}

function Card(title) {
  const [count, setCount] = React.useState(new Count(0))
  const [callbackSetup, setCallbackSetup] = React.useState(false)
  
  function setupConsoleCallback(callback) {
    console.log("Setting up callback")
    setInterval(callback, 3000)
  }

  function clickHandler() {
    setCount(count.update(1));
    if (!callbackSetup) {
      setupConsoleCallback(() => {console.log(`Count is: ${count.get()}`)})
      setCallbackSetup(true)
    }
  }
  
  
  return (
    <div>
      Active count {count.get()} <br/>
      <button onClick={clickHandler}>Increment</button>
    </div>
  )
}

const el = document.querySelector("#root");
ReactDOM.render(<Card title='Example Component' />, el);

You can see there that simply by having the state point to a reference, that doesn't change, and mutating the underlying values that the reference points to, you get the behaviour you're after both in the setInterval closure and in the React component.

Again, this is not idiomatic React, but just illustrates the point about references being the ultimate issue here. Hope it's helpful!

---

Instead of trying to access the most recent state within a callback, use useEffect. Setting your state with the function returned from setState will not immediately update your value. The state updates are batched and updated

It may help if you think of useEffect() like setState's second parameter (from class based components).

If you want to do an operation with the most recent state, use useEffect() which will be hit when the state changes:

const { useState, useEffect } = React; function App() { const [count, setCount] = useState(0); const decrement = () => setCount(count-1); const increment = () => setCount(count+1); useEffect(() => { console.log("useEffect", count); }, [count]); console.log("render", count); return (

); } const rootElement = document.getElementById("root"); ReactDOM.render( < App / > , rootElement);

Update

You can create a hook for your setInterval and call it like this:

const { useState, useEffect, useRef } = React; function useInterval(callback, delay) { const savedCallback = useRef(); // Remember the latest callback. useEffect(() => { savedCallback.current = callback; }, [callback]); // Set up the interval. useEffect(() => { function tick() { savedCallback.current(); } if (delay !== null) { let id = setInterval(tick, delay); return () => clearInterval(id); } }, [delay]); } function Card(title) { const [count, setCount] = useState(0); const callbackFunction = () => { console.log(count); }; useInterval(callbackFunction, 3000); useEffect(()=>{ console.log('Count has been updated!'); }, [count]); return (

); } const el = document.querySelector("#root"); ReactDOM.render(, el);

Some further info on useEffect()

---

Thanks for your answer @miroslav. I think I need to explain a bit more: The code I provided was just a toy example to illustrate the problem I've encountered. The issue I have is that an event fired from a third-party library needs to access the latest React state. To do so, I pass a callback to this library for it that can get the state. With react classes, I would have bound the callback to this - the component class. But with react effects, I have the unique problem that the state at the time of setting up the callback gets enclosed within the callback - leading to stale state.

so far. The only solution I have found is to set the state (even though I have no desire to change the state), and use the state-setting function as a workaround to reading the state, setting the "new" state to being the same as the previous state (setupConsoleCallback(() => {setCount(prevCount => {console.log(`Count is: ${prevCount}`); return prevCount})}). But that really seems counter-intuitive. I wonder if it would also trigger an unecessary re-render to any effects that listed count as a dependency. Though I'm not too sure about that point.

@rod got the same issue, working with a callback to another library. Did you every figure out a way to do it other than setting the state to read the value?

@Tom no I stuck with the approach I provided at the end of my question: using a setter function from the state hook to read the latest state (from the passed variable) without actually changing it. There may be a better way of doing this now, but at the time I believe this was the best approach.

You can access the latest state in setState callback. But the intention is not clear, we never want to setState in this case, it may confuse other people when they read your code. So you may want to wrap it in another hook that can express what you want better

function useExtendedState<T>(initialState: T) {
  const [state, setState] = React.useState<T>(initialState);
  const getLatestState = () => {
    return new Promise<T>((resolve, reject) => {
      setState((s) => {
        resolve(s);
        return s;
      });
    });
  };

  return [state, setState, getLatestState] as const;
}

Usage

const [counter, setCounter, getCounter] = useExtendedState(0);

...

getCounter().then((counter) => /* ... */)

// you can also use await in async callback
const counter = await getCounter();

Live Demo

https://codesandbox.io/static/img/play-codesandbox.svg

---

The only method i know is to call the setState(current_value => ...) function and use the current_value in ur logic. Just make sure you return it back. Ex:

const myPollingFunction = () => {
    setInterval(() => {
        setState(latest_value => {
            // do something with latest_value
            return latest_value;    
        }
    }, 1000);
};

---

I really like @davnicwil's answer and hopefully with the source code of useState, it might be more clear what he meant.

  // once when the component is mounted
  constructor(initialValue) {
    this.args = Object.freeze([initialValue, this.updater]);
  }

  // every time the Component is invoked
  update() {
    return this.args
  }

  // every time the setState is invoked
  updater(value) {
    this.args = Object.freeze([value, this.updater]);
    this.el.update()
  }

In the usage, if initialValue starts as a number or string, ex. 1.

  const Component = () => {
    const [state, setState] = useState(initialValue)
  }

Walkthrough

first time you run useState, this.args = [1, ] second time you run useState, this.args no change if setState is invoked with 2, this.args = [2, ] next time you run useState, this.args no change

Now, if you do something especially with a deferred usage of the value.

  function doSomething(v) {
    // I need to wait for 10 seconds
    // within this period of time
    // UI has refreshed multiple times
    // I'm in step 4)
    console.log(v)
  }

  // Invoke this function in step 1)
  doSomething(value)

You will be given an "old" value, because you pass the current copy (at that time) to it at the first place. Although this.args gets the latest copy every time, it doesn't mean an old copy get changed. The value you passed isn't reference based. This could be a feature!!

Summary

In order to change it,

Use the value without passing it;

Use an object for the value;

Use useRef to get latest value;

Or design another hook.

Although the approach above all fix it (in other answers), the root cause of the problem is that you pass an old value to a function and expect it to run with a future value. I think this is where it went wrong at the first place, which isn't very clear if you just look at the solutions.

---

I had similar problem but in my case I was using redux with hooks and also I was not mutating state in the same component. I had method which on callback used state value score and it (callback) had old score. So my sollution for that is quite simple. It's not as ellegant as previous ones but in my case it did the job so I'm putting it here with hope that it will help someone.

const score = useSelector((state) => state.score);
const scoreRef = useRef(score);

useEffect(() => {
    scoreRef.current = score ;
}, [score])

General idea is to store newest state in ref :)

---

I would use a combination of setInterval() and useEffect().

setInterval() on its own is problematic, as it might pop after the component has been unmounted. In your toy example this is not a problem, but in the real world it's likely that your callback will want to mutate your component's state, and then it would be a problem.

useEffect() on its own isn't enough to cause something to happen in some period of time.

useRef() is really for those rare occasions where you need to break React's functional model because you have to work with some functionality that doesn't fit (e.g. focusing an input or something), and I would avoid it for situations like this.

Your example isn't doing anything very useful, and I'm not sure whether you care about how regular the timer pops are. So the simplest way of achieving roughly what you want using this technique is as follows:

import React from 'react';

const INTERVAL_FOR_TIMER_MS = 3000;

export function Card({ title }) {
  const [count, setCount] = React.useState(0)

  React.useEffect(
    () => {
      const intervalId = setInterval(
        () => console.log(`Count is ${count}`),
        INTERVAL_FOR_TIMER_MS,
      );
      return () => clearInterval(intervalId);
    },
    // you only want to restart the interval when count changes
    [count],
  );

  function clickHandler() {
    // I would also get in the habit of setting this way, which is safe
    // if the increment is called multiple times in the same callback
    setCount(num => num + 1);
  }

  return (
    <div>
      Active count {count} <br/>
      <button onClick={clickHandler}>Increment</button>
    </div>
  );
}

The caveat is that if the timer pops, then you click a second later, then the next log will be 4 seconds after the previous log because the timer is reset when you click.

If you want to solve that problem, then the best thing will probably be to use Date.now() to find the current time and use a new useState() to store the next pop time you want, and use setTimeout() instead of setInterval().

It's a bit more complicated as you have to store the next timer pop, but not too bad. Also that complexity can be abstracted by simply using a new function. So to sum up here's a safe "Reacty" way of starting a periodic timer using hooks.

import React from 'react';

const INTERVAL_FOR_TIMER_MS = 3000;

const useInterval = (func, period, deps) => {
  const [nextPopTime, setNextPopTime] = React.useState(
    Date.now() + period,
  );
  React.useEffect(() => {
    const timerId = setTimeout(
      () => {
        func();
        
        // setting nextPopTime will cause us to run the 
        // useEffect again and reschedule the timeout
        setNextPopTime(popTime => popTime + period);
      },
      Math.max(nextPopTime - Date.now(), 0),
    );
    return () => clearTimeout(timerId);
  }, [nextPopTime, ...deps]);
};

export function Card({ title }) {
  const [count, setCount] = React.useState(0);

  useInterval(
    () => console.log(`Count is ${count}`),
    INTERVAL_FOR_TIMER_MS,
    [count],
  );

  return (
    <div>
      Active count {count} <br/>
      <button onClick={() => setCount(num => num + 1)}>
        Increment
      </button>
    </div>
  );
}

And as long as you pass all the dependencies of the interval function in the deps array (exactly like with useEffect()), you can do whatever you like in the interval function (set state etc.) and be confident nothing will be out of date.

---

onClick={() => { clickHandler(); }}

This way you run the function as defined when you click it not when you declared the onClick handler.

React re-runs the hook function every time there is a change, and when it does so it re-defines your clickHandler() function.

For the record, you could clean that up. Since you don't care what your arrow function returns you could write it as such.

onClick={e => clickHandler()}

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