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

javascriptreact-hooksreactjs

EDIT (22 June 2020): as this question has some renewed interest, I realise there may be a few points of confusion. So I would like to highlight: the example in the question is intended as a toy example. It is not reflective of the problem. The problem which spurred this question, is in the use a third party library (over which there is limited control) that takes a callback as argument to a function. What is the correct way to provide that callback with the latest state. In react classes, this would be done through the use of this. In React hooks, due to the way state is encapsulated in the functions of React.useState(), if a callback gets the state through React.useState(), it will be stale (the value when the callback was setup). But if it sets the state, it will have access to the latest state through the passed argument. This means we can potentially get the latest state in such a callback with React hooks by setting the state to be the same as it was. This works, but is counter-intuitive.

— Original question continues below —

I am using React hooks and trying to read state from within a callback. Every time the callback accesses it, it's back at its default value.

With the following code. The console will keep printing Count is: 0 no matter how many times I click.

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

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

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

You can find this code here

I've had no problem setting state within a callback, only in accessing the latest state.

If I was to take a guess, I'd think that any change of state creates a new instance of the Card function. And that the callback is referring to the old one. Based on the documentation at https://reactjs.org/docs/hooks-reference.html#functional-updates, I had an idea to take the approach of calling setState in the callback, and passing a function to setState, to see if I could access the current state from within setState. Replacing

setupConsoleCallback(() => {console.log(`Count is: ${count}`)})

with

setupConsoleCallback(() => {setCount(prevCount => {console.log(`Count is: ${prevCount}`); return prevCount})})

You can find this code here

That approach hasn't worked either.
EDIT: Actually that second approach does work. I just had a typo in my callback. This is the correct approach. I need to call setState to access the previous state. Even though I have no intention of setting the state.

I feel like I've taken similar approaches with React classes, but. For code consistency, I need to stick with React Effects.

How can I access the latest state information from within a callback?

Best Answer

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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