Where is component state stored?

ReactJunior~2 min read

1. At the code level

When you write, for example:

javascript

function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

It seems as if count is stored simply in a variable inside the function. But this is not the case, because the function is called again on every render, which means all local variables inside it are reset.

2. Where `state` is actually stored

In fact, the state is stored inside React itself, not in the component directly.

During render, React creates an internal structure (Fiber node) for each component, where it keeps all the internal-service information:

the current state (state)
hooks (useState, useEffect, useRef, etc.)
references to child components
previous props and state
flags for updates, and so on

That is, useState does not create a variable in your function, it registers it inside React Fiber.

3. What happens on `setState` / `setCount`

When you call setCount(newValue):

React saves the new state value in its internal structure (Fiber).
Marks the component as "needing an update".
Re-renders the component (will call the Counter() function again).
On the new render, React retrieves the saved state and substitutes it into [count, setCount].

So the value of count "remembers" its past, even though the Counter function is called again and again.

4. Where exactly it is stored (simplified)

If you picture it as a structure:

javascript

FiberNode = {
  type: Counter,
  stateHooks: [
    { state: 0, queue: [updateFn1, updateFn2, ...] }
  ],
  props: {},
  memoizedState: ...,
  ...
}

Then the state is stored in stateHooks or memoizedState of the Fiber object, not in your function directly.

5. Summary:

Question	Answer
Where is the state stored?	Inside React (in the Fiber structure associated with the component).
Why not in the function itself?	Because a component is just a function that gets called every render.
How does React "remember" past state?	It stores it between function calls, in its internal component tree.

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Where is component state stored?

#1. At the code level

#2. Where state is actually stored

#3. What happens on setState / setCount

#4. Where exactly it is stored (simplified)

#5. Summary: