Implementation details of setStateVariable function in useState hook

Question

We know that useState is a hook in FC that returns an array consists of two elements. The first one is a state variable and the second is a function to update the state variable.

const initialStateVariableValue = 0; // any primitive value
const [StateVariable, setStateVariable] = useState(initialStateVariableValue);

Here I would like to know what is the implementation details of setStateVariable function? How it is updating the state variable?

Answer 1

If you check the implementation on React github you notice that useState is just calling useReducer with a basic reducer:

export function useState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return useReducer(
    basicStateReducer,
    (initialState: any),
  );
}

So looking for the useReducer implementation, we see that the setter function is the dispatch function which changing depending on the lifecycle we currently in

export function useReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  ...
    // dispatch depends on lifecycle
    return [..., dispatch];
  }
}

You can see the full useReducer implementation here.

For detailed implementation, you should try and Build your own React which eventually will lead to a simplified version of this hook:

function useState(initial) {
  const oldHook =
    wipFiber.alternate &&
    wipFiber.alternate.hooks &&
    wipFiber.alternate.hooks[hookIndex];
  const hook = {
    state: oldHook ? oldHook.state : initial,
    queue: []
  };

  const actions = oldHook ? oldHook.queue : [];
  actions.forEach(action => {
    hook.state = action(hook.state);
  });

  const setState = action => {
    hook.queue.push(action);
    wipRoot = {
      dom: currentRoot.dom,
      props: currentRoot.props,
      alternate: currentRoot
    };
    nextUnitOfWork = wipRoot;
    deletions = [];
  };

  wipFiber.hooks.push(hook);
  hookIndex++;
  return [hook.state, setState];
}

In simple words: every hook is saved in "React hooks array" (that's why the call order is essential because hooks saved in array's indexes - see Rules of Hooks), and depending on the hook's index, whenever called it mutates the state object associated with the current component.