Manage global state with React Hooks

global-reducer.js

const initialState = { count: 0 };

function reducer(state, action) {
  switch (action.type) {
    case 'reset':
      return initialState;
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
  }
}

function Counter() {
  const [ state, dispatch ] = useReducer(
    reducer,
    {count: initialCount}
  );
  const reset = () => dispatch({ type: 'reset' });
  const increment = () => dispatch({ type: 'increment' });
  const decrement = () => dispatch({ type: 'decrement' });
  return (
    <>
      Count: {state.count}
      <button onClick={reset}>Reset</button>
      <button onClick={increment}>+</button>
      <button onClick={decrement}>-</button>
    </>
  );
}

useReducer hooks second argument should be initialState (which you have already declared) and not {count: initialCount}.