Counter with Context and useReducer

CounterWithContextAndUsereducer.tsx

import React, { useReducer, createContext, useContext } from 'react';

type CounterProps = {
  initialCount: number
};

type CounterState = {
  count: number
};

type CounterAction = {
  type: 'increment' | 'decrement' | 'reset',
  payload?: number
};

function init (initialCount: number): CounterState {
  return { count: initialCount };
}

function reducer (state: CounterState, action: CounterAction) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    case 'reset':
      return init(action.payload as number);
    default:
      throw new Error();
  }
}

const initialCount = 10;

type CounterContextProps = {
  state: CounterState,
  dispatch: React.Dispatch<CounterAction>
};

const contextStub: CounterContextProps = {
  state: { count: 0 },
  dispatch: () => { return; }
};

const CounterContext = createContext<CounterContextProps>(contextStub);

export function CounterProvider (props: React.PropsWithChildren<{}>) {
  const [state, dispatch] = useReducer(reducer, initialCount, init);
  return (
    <CounterContext.Provider value={{ state, dispatch }}>
      {props.children}
    </CounterContext.Provider>
  );
}

export function Counter ({ initialCount }: CounterProps) {
  const { state, dispatch } = useContext(CounterContext);
  return (
    <>
      Count: { state.count }
      <button onClick={() => dispatch({ type: 'reset', payload: initialCount })}>
        Reset
      </button>
      <button onClick={() => dispatch({ type: 'increment' })}>+</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
    </>
  );
}