context-useReducer.jsx

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

export const StateContext = createContext();
export const StateProvider = ({ reducer, initialState, children }) => (
  <StateContext.Provider value={useReducer(reducer, initialState)}>
    {children}
  </StateContext.Provider>
);
export const useStore = () => useContext(StateContext);

//-----------------------------------------------------//
const store = {
  initialState: {
    count: 0
  },
  reducer: (state, action) => {
    switch (action.type) {
      case "increment":
        return { ...state, count: state.count + 1 };
      case "decrement":
        return { ...state, count: state.count - 1 };
      default:
        return state;
    }
  }
};

export default function App() {
  return (
    <StateProvider {...store}>
      <Counter />
    </StateProvider>
  );
}

export function Counter() {
  const [state, dispatch] = useStore();
  console.log(state, dispatch);
  return (
    <div>
      {state.count}
      <button onClick={e => dispatch({ type: "increment" })}>+</button>
      <button onClick={e => dispatch({ type: "decrement" })}>-</button>
    </div>
  );
}