full.js

import { render } from "react-dom";
import React, { useState, useCallback } from "react";

const initialState = { count: 0 };

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

function useReducer(reducer, initState) {
  const [state, setState] = useState(initState);

  const dispatch = useCallback((action) => {
    const nextState = reducer(state, action)
    setState(nextState)
  }, [setState, state])

  return [state, dispatch];
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <React.Fragment>
      Count: {state.count}
      <button onClick={() => dispatch({ type: "increment" })}>+</button>
      <button onClick={() => dispatch({ type: "decrement" })}>-</button>
    </React.Fragment>
  );
}