react useEffect & useContext example

App.js

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

function Counter() {
  const { count, setCount } = useContext(CountContext)

  useEffect(() => {
    document.title = `You clicked ${count} times`
  }, [count])

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  )
}

const CountContext = createContext()
function countReducer(state, count) {
  console.log("count=", count)
  console.log("state=", state)
  return count
}
function App() {
  const initialvalue = 1
  const [count, setCount] = useReducer(countReducer, initialvalue)
  return (
    <CountContext.Provider value={{ count, setCount }}>
      <Counter></Counter>
    </CountContext.Provider>
  )
}

export default App