counter.jsx

import React, { Fragment, useState } from 'react'
import Button from '../button/button'

function Counter (props) {
  const [count, setCount] = useState(props.initialCount)

  function increment () {
    setCount(count + 1)
  }

  function decrement () {
    setCount(count - 1)
  }

  return (
    <Fragment>
      I've counted {count}.
      <div>
        <Button onClick={increment}>+</Button>
        <Button onClick={decrement}>-</Button>
      </div>
    </Fragment>
  )
}

Counter.defaultProps = {
  initialCount: 0
}

export default Counter