counter-setstate.js

import React, { Component } from 'react'

export default class Counter extends Component {
  constructor (props) {
    this.state = {
      counter: 0
    }
  }

  handleIncrement = (amount) => {
    const { counter } = this.state
    this.setState({ counter: counter + amount })
  }

  handleDecrement = (amount) => {
    const { counter } = this.state
    this.setState({ counter: counter - amount })
  }

  render () {
    const { counter } = this.state

    return (
      <div>
        <p>Count: {counter}</p>
        <button onClick={() => this.handleIncrement(1)}>Increment</button>
        <button onClick={() => this.handleDecrement(1)}>Decrement</button>
      </div>
    )
  }
}