app.jsx

import React from 'react'

let counter = 0

class App extends React.Component {
  constructor(props) {
    super(props)
    this.addOne = this.addOne.bind(this)
  }
  
  addOne() {
    counter += 1
    this.forceUpdate()
  }
  
  render() {
    return (
      <div>
        <button
          onClick={ this.addOne }>
          Increment
        </button>
        { counter }
      </div>
    )
  }
}

export default App