App.js

// signature is (props:object, state:object, setState:function)
function App({ label }, { count = 0 }, setState) {
  return (
    <div>
      <button
        onClick={() => setState({ count: count + 1 })}>
        {label + " " + count}
      </button>
    </div>
  )
}

export default toReactClass(App)

index.js

React.render(
  <App
    label="clicked" />,
  document.getElementById("App")
)

toReactClass.js

function toReactClass(func) {
  return class extends React.Component {

    state = {}

    render() {
      return func(this.props, this.state, ::this.setState)
    }
  }
}