// use Prompt like normal... magic happens in getUserConfirmation
class App extends Component {
  render () {
    return (
      <Router getUserConfirmation={getUserConfirmation}>
        {...}
        <Prompt
          when={formIsHalfFilledOut}
          message="Are you sure you want to leave?" 
        />
      </Router>
    )
  }
}

ReactDOM.render(
  <App />, 
  document.getElementById('root')
)