If the app has not been setup yet (based on the /setup/setup-config/ express endpoint), redirect them to the setup page no matter what URL they go to.

App.js

import React, {Component} from 'react'
import {BrowserRouter, Switch, Route} from 'react-router-dom'
import Setup from '../pages/Setup'

import Routes from '../routes'
class App extends Component {
  constructor (props) {
    super(props)
    this.state = {setup: true} // assume true first...
  }
  componentDidMount () {
    fetch('/setup/setup-config') // this is an express url that returns {setup: false} or {setup: true}
    .then((responseJson) => {
      if (responseJson.ok) {
        responseJson.json().then(json => {
          this.setState({
            setup: json.setup
          })
        })
      }
    })
    .catch((error) => {
      console.error(error)
    })
  }
  render () {
    if (this.state.setup) {
      return (<BrowserRouter>
        <Routes />
      </ BrowserRouter>)
    }
    return (<BrowserRouter>
      <Switch>
        <Route exact path='*' component={Setup} />
      </Switch>
    </ BrowserRouter>)
  }
}

export default App

gistfile1.txt

import React from 'react'

export default () => {
  return (<h2>Setup Page</h2>)
}

Home.js

import React, {Component} from 'react'


class Home extends Component {
  render () {
    return (
      <div>
        Welcome To Home Page
      </div>
    )
  }
}

export default Home

index.js

import React from 'react'
import ReactDOM from 'react-dom'
import registerServiceWorker from './registerServiceWorker'
import App from './components/App'

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

NotFound.js

import React from 'react'

export default () => {
  return (<h2>Not Found</h2>)
}

routes.js

import React from 'react'
import { Switch, Route } from 'react-router-dom'

import Setup from './pages/Setup'
import Home from './pages/Home'
import NotFound from './pages/NotFound'

const Routes = () => (
  <Switch>
    <Route exact path='/' component={Home} />
    <Route exact path='/setup' component={Setup} />
    <Route exact path='*' component={NotFound} />
  </Switch>
)

export default Routes

gistfile1.txt should be Setup.js, the setup page