R_on_R => RR v4 + SSR = :(

Routes.js

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

import PageNotFound from "routing/pages/PageNotFound"
import LandingPage from "./pages/LandingPage"
import LoginPage from "./pages/LoginPage"
import SignupPage from "./pages/SignupPage"

const Routes = () =>
  <Switch>
    <Route exact path="/" component={LandingPage} />
    <Route exact path="/login" component={LoginPage} />
    <Route exact path="/signup" component={SignupPage} />
    <Route component={PageNotFound} />
  </Switch>

export default Routes

routes.rb

Rails.application.routes.draw do
  root to: "pages#marketing"

  get "/platform", to: "pages#platform
  get "/platform/*path", to: "pages#platform"

  get "*path", to: "pages#marketing"
end

server.js

import ReactOnRails from "react-on-rails"

import ServerApp from "./ServerApp"

ReactOnRails.register({
  MarketingApp: ServerApp,
})

ServerApp.js

import React from "react"
import { StaticRouter } from "react-router"

import Routes from "./Routes"

const ServerApp = (
  props,
  railsContext,
) =>
  <StaticRouter
    location={railsContext.location}
  >
    <Routes />
  </StaticRouter>

export default ServerApp