title: How to put a React application in a maintenance mode?
body: <p>I have a React application that lives separately from the backend Rails API application and I would like to introduce a feature of putting the frontend app into maintenance mode and I can't come up with any good idea.</p>
  <p>My client wants to have a button to click and so that when he clicks it, the app would go to a maintenance mode.</p>
  <p>Since this is a React app, obviously the HTML is not rendered by any backend server (the backend API serves only for providing data to the frontend and as an admin panel) but by JS. I use <code>react-router</code> and I have many routes which render different components, those, in turn, make several API calls to the backend. I could put the backend in a maintenance mode and make the frontend app react on responses from the backend (503 Service Unavailable) but it would take a lot of effort and time since I'd have to modify all routes and sagas to make them react appropriately to the responses from the backend.</p>
  <p>I am thinking of creating a separate branch holding just a maintenance page and have the customer to deploy this branch whenever he wants to instead of having the button to click. I've tried using Express.js middleware <a href="https://github.com/likeastore/maintenance" rel="nofollow noreferrer">Maintenance</a> but either it doesn't work or I don't know how to use it (I am pretty new to this). The app runs on an Express server on Node. How can I solve this problem?</p>