Relay Modern (or anything else) server side rendering for bots.... only.

appname.js

#!/usr/bin/env node

const fs = require("fs");
const path = require("path");
const express = require("express");
const jsdom = require("jsdom");
const isBot = require("isbot");
const { Script } = require("vm");

const app = express();

const INDEX_FILE = path.normalize(`${__dirname}/../src/www/_index.html`);
const APP_PATH = path.normalize(`${__dirname}/../src/www`);

const readFile = function(path) {
  return new Promise((resolve, reject) => {
    fs.readFile(path, "utf8", (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
};

// Server up public files from www/. Notice how index.html is _index.html, so it
// it doesn't get resolved here.
app.use(express.static(APP_PATH));

app.get("*", async function(req, res) {
  // If we're in bot territory, serve them a rendered html view so they can
  // scrape away. Otherwise, give the user a good experience.
  if (isBot(req.headers["user-agent"])) {
    console.info("Bot detected. Sending html: " + req.headers["user-agent"]);

    const html = await readFile(INDEX_FILE);

    // Run scripts, dangerously.
     const dom = new jsdom.JSDOM(html, {
      url: process.env.APP_URL + req.originalUrl,
      pretendToBeVisual: true,
      resources: "usable",
      runScripts: "dangerously",
      beforeParse(window) {
        // Add any other window specific func's your app may use.
        window.scrollTo = function() {};
        window.alert = function() {};
      }

    });

    // You may need to adjust the interval timeout needed for your app
    // depending on the kinds of things it's doing to the DOM.
    setTimeout(() => res.send(dom.serialize(), 10);
  } else {
    res.sendFile(INDEX_FILE);
  }
});

app.listen(process.env.PORT || 3000);

console.info("Environment:", process.env.NODE_ENV);
console.info("Listening on port ", process.env.PORT);