Heroku

1. In the server folder, package.json folder, add these node values:
 "engines": {
    "node": "12.13.0",
    "npm": "6.12.0"
  },
 and 
2. "heroku-postbuild": "NPM_CONFIG_PRODUCTION=false npm i --prefix client && npm run build --prefix client"

3. if (process.env.NODE_ENV === "production") {
  // Express will serve up production assets
  // like our main.js file or main.css file
  app.use(express.static("client/build"));

  // Express will serve up the index.html file if it doesn't recognize the route
  const path = require("path");
  app.get("*", (req, res) => {
    res.sendFile(path.resolve(__dirname, "client", "build", "index.html"));
  });
}

4. In the server, check package.json, see if we have start script:
"start": "node index.js",

5. Don't forget any REACT_APP_ environment variable if there is !!!!

*****************************************************

heroku login -i
heroku create
heroku apps

If app already mapped, remap it:
heroku git:remote <new_app>

If not, push to heroku master
git push heroku master

Link: https://devcenter.heroku.com/articles/heroku-cli-commands
https://devcenter.heroku.com/articles/maintenance-mode
https://dev.to/lawrence_eagles/causes-of-heroku-h10-app-crashed-error-and-how-to-solve-them-3jnl