MERN

Client

npx create-react-app client

Start

npm start

conect server nodejs

{
  ...
  "proxy": "http://localhost:5000/",
  ...
}

Server

port

var port = normalizePort(process.env.PORT || "5000");

MERN

start mern

Build client:

cd client && npm install && npm install --only=dev --no-shrinkwrap && npm run build

React build:

app.use(express.static("./../client/build"));

Run server

cd server && npm start