Setup the an express server

index.js

const express = require("express");
const mongoose = require("mongoose");

const port = process.env.PORT || 4000;
const uri = process.env.MONGODB_URI;

const options = {
  useCreateIndex: true,
  useNewUrlParser: true,
  useFindAndModify: false,
  useUnifiedTopology: true
};

mongoose
  .connect(uri, options)
  .then(() => console.log("Connected to the DB"))
  .catch(err => {
    console.log(`Could not connect to DB: ${err}`);
    process.exit(1);
  });

const app = express();

//create a server object:
app.listen(port, err => {
  if (err) {
    console.log(`Could not start the server: ${err}`);
    process.exit(1);
  }

  console.log(`server started at http://localhost:${port}`);
});