Why should you separate express app and server?

Keeping the API declaration separated from the network related configuration (port, protocol, etc) allows testing the API in-process, without performing network calls, with all the benefits that it brings to the table: fast testing execution and getting coverage metrics of the code. It also allows deploying the same API under flexible and different network conditions.

API declaration, should reside in app.js:

const app = express();

app.use(bodyParser.json());
app.use("/api/events", events.API);
app.use("/api/forms", forms);

Server network declaration

const app = require('../app');
const http = require('http');

// Get port from environment and store in Express.
const port = normalizePort(process.env.PORT || '3000');
app.set('port', port);

// Create HTTP server.
const server = http.createServer(app);