Express server manage used port

express_with_used_port_management_server.js

const express = require('express');
const app = express();
const port = process.env.PORT || 3000;

// Start the server using managed startup process
startServer()

// Since express is just a wrapper around http and extends the EventEmitter, one can catch the port error using on('error').
function startServer(app, port) {
  // process.env props are usually sent as string
  port = Number(port);
  
  const server = app
    .listen(port, () => {
      console.log(`Example app listening on port ${port}!`);
    })
    .on("error", err => {
      if (err.code === "EADDRINUSE") {
        server.close();
        startServer(app, port + 1);
      }
    });
}

Usually process.env parameters send as string. So it's better that convert port to integer, first.

Good catch. Updated the gist.