index.js

const http = require("http"),
   app = require("./app");
server = http.createServer(app);

const PORT = process.env.PORT || 3001;

app.set("port", PORT);

server.listen(PORT);

server.on("listening", onListen);
server.on("error", onError);

/**
 * Event listener for HTTP server "error" event.
 */

function onError(error) {
   if (error.syscall !== "listen") {
      throw error;
   }

   var bind = typeof PORT === "string" ? "Pipe " + PORT : "Port " + PORT;

   // handle specific listen errors with friendly messages
   switch (error.code) {
      case "EACCES":
         console.error(bind + " requires elevated privileges");
         process.exit(1);
         break;
      case "EADDRINUSE":
         console.error(bind + " is already in use");
         process.exit(1);
         break;
      default:
         throw error;
   }
}

/**
 * Event listener for HTTP server "listening" event.
 */

function onListen() {
   console.log(
      `Server listening on port ${PORT} in ${process.env.NODE_ENV} mode`
   );
}

// handling uncaughtException and unhandledRejection
process
   .on("unhandledRejection", (reason, promise) => {
      console.error("Unhandled Rejection at:", promise, "reason:", reason);
   })
   .on("uncaughtException", (err, origin) => {
      console.error("Uncaught Exception thrown", err, "on", origin);
   });