How does this variable have a function called `listen` after importing it into index.ts?

Question

I'm running a Github repo I found. With regard to the line import server from './server' - how does this API know that server has a function called listen? When I go to the server.ts file located in the same folder, there is no listen function defined. If I right click the server variable after the import keyword and click Go To Definition (using VS Code), it points to the line const app = express() in server.ts, so this is definitely the correct file.

index.ts:

import './pre-start'; // Must be the first import
import logger from 'jet-logger';

import EnvVars from '@src/constants/EnvVars';
import server from './server';


// **** Run **** //

const SERVER_START_MSG = ('Express server started on port: ' + 
  EnvVars.Port.toString());

server.listen(EnvVars.Port, () => logger.info(SERVER_START_MSG));

server.ts:

/**
 * Setup express server.
 */

import cookieParser from 'cookie-parser';
import morgan from 'morgan';
import path from 'path';
import helmet from 'helmet';
import express, { Request, Response, NextFunction } from 'express';
import logger from 'jet-logger';

import 'express-async-errors';

import BaseRouter from '@src/routes/api';
import Paths from '@src/routes/constants/Paths';

import EnvVars from '@src/constants/EnvVars';
import HttpStatusCodes from '@src/constants/HttpStatusCodes';

import { NodeEnvs } from '@src/constants/misc';
import { RouteError } from '@src/other/classes';


// **** Variables **** //

const app = express();


// **** Setup **** //

// Basic middleware
app.use(express.json());
app.use(express.urlencoded({extended: true}));
app.use(cookieParser(EnvVars.CookieProps.Secret));

// Show routes called in console during development
if (EnvVars.NodeEnv === NodeEnvs.Dev) {
  app.use(morgan('dev'));
}

// Security
if (EnvVars.NodeEnv === NodeEnvs.Production) {
  app.use(helmet());
}

// Add APIs, must be after middleware
app.use(Paths.Base, BaseRouter);

// Add error handler
app.use((
  err: Error,
  _: Request,
  res: Response,
  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  next: NextFunction,
) => {
  if (EnvVars.NodeEnv !== NodeEnvs.Test) {
    logger.err(err, true);
  }
  let status = HttpStatusCodes.BAD_REQUEST;
  if (err instanceof RouteError) {
    status = err.status;
  }
  return res.status(status).json({ error: err.message });
});


// ** Front-End Content ** //

// Set views directory (html)
const viewsDir = path.join(__dirname, 'views');
app.set('views', viewsDir);

// Set static directory (js and css).
const staticDir = path.join(__dirname, 'public');
app.use(express.static(staticDir));

// Nav to login pg by default
app.get('/', (_: Request, res: Response) => {
  res.sendFile('login.html', { root: viewsDir });
});

// Redirect to login if not logged in.
app.get('/users', (req: Request, res: Response) => {
  const jwt = req.signedCookies[EnvVars.CookieProps.Key];
  if (!jwt) {
    res.redirect('/');
  } else {
    res.sendFile('users.html', {root: viewsDir});
  }
});


// **** Export default **** //

export default app;

Answer 1

server is the value imported from the module.

That value is app (because the default export is defined such: export default app;)

The value of app is the return value of calling the function express (because const app = express();).

express is imported from a different module (import express, { Request, Response, NextFunction } from 'express';).

… and so on.

---

You need to trace the variable back further than a single file to find the definition of the listen method.