When building a Node.js application using Express, one of the key decisions you'll need to make is whether to keep your application logic in a single file, such as app.js, or to separate it out into multiple files, including a dedicated server.js file. While it may be tempting to keep everything in one place for the sake of simplicity, separating your code into distinct files can actually improve the quality and maintainability of your application in the long run.

While separating your code into different files is a good idea in general, there's also a specific reason why you should consider separating app.js and server.js. In this blog post, we'll explore why separating app.js and server.js is a good practice and provide examples and code snippets to illustrate the advantages.

Separation of concerns

One of the main reasons to separate app.js and server.js is to divide the application's concerns. app.js is responsible for defining the routes, middleware, and other application-level functionality. server.js, on the other hand, is responsible for creating the server, listening for incoming requests, and handling errors.

Thus we can keep our code organized and maintainable. If we need to make changes to the application logic or routes, we can do so in the app.js file without worrying about the server setup. Similarly, if we need to make changes to the server configuration, we can do so in the server.js file without affecting the application logic.

Here is an example of what a app.js and server.js file might look like:

app.js file:

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  res.send('Hello World!');
});

module.exports = app;

server.js file:

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

const PORT = process.env.PORT || 3000;

app.listen(PORT, () => {
  console.log(`Server listening on port ${PORT}`);
});

In this code snippet, the app.js file is responsible for defining the Express application and its routes, while server.js creates a new HTTP server instance and passes it to the app module. This approach separates the server configuration from the application's functionality.

Easier testing

Separating app.js and server.js not only draws a clean separation of concerns but also significantly eases mocking and testing the system by 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.

For example, you can write unit tests for app.js to ensure that each route and middleware function is working as intended. You can also write integration tests for server.js to ensure that the server is properly configured and handling requests correctly.

Here is an example of a test file for app.js:

const request = require('supertest');
const app = require('../app');

describe('GET /', () => {
  it('should respond with "Hello, world!"', (done) => {
    request(app)
      .get('/')
      .expect(200)
      .end((err, res) => {
        if (err) return done(err);
        expect(res.text).toBe('Hello, world!');
        done();
      });
  });
});

In this code snippet, we're using the supertest library to make HTTP requests to our app module and test its responses.

Improved scalability

Finally, separating app.js and server.js can improve the application's scalability. By breaking up the code into smaller modules, it becomes easier to add new features or modify existing ones without having to touch the entire application codebase.

In summary, separating the app.js and server.js files in an Express.js application can improve code organization, maintainability, and testability.