Project structure

So far the whole server fits in one file, but as the API grows it is easier to split it. The usual approach separates building the application (src/app.js) from starting it (src/server.js): this way tests can create the application without taking a port.

Project folders

blog-api/
  package.json
  src/
    app.js          builds the application
    server.js       starts it: listen
    routes/
      posts.js      post routes
      comments.js   comment routes
    plugins/
      db.js         shared parts: the database and so on
  test/
    posts.test.js   tests

You create the routes and plugins folders yourself — Fastify does not require a fixed structure, this is a common convention.

A routes file

Create src/routes/posts.js. In Fastify routes are written as a function that receives the application instance:

// src/routes/posts.js
export default async function postRoutes(app) {
  app.get('/posts', async () => {
    return [{ id: 1, title: 'First post' }];
  });
}

Such a function is called a plugin. Plugins get a whole chapter later; for now it is enough to know that they are connected with register.

Building the application

Create src/app.js. It creates the instance, registers the routes and returns it without starting:

// src/app.js
import Fastify from 'fastify';
import postRoutes from './routes/posts.js';
 
export function buildApp(options = {}) {
  const app = Fastify(options);
 
  app.register(postRoutes);
 
  return app;
}

The function takes options, so the server can turn the logger on while a test leaves it off.

Starting

Replace the contents of src/server.js:

// src/server.js
import { buildApp } from './app.js';
 
const app = buildApp({ logger: true });
 
try {
  await app.listen({ port: 3000 });
} catch (error) {
  app.log.error(error);
  process.exit(1);
}

Run npm run dev and open http://localhost:3000/posts — you see an array with one post.

What goes where

Common mistake

In ES modules the path to your own file includes the extension: import postRoutes from './routes/posts.js'. Without .js Node.js fails with ERR_MODULE_NOT_FOUND.