Express 5 vs Fastify Comparison

// Express 5 - Async route handler + automatic error catching
import express from 'express';

const app = express();
app.use(express.json());

// A simple error class — for a meaningful message at the API boundary
class NotFoundError extends Error {
  status = 404;
}

async function getUser(id) {
  const user = await db.users.findById(id);
  if (!user) throw new NotFoundError('User not found');
  return user;
}

// Express 5: if the async handler rejects/throws, next(err) is called automatically.
// Don't forget to return the Promise — otherwise the rejection can stay unhandled.
app.get('/api/users/:id', async (req, res) => {
  const user = await getUser(req.params.id);
  res.json(user);
});

// Central error middleware — added at the end of the global stack
app.use((err, req, res, next) => {
  const status = err.status ?? 500;
  res.status(status).json({ error: err.message });
});

app.listen(3000, () => {
  console.log('Express server listening on port 3000');
});

// Fastify 5 - Schema validation + automatic serialization
import Fastify from 'fastify';

const fastify = Fastify({ logger: true });

// Response schema: fast-json-stringify compiles a serialization
// function from this schema — faster than general-purpose JSON.stringify.
const userSchema = {
  type: 'object',
  properties: {
    id: { type: 'string' },
    name: { type: 'string' },
    email: { type: 'string' },
  },
  required: ['id', 'name', 'email'],
};

const getUserOpts = {
  schema: {
    params: {
      type: 'object',
      properties: { id: { type: 'string' } },
      required: ['id'],
    },
    response: {
      200: userSchema,
    },
  },
};

// Encapsulation: hooks/decorators registered inside this plugin
// only leak into routes within this context.
fastify.register(async function userRoutes(instance) {
  instance.get('/api/users/:id', getUserOpts, async (request, reply) => {
    const user = await db.users.findById(request.params.id);
    if (!user) {
      return reply.code(404).send({ error: 'User not found' });
    }
    request.log.info({ userId: user.id }, 'user fetched');
    return user;
  });
});

fastify.setErrorHandler((error, request, reply) => {
  reply.status(error.statusCode ?? 500).send({ error: error.message });
});

await fastify.listen({ port: 3000 });