# Plugins & Decorators — Fastify

Source: https://www.geekswithgeeks.com/en/fastify/ff-plugins

> Extend Fastify with reusable plugins and decorators.

## What are plugins?

Plugins are async functions that encapsulate functionality—database clients, authentication, middleware—and register routes, decorators, or hooks. They enable modular server design.

## Writing a plugin

A plugin receives the Fastify instance and an options object. Register it with `fastify.register(plugin, opts)`.

```javascript
// plugins/database.js
export default async function dbPlugin(fastify, opts) {
  const db = { getUser: (id) => ({ id, name: 'Alice' }) };
  fastify.decorate('db', db);
}

// server.js
fastify.register(dbPlugin);
fastify.get('/users/:id', async (req, reply) => {
  const user = await fastify.db.getUser(req.params.id);
  return user;
});
```

## Decorators add properties

`fastify.decorate()` adds a property to the Fastify instance, available in all routes. `fastify.decorateRequest()` adds to the request object, `fastify.decorateReply()` to the reply.