Decorators

Decorators

The decorators API customizes core Fastify objects, such as the server instance and any request and reply objects used during the HTTP request lifecycle. It can attach any type of property to core objects, e.g., functions, plain objects, or native types.

This API is synchronous. Defining a decoration asynchronously could result in the Fastify instance booting before the decoration completes. To register an asynchronous decoration, use the register API with fastify-plugin. See the Plugins documentation for more details.

Decorating core objects with this API allows the underlying JavaScript engine to optimize the handling of server, request, and reply objects. This is accomplished by defining the shape of all such object instances before they are instantiated and used. As an example, the following is not recommended because it will change the shape of objects during their lifecycle:

// Bad example! Continue reading.

// Attach a user property to the incoming request before the request
// handler is invoked.
fastify.addHook('preHandler', function (req, reply, done) {
  req.user = 'Bob Dylan'
  done()
})

// Use the attached user property in the request handler.
fastify.get('/', function (req, reply) {
  reply.send(`Hello, ${req.user}`)
})

The above example mutates the request object after instantiation, causing the JavaScript engine to deoptimize access. Using the decoration API avoids this deoptimization:

// Decorate request with a 'user' property
fastify.decorateRequest('user', '')

// Update our property
fastify.addHook('preHandler', (req, reply, done) => {
  req.user = 'Bob Dylan'
  done()
})
// And finally access it
fastify.get('/', (req, reply) => {
  reply.send(`Hello, ${req.user}!`)
})

Keep the initial shape of a decorated field close to its future dynamic value. Initialize a decorator as '' for strings and null for objects or functions. This works only with value types; reference types will throw an error during Fastify startup. See decorateRequest and JavaScript engine fundamentals: Shapes and Inline Caches for more information.

Usage

decorate(name, value, [dependencies])

This method customizes the Fastify server instance.

For example, to attach a new method to the server instance:

fastify.decorate('utility', function () {
  // Something very useful
})

Non-function values can also be attached to the server instance:

fastify.decorate('conf', {
  db: 'some.db',
  port: 3000
})

To access decorated properties, use the name provided to the decoration API:

fastify.utility()

console.log(fastify.conf.db)

The decorated Fastify server is bound to this in route handlers:

fastify.decorate('db', new DbConnection())

fastify.get('/', async function (request, reply) {
  // using return
  return { hello: await this.db.query('world') }

  // or
  // using reply.send()
  reply.send({ hello: await this.db.query('world') })
  await reply
})

The dependencies parameter is an optional list of decorators that the decorator being defined relies upon. This list contains the names of other decorators. In the following example, the "utility" decorator depends on the "greet" and "hi" decorators:

async function greetDecorator (fastify, opts) {
  fastify.decorate('greet', () => {
    return 'greet message'
  })
}

async function hiDecorator (fastify, opts) {
  fastify.decorate('hi', () => {
    return 'hi message'
  })
}

async function utilityDecorator (fastify, opts) {
  fastify.decorate('utility', () => {
    return `${fastify.greet()} | ${fastify.hi()}`
  })
}

fastify.register(fastifyPlugin(greetDecorator, { name: 'greet' }))
fastify.register(fastifyPlugin(hiDecorator, { name: 'hi' }))
fastify.register(fastifyPlugin(utilityDecorator, { dependencies: ['greet', 'hi'] }))

fastify.get('/', function (req, reply) {
  // Response: {"hello":"greet message | hi message"}
  reply.send({ hello: fastify.utility() })
})

fastify.listen({ port: 3000 }, (err, address) => {
  if (err) throw err
})

Using an arrow function breaks the binding of this to the FastifyInstance.

If a dependency is not satisfied, the decorate method throws an exception. The dependency check occurs before the server instance boots, not during runtime.

decorateReply(name, value, [dependencies])

This API adds new methods/properties to the core Reply object:

fastify.decorateReply('utility', function () {
  // Something very useful
})

Using an arrow function will break the binding of this to the Fastify Reply instance.

Using decorateReply will throw and error if used with a reference type:

// Don't do this
fastify.decorateReply('foo', { bar: 'fizz'})

In this example, the object reference would be shared with all requests, and any mutation will impact all requests, potentially creating security vulnerabilities or memory leaks. Fastify blocks this.

To achieve proper encapsulation across requests configure a new value for each incoming request in the 'onRequest' hook.

const fp = require('fastify-plugin')

async function myPlugin (app) {
  app.decorateReply('foo')
  app.addHook('onRequest', async (req, reply) => {
    reply.foo = { bar: 42 }
  })
}

module.exports = fp(myPlugin)

See decorate for information about the dependencies parameter.

decorateRequest(name, value, [dependencies])

As with decorateReply, this API adds new methods/properties to the core Request object:

fastify.decorateRequest('utility', function () {
  // something very useful
})

Using an arrow function will break the binding of this to the Fastify Request instance.

Using decorateRequest will emit an error if used with a reference type:

// Don't do this
fastify.decorateRequest('foo', { bar: 'fizz'})

In this example, the object reference would be shared with all requests, and any mutation will impact all requests, potentially creating security vulnerabilities or memory leaks. Fastify blocks this.

To achieve proper encapsulation across requests configure a new value for each incoming request in the 'onRequest' hook.

Example:

const fp = require('fastify-plugin')

async function myPlugin (app) {
  app.decorateRequest('foo')
  app.addHook('onRequest', async (req, reply) => {
    req.foo = { bar: 42 }
  })
}

module.exports = fp(myPlugin)

The hook solution is more flexible and allows for more complex initialization because more logic can be added to the onRequest hook.

Another approach is to use the getter/setter pattern, but it requires 2 decorators:

fastify.decorateRequest('my_decorator_holder') // define the holder
fastify.decorateRequest('user', {
  getter () {
    this.my_decorator_holder ??= {} // initialize the holder
    return this.my_decorator_holder
  }
})

fastify.get('/', async function (req, reply) {
  req.user.access = 'granted'
  // other code
})

This ensures that the user property is always unique for each request.

See decorate for information about the dependencies parameter.

hasDecorator(name)

Used to check for the existence of a server instance decoration:

fastify.hasDecorator('utility')

hasRequestDecorator

Used to check for the existence of a Request decoration:

fastify.hasRequestDecorator('utility')

hasReplyDecorator

Used to check for the existence of a Reply decoration:

fastify.hasReplyDecorator('utility')

Decorators and Encapsulation

Defining a decorator (using decorate, decorateRequest, or decorateReply) with the same name more than once in the same encapsulated context will throw an exception. For example, the following will throw:

const server = require('fastify')()

server.decorateReply('view', function (template, args) {
  // Amazing view rendering engine
})

server.get('/', (req, reply) => {
  reply.view('/index.html', { hello: 'world' })
})

// Somewhere else in our codebase, we define another
// view decorator. This throws.
server.decorateReply('view', function (template, args) {
  // Another rendering engine
})

server.listen({ port: 3000 })

But this will not:

const server = require('fastify')()

server.decorateReply('view', function (template, args) {
  // Amazing view rendering engine.
})

server.register(async function (server, opts) {
  // We add a view decorator to the current encapsulated
  // plugin. This will not throw as outside of this encapsulated
  // plugin view is the old one, while inside it is the new one.
  server.decorateReply('view', function (template, args) {
    // Another rendering engine
  })

  server.get('/', (req, reply) => {
    reply.view('/index.page', { hello: 'world' })
  })
}, { prefix: '/bar' })

server.listen({ port: 3000 })

Getters and Setters

Decorators accept special "getter/setter" objects with getter and optional setter functions. This allows defining properties via decorators, for example:

fastify.decorate('foo', {
  getter () {
    return 'a getter'
  }
})

Will define the foo property on the Fastify instance:

console.log(fastify.foo) // 'a getter'

getDecorator<T> API

Fastify's getDecorator<T> API retrieves an existing decorator from the Fastify instance, Request, or Reply. If the decorator is not defined, an FST_ERR_DEC_UNDECLARED error is thrown.

Use cases

Early Plugin Dependency Validation

getDecorator<T> on Fastify instance verifies that required decorators are available at registration time.

For example:

fastify.register(async function (fastify) {
  const usersRepository = fastify.getDecorator('usersRepository')

  fastify.get('/users', async function (request, reply) {
    // We are sure `usersRepository` exists at runtime
    return usersRepository.findAll()
  })
})

Handling Missing Decorators

Directly accessing a decorator may lead to unexpected behavior if it is not declared:

const user = request.user;
if (user && user.isAdmin) {
  // Execute admin tasks.
}

If request.user doesn't exist, then user will be set to undefined. This makes it unclear whether the user is unauthenticated or the decorator is missing.

Using getDecorator enforces runtime safety:

// If the decorator is missing, an explicit `FST_ERR_DEC_UNDECLARED` 
// error is thrown immediately.
const user = request.getDecorator('user');
if (user && user.isAdmin) {
  // Execute admin tasks.
}

Alternative to Module Augmentation

Decorators are typically typed via module augmentation:

declare module 'fastify' {
  interface FastifyInstance {
    usersRepository: IUsersRepository
  }
  interface FastifyRequest {
    session: ISession
  }
  interface FastifyReply {
    sendSuccess: SendSuccessFn
  }
}

This approach modifies the Fastify instance globally, which may lead to conflicts and inconsistent behavior in multi-server setups or with plugin encapsulation.

Using getDecorator<T> allows to limit types scope:

serverOne.register(async function (fastify) {
  const usersRepository = fastify.getDecorator<PostgreUsersRepository>(
    'usersRepository'
  )

  fastify.decorateRequest('session', null)
  fastify.addHook('onRequest', async (req, reply) => {
    // Yes, the request object has a setDecorator method. 
    // More information will be provided soon.
    req.setDecorator('session', { user: 'Jean' })
  })

  fastify.get('/me', (request, reply) => {
    const session = request.getDecorator<ISession>('session')
    reply.send(session)
  })
})

serverTwo.register(async function (fastify) {
  const usersRepository = fastify.getDecorator<SqlLiteUsersRepository>(
    'usersRepository'
  )

  fastify.decorateReply('sendSuccess', function (data) {
    return this.send({ success: true })
  })

  fastify.get('/success', async (request, reply) => {
    const sendSuccess = reply.getDecorator<SendSuccessFn>('sendSuccess')
    await sendSuccess()
  })
})

Bound functions inference

To save time, it's common to infer function types instead of writing them manually:

function sendSuccess (this: FastifyReply) {
  return this.send({ success: true })
}

export type SendSuccess = typeof sendSuccess

However, getDecorator returns functions with the this context already bound, meaning the this parameter disappears from the function signature.

To correctly type it, you should use OmitThisParameter utility:

function sendSuccess (this: FastifyReply) {
  return this.send({ success: true })
}

type BoundSendSuccess = OmitThisParameter<typeof sendSuccess>

fastify.decorateReply('sendSuccess', sendSuccess)
fastify.get('/success', async (request, reply) => {
  const sendSuccess = reply.getDecorator<BoundSendSuccess>('sendSuccess')
  await sendSuccess()
})

Request.setDecorator<T> Method

The setDecorator<T> method provides a safe and convenient way to update the value of a Request decorator.
If the decorator does not exist, a FST_ERR_DEC_UNDECLARED error is thrown.

Use Cases

Runtime Safety

A typical way to set a Request decorator looks like this:

fastify.decorateRequest('user', '')
fastify.addHook('preHandler', async (req, reply) => {
  req.user = 'Bob Dylan'
})

However, there is no guarantee that the decorator actually exists unless you manually check beforehand.
Additionally, typos are common, e.g. account, acount, or accout.

By using setDecorator, you are always sure that the decorator exists:

fastify.decorateRequest('user', '')
fastify.addHook('preHandler', async (req, reply) => {
  // Throws FST_ERR_DEC_UNDECLARED if the decorator does not exist
  req.setDecorator('user-with-typo', 'Bob Dylan')
})

---

Type Safety

If the FastifyRequest interface does not declare the decorator, you would typically need to use type assertions:

fastify.addHook('preHandler', async (req, reply) => {
  (req as typeof req & { user: string }).user = 'Bob Dylan'
})

The setDecorator<T> method eliminates the need for explicit type assertions while allowing type safety:

fastify.addHook('preHandler', async (req, reply) => {
  req.setDecorator<string>('user', 'Bob Dylan')
})