Decorators

Decorators

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

This API is a synchronous API. Attempting to define a decoration asynchronously could result in the Fastify instance booting prior to the decoration completing its initialization. To avoid this issue, and register an asynchronous decoration, the register API, in combination with fastify-plugin, must be used instead. To learn more, see the Plugins documentation.

Decorating core objects with this API allows the underlying JavaScript engine to optimize handling of the 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}`)
})

Since the above example mutates the request object after it has already been instantiated, the JavaScript engine must deoptimize access to the request object. By using the decoration API this deoptimization is avoided:

// 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}!`)
})

See JavaScript engine fundamentals: Shapes and Inline Caches for more information on this topic.

Usage

decorate(name, value, [dependencies])

This method is used to customize the Fastify server instance.

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

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

As mentioned above, non-function values can be attached:

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

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

fastify.utility()

console.log(fastify.conf.db)

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

fastify.decorate('utility', fn, ['greet', 'log'])

If a dependency is not satisfied, the decorate method will throw an exception. The dependency check is peformed before the server instance is booted. Thus, it cannot occur during runtime.

decorateReply(name, value, [dependencies])

As the name suggests, this API is used to add new methods/properties to the core Reply object:

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

Note: using an arrow function will break the binding of this to the Fastify Reply instance.

See decorate for information about the dependencies parameter.

decorateRequest(name, value, [dependencies])

As above with decorateReply, this API is used add new methods/properties to the core Request object:

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

Note: using an arrow function will break the binding of this to the Fastify Request instance.

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.

As an 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(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(3000)

Getters and Setters

Decorators accept special "getter/setter" objects. These objects have functions named getter and setter (though, the setter function is optional). 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'