Plugins

Fastify allows the user to extend its functionalities with plugins. A plugin can be a set of routes, a server decorator, or whatever. The API that you will need to use one or more plugins, is register.

By default, register creates a new scope, this means that if you make some changes to the Fastify instance (via decorate), this change will not be reflected by the current context ancestors, but only by its descendants. This feature allows us to achieve plugin encapsulation and inheritance, in this way we create a directed acyclic graph (DAG) and we will not have issues caused by cross dependencies.

You may have already seen in the Getting Started guide how easy it is to use this API:

fastify.register(plugin, [options])

Plugin Options

The optional options parameter for fastify.register supports a predefined set of options that Fastify itself will use, except when the plugin has been wrapped with fastify-plugin. This options object will also be passed to the plugin upon invocation, regardless of whether or not the plugin has been wrapped. The currently supported list of Fastify specific options is:

Note: Those options will be ignored when used with fastify-plugin

It is possible that Fastify will directly support other options in the future. Thus, to avoid collisions, a plugin should consider namespacing its options. For example, a plugin foo might be registered like so:

fastify.register(require('fastify-foo'), {
  prefix: '/foo',
  foo: {
    fooOption1: 'value',
    fooOption2: 'value'
  }
})

If collisions are not a concern, the plugin may simply accept the options object as-is:

fastify.register(require('fastify-foo'), {
  prefix: '/foo',
  fooOption1: 'value',
  fooOption2: 'value'
})

The options parameter can also be a Function that will be evaluated at the time the plugin is registered while giving access to the Fastify instance via the first positional argument:

const fp = require('fastify-plugin')

fastify.register(fp((fastify, opts, done) => {
  fastify.decorate('foo_bar', { hello: 'world' })

  done()
}))

// The opts argument of fastify-foo will be { hello: 'world' }
fastify.register(require('fastify-foo'), parent => parent.foo_bar)

The Fastify instance passed on to the function is the latest state of the external Fastify instance the plugin was declared on, allowing access to variables injected via decorate by preceding plugins according to the order of registration. This is useful in case a plugin depends on changes made to the Fastify instance by a preceding plugin i.e. utilizing an existing database connection to wrap around it.

Keep in mind that the Fastify instance passed on to the function is the same as the one that will be passed into the plugin, a copy of the external Fastify instance rather than a reference. Any usage of the instance will behave the same as it would if called within the plugins function i.e. if decorate is called, the decorated variables will be available within the plugins function unless it was wrapped with fastify-plugin.

Route Prefixing option

If you pass an option with the key prefix with a string value, Fastify will use it to prefix all the routes inside the register, for more info check here.

Be aware that if you wrap your routes with fastify-plugin, this option will not work (there is a workaround available).

Error handling

The error handling is done by avvio.

As a general rule, it is highly recommended that you handle your errors in the next after or ready block, otherwise you will get them inside the listen callback.

fastify.register(require('my-plugin'))

// `after` will be executed once
// the previous declared `register` has finished
fastify.after(err => console.log(err))

// `ready` will be executed once all the registers declared
// have finished their execution
fastify.ready(err => console.log(err))

// `listen` is a special ready,
// so it behaves in the same way
fastify.listen({ port: 3000 }, (err, address) => {
  if (err) console.log(err)
})

async/await

async/await is supported by after, ready, and listen, as well as fastify being a Thenable.

await fastify.register(require('my-plugin'))

await fastify.after()

await fastify.ready()

await fastify.listen({ port: 3000 })

Note: Using await when registering a plugin loads the plugin and the underlying dependency tree, "finalizing" the encapsulation process. Any mutations to the plugin after it and its dependencies have been loaded will not be reflected in the parent instance.

ESM support

ESM is supported as well from Node.js v13.3.0 and above!

// main.mjs
import Fastify from 'fastify'
const fastify = Fastify()

fastify.register(import('./plugin.mjs'))

fastify.listen({ port: 3000 }, console.log)


// plugin.mjs
async function plugin (fastify, opts) {
  fastify.get('/', async (req, reply) => {
    return { hello: 'world' }
  })
}

export default plugin

Create a plugin

Creating a plugin is very easy, you just need to create a function that takes three parameters, the fastify instance, an options object, and the done callback.

Example:

module.exports = function (fastify, opts, done) {
  fastify.decorate('utility', function () {})

  fastify.get('/', handler)

  done()
}

You can also use register inside another register:

module.exports = function (fastify, opts, done) {
  fastify.decorate('utility', function () {})

  fastify.get('/', handler)

  fastify.register(require('./other-plugin'))

  done()
}

Sometimes, you will need to know when the server is about to close, for example, because you must close a connection to a database. To know when this is going to happen, you can use the 'onClose' hook.

Do not forget that register will always create a new Fastify scope, if you do not need that, read the following section.

Handle the scope

If you are using register only for extending the functionality of the server with decorate, it is your responsibility to tell Fastify not to create a new scope. Otherwise, your changes will not be accessible by the user in the upper scope.

You have two ways to tell Fastify to avoid the creation of a new context:

Use the fastify-plugin module
Use the 'skip-override' hidden property

We recommend using the fastify-plugin module, because it solves this problem for you, and you can pass a version range of Fastify as a parameter that your plugin will support.

const fp = require('fastify-plugin')

module.exports = fp(function (fastify, opts, done) {
  fastify.decorate('utility', function () {})
  done()
}, '0.x')

Check the fastify-plugin documentation to learn more about how to use this module.

If you do not use the fastify-plugin module, you can use the 'skip-override' hidden property, but we do not recommend it. If in the future the Fastify API changes it will be your responsibility to update the module, while if you use fastify-plugin, you can be sure about backward compatibility.

function yourPlugin (fastify, opts, done) {
  fastify.decorate('utility', function () {})
  done()
}
yourPlugin[Symbol.for('skip-override')] = true
module.exports = yourPlugin

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