fastify-plugin is a plugin helper for Fastify.

When you build plugins for Fastify and you want them to be accessible in the same context where you require them, you have two ways:

1. Use the skip-override hidden property
2. Use this module

Note: the v4.x series of this module covers Fastify v4 Note: the v2.x & v3.x series of this module covers Fastify v3. For Fastify v2 support, refer to the v1.x series.

npm i fastify-plugin

fastify-plugin can do three things for you:

• Add the skip-override hidden property
• Check the bare-minimum version of Fastify
• Pass some custom metadata of the plugin to Fastify

Example using a callback:

const fp = require('fastify-plugin')

module.exports = fp(function (fastify, opts, done) {
  // your plugin code
  done()
})

Example using an async function:

const fp = require('fastify-plugin')

// A callback function param is not required for async functions
module.exports = fp(async function (fastify, opts) {
  // Wait for an async function to fulfill promise before proceeding
  await exampleAsyncFunction()
})

In addition, if you use this module when creating new plugins, you can declare the dependencies, the name, and the expected Fastify version that your plugin needs.

If you need to set a bare-minimum version of Fastify for your plugin, just add the semver range that you need:

const fp = require('fastify-plugin')

module.exports = fp(function (fastify, opts, done) {
  // your plugin code
  done()
}, { fastify: '5.x' })

If you need to check the Fastify version only, you can pass just the version string.

You can check here how to define a semver range.

Fastify uses this option to validate the dependency graph, allowing it to ensure that no name collisions occur and making it possible to perform dependency checks.

const fp = require('fastify-plugin')

function plugin (fastify, opts, done) {
  // your plugin code
  done()
}

module.exports = fp(plugin, {
  fastify: '5.x',
  name: 'your-plugin-name'
})

You can also check if the plugins and decorators that your plugin intend to use are present in the dependency graph.

Note: This is the point where registering name of the plugins become important, because you can reference plugin dependencies by their name.

const fp = require('fastify-plugin')

function plugin (fastify, opts, done) {
  // your plugin code
  done()
}

module.exports = fp(plugin, {
  fastify: '5.x',
  decorators: {
    fastify: ['plugin1', 'plugin2'],
    reply: ['compress']
  },
  dependencies: ['plugin1-name', 'plugin2-name']
})

By default, fastify-plugin breaks the encapsulation but you can optionally keep the plugin encapsulated. This allows you to set the plugin's name and validate its dependencies without making the plugin accessible.

const fp = require('fastify-plugin')

function plugin (fastify, opts, done) {
  // the decorator is not accessible outside this plugin
  fastify.decorate('util', function() {})
  done()
}

module.exports = fp(plugin, {
  name: 'my-encapsulated-plugin',
  fastify: '5.x',
  decorators: {
    fastify: ['plugin1', 'plugin2'],
    reply: ['compress']
  },
  dependencies: ['plugin1-name', 'plugin2-name'],
  encapsulate: true
})

fastify-plugin adds a .default and [name] property to the passed in function. The type definition would have to be updated to leverage this.

Documentation Reference

It is common for developers to inline their plugin with fastify-plugin such as:

fp((fastify, opts, done) => { done() })
fp(async (fastify, opts) => { return })

TypeScript can sometimes infer the types of the arguments for these functions. Plugins in Fastify are recommended to be typed using either FastifyPluginCallback or FastifyPluginAsync. These two definitions only differ in two ways:

1. The third argument done (the callback part)
2. The return type FastifyPluginCallback or FastifyPluginAsync

At this time, TypeScript inference is not smart enough to differentiate by definition argument length alone.

Thus, if you are a TypeScript developer please use on the following patterns instead:

// Callback

// Assign type directly
const pluginCallback: FastifyPluginCallback = (fastify, options, done) => { }
fp(pluginCallback)

// or define your own function declaration that satisfies the existing definitions
const pluginCallbackWithTypes = (fastify: FastifyInstance, options: FastifyPluginOptions, done: (error?: FastifyError) => void): void => { }
fp(pluginCallbackWithTypes)
// or inline
fp((fastify: FastifyInstance, options: FastifyPluginOptions, done: (error?: FastifyError) => void): void => { })

// Async

// Assign type directly
const pluginAsync: FastifyPluginAsync = async (fastify, options) => { }
fp(pluginAsync)

// or define your own function declaration that satisfies the existing definitions
const pluginAsyncWithTypes = async (fastify: FastifyInstance, options: FastifyPluginOptions): Promise<void> => { }
fp(pluginAsyncWithTypes)
// or inline
fp(async (fastify: FastifyInstance, options: FastifyPluginOptions): Promise<void> => { })

This project is kindly sponsored by:

• nearForm
• LetzDoIt

Licensed under MIT.