Documentation

Getting Started

Hello! Thank you for checking out Fastify!
This document aims to be a gentle introduction to the framework and its features. It is an elementary preface with examples and links to other parts of the documentation.
Let's start!

Install

Install with npm:

npm i fastify --save

Install with yarn:

yarn add fastify

Your first server

Let's write our first server:

// Require the framework and instantiate it
const fastify = require('fastify')({
  logger: true
})

// Declare a route
fastify.get('/', function (request, reply) {
  reply.send({ hello: 'world' })
})

// Run the server!
fastify.listen(3000, function (err, address) {
  if (err) {
    fastify.log.error(err)
    process.exit(1)
  }
  fastify.log.info(`server listening on ${address}`)
})

Do you prefer to use async/await? Fastify supports it out-of-the-box.
(We also suggest using make-promises-safe to avoid file descriptor and memory leaks.)

const fastify = require('fastify')()

fastify.get('/', async (request, reply) => {
  return { hello: 'world' }
})

const start = async () => {
  try {
    await fastify.listen(3000)
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

Awesome, that was easy.
Unfortunately, writing a complex application requires significantly more code than this example. A classic problem when you are building a new application is how to handle multiple files, asynchronous bootstrapping and the architecture of your code.
Fastify offers an easy platform that helps to solve all of the problems outlined above, and more!

Note

The above examples, and subsequent examples in this document, default to listening only on the localhost 127.0.0.1 interface. To listen on all available IPv4 interfaces the example should be modified to listen on 0.0.0.0 like so:

fastify.listen(3000, '0.0.0.0', function (err, address) {
  if (err) {
    fastify.log.error(err)
    process.exit(1)
  }
  fastify.log.info(`server listening on ${address}`)
})

Similarly, specify ::1 to accept only local connections via IPv6. Or specify :: to accept connections on all IPv6 addresses, and, if the operating system supports it, also on all IPv4 addresses.

When deploying to a Docker (or other type of) container using 0.0.0.0 or :: would be the easiest method for exposing the application.

Your first plugin

As with JavaScript, where everything is an object, with Fastify everything is a plugin.
Before digging into it, let's see how it works!
Let's declare our basic server, but instead of declaring the route inside the entry point, we'll declare it in an external file (check out the route declaration docs).

const fastify = require('fastify')({
  logger: true
})

fastify.register(require('./our-first-route'))

fastify.listen(3000, function (err, address) {
  if (err) {
    fastify.log.error(err)
    process.exit(1)
  }
  fastify.log.info(`server listening on ${address}`)
})
// our-first-route.js

async function routes (fastify, options) {
  fastify.get('/', async (request, reply) => {
    return { hello: 'world' }
  })
}

module.exports = routes

In this example, we used the register API, which is the core of the Fastify framework. It is the only way to add routes, plugins, et cetera.

At the beginning of this guide, we noted that Fastify provides a foundation that assists with asynchronous bootstrapping of your application. Why is this important? Consider the scenario where a database connection is needed to handle data storage. Obviously, the database connection needs to be available before the server is accepting connections. How do we address this problem?
A typical solution is to use a complex callback, or promises - a system that will mix the framework API with other libraries and the application code.
Fastify handles this internally, with minimum effort!

Let's rewrite the above example with a database connection.
(we will use a simple example, for a robust solution consider using fastify-mongo or another in the Fastify ecosystem)

First, install fastify-plugin:

npm install --save fastify-plugin

server.js

const fastify = require('fastify')({
  logger: true
})

fastify.register(require('./our-db-connector'), {
  url: 'mongodb://localhost:27017/'
})
fastify.register(require('./our-first-route'))

fastify.listen(3000, function (err, address) {
  if (err) {
    fastify.log.error(err)
    process.exit(1)
  }
  fastify.log.info(`server listening on ${address}`)
})

our-db-connector.js

const fastifyPlugin = require('fastify-plugin')
const MongoClient = require('mongodb').MongoClient

async function dbConnector (fastify, options) {
  const url = options.url
  delete options.url

  const db = await MongoClient.connect(url, options)
  fastify.decorate('mongo', db)
}

// Wrapping a plugin function with fastify-plugin exposes the decorators,
// hooks, and middlewares declared inside the plugin to the parent scope.
module.exports = fastifyPlugin(dbConnector)

our-first-route.js

async function routes (fastify, options) {
  const database = fastify.mongo.db('db')
  const collection = database.collection('test')

  fastify.get('/', async (request, reply) => {
    return { hello: 'world' }
  })

  fastify.get('/search/:id', async (request, reply) => {
    const result = await collection.findOne({ id: request.params.id })
    if (result.value === null) {
      throw new Error('Invalid value')
    }
    return result.value
  })
}

module.exports = routes

Wow, that was fast!
Let's recap what we have done here since we've introduced some new concepts.
As you can see, we used register both for the database connector and the registration of the routes. This is one of the best features of Fastify, it will load your plugins in the same order you declare them, and it will load the next plugin only once the current one has been loaded. In this way, we can register the database connector in the first plugin and use it in the second (read here to understand how to handle the scope of a plugin). Plugin loading starts when you call fastify.listen(), fastify.inject() or fastify.ready()

We have also used the decorate API to add custom objects to the Fastify namespace, making them available for use everywhere. Use of this API is encouraged to faciliate easy code reuse and to decrease code or logic duplication.

To dig deeper into how Fastify plugins work, how to develop new plugins, and for details on how to use the whole Fastify API to deal with the complexity of asynchronously bootstrapping an application, read the hitchhiker's guide to plugins.

Loading order of your plugins

To guarantee a consistent and predictable behaviour of your application, we highly recommend to always load your code as shown below:

└── plugins (from the Fastify ecosystem)
└── your plugins (your custom plugins)
└── decorators
└── hooks and middlewares
└── your services

In this way, you will always have access to all of the properties declared in the current scope.
As discussed previously, Fastify offers a solid encapsulation model, to help you build your application as single and independent services. If you want to register a plugin only for a subset of routes, you just have to replicate the above structure.

└── plugins (from the Fastify ecosystem)
└── your plugins (your custom plugins)
└── decorators
└── hooks and middlewares
└── your services
    │
    └──  service A
    │     └── plugins (from the Fastify ecosystem)
    │     └── your plugins (your custom plugins)
    │     └── decorators
    │     └── hooks and middlewares
    │     └── your services
    │
    └──  service B
          └── plugins (from the Fastify ecosystem)
          └── your plugins (your custom plugins)
          └── decorators
          └── hooks and middlewares
          └── your services

Validate your data

Data validation is extremely important and a core concept of the framework.
To validate incoming requests, Fastify uses JSON Schema. Let's look at an example demonstrating validation for routes:

const opts = {
  schema: {
    body: {
      type: 'object',
      properties: {
        someKey: { type: 'string' },
        someOtherKey: { type: 'number' }
      }
    }
  }
}

fastify.post('/', opts, async (request, reply) => {
  return { hello: 'world' }
})

This example shows how to pass an options object to the route, which accepts a schema key, that contains all of the schemas for route, body, querystring, params and headers.
Read Validation and Serialization to learn more.

Serialize your data

Fastify has first class support for JSON. It is extremely optimized to parse JSON bodies and to serialize JSON output.
To speed up JSON serialization (yes, it is slow!) use the response key of the schema option as shown in the following example:

const opts = {
  schema: {
    response: {
      200: {
        type: 'object',
        properties: {
          hello: { type: 'string' }
        }
      }
    }
  }
}

fastify.get('/', opts, async (request, reply) => {
  return { hello: 'world' }
})

Simply by specifying a schema as shown, you can speed up serialization by a factor of 2-3. This also helps to protect against leakage of potentially sensitive data, since Fastify will serialize only the data present in the response schema. Read Validation and Serialization to learn more.

Extend your server

Fastify is built to be extremely extensible and minimal, we believe that a bare bones framework is all that is necessary to make great applications possible.
In other words, Fastify is not a "batteries included" framework, and relies on an amazing ecosystem!

Test your server

Fastify does not offer a testing framework, but we do recommend a way to write your tests that uses the features and architecture of Fastify.
Read the testing documentation to learn more!

Run your server from CLI

Fastify also has CLI integration thanks to fastify-cli.

First, install fastify-cli:

npm i fastify-cli

You can also install it globally with -g.

Then, add the following lines to package.json:

{
  "scripts": {
    "start": "fastify start server.js"
  }
}

And create your server file(s):

// server.js
'use strict'

module.exports = async function (fastify, opts) {
  fastify.get('/', async (request, reply) => {
    return { hello: 'world' }
  })
}

Then run your server with:

npm start

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