fastify/docs/Guides/Plugins-Guide.md

Fast and low overhead web framework, for Node.js

<h1 align="center">Fastify</h1>

# The hitchhiker's guide to plugins
First of all, `DON'T PANIC`!

Fastify was built from the beginning to be an extremely modular system. We built
a powerful API that allows you to add methods and utilities to Fastify by
creating a namespace. We built a system that creates an encapsulation model,
which allows you to split your application into multiple microservices at any
moment, without the need to refactor the entire application.

**Table of contents**
- [The hitchhiker's guide to plugins](#the-hitchhikers-guide-to-plugins)
  - [Register](#register)
  - [Decorators](#decorators)
  - [Hooks](#hooks)
  - [How to handle encapsulation and
    distribution](#how-to-handle-encapsulation-and-distribution)
  - [ESM support](#esm-support)
  - [Handle errors](#handle-errors)
  - [Custom errors](#custom-errors)
  - [Emit Warnings](#emit-warnings)
  - [Let's start!](#lets-start)

## Register
<a id="register"></a>

As with JavaScript, where everything is an object, in Fastify everything is a
plugin.

Your routes, your utilities, and so on are all plugins. To add a new plugin,
whatever its functionality may be, in Fastify you have a nice and unique API:
[`register`](../Reference/Plugins.md).
```js
fastify.register(
  require('./my-plugin'),
  { options }
)
```
`register` creates a new Fastify context, which means that if you perform any
changes on the Fastify instance, those changes will not be reflected in the
context's ancestors. In other words, encapsulation!

*Why is encapsulation important?*

Well, let's say you are creating a new disruptive startup, what do you do? You
create an API server with all your stuff, everything in the same place, a
monolith!

Ok, you are growing very fast and you want to change your architecture and try
microservices. Usually, this implies a huge amount of work, because of cross
dependencies and a lack of separation of concerns in the codebase.

Fastify helps you in that regard. Thanks to the encapsulation model, it will
completely avoid cross dependencies and will help you structure your code into
cohesive blocks.

*Let's return to how to correctly use `register`.*

As you probably know, the required plugins must expose a single function with
the following signature
```js
module.exports = function (fastify, options, done) {}
```
Where `fastify` is the encapsulated Fastify instance, `options` is the options
object, and `done` is the function you **must** call when your plugin is ready.

Fastify's plugin model is fully reentrant and graph-based, it handles
asynchronous code without any problems and it enforces both the load and close
order of plugins. *How?* Glad you asked, check out
[`avvio`](https://github.com/mcollina/avvio)! Fastify starts loading the plugin
__after__ `.listen()`, `.inject()` or `.ready()` are called.

Inside a plugin you can do whatever you want, register routes, utilities (we
will see this in a moment) and do nested registers, just remember to call `done`
when everything is set up!
```js
module.exports = function (fastify, options, done) {
  fastify.get('/plugin', (request, reply) => {
    reply.send({ hello: 'world' })
  })

  done()
}
```

Well, now you know how to use the `register` API and how it works, but how do we
add new functionality to Fastify and even better, share them with other
developers?

## Decorators
<a id="decorators"></a>

Okay, let's say that you wrote a utility that is so good that you decided to
make it available along with all your code. How would you do it? Probably
something like the following:
```js
// your-awesome-utility.js
module.exports = function (a, b) {
  return a + b
}
```
```js
const util = require('./your-awesome-utility')
console.log(util('that is ', 'awesome'))
```
Now you will import your utility in every file you need it in. (And do not
forget that you will probably also need it in your tests).

Fastify offers you a more elegant and comfortable way to do this, *decorators*.
Creating a decorator is extremely easy, just use the
[`decorate`](../Reference/Decorators.md) API:
```js
fastify.decorate('util', (a, b) => a + b)
```
Now you can access your utility just by calling `fastify.util` whenever you need
it - even inside your test.

And here starts the magic; do you remember how just now we were talking about
encapsulation? Well, using `register` and `decorate` in conjunction enable
exactly that, let me show you an example to clarify this:
```js
fastify.register((instance, opts, done) => {
  instance.decorate('util', (a, b) => a + b)
  console.log(instance.util('that is ', 'awesome'))

  done()
})

fastify.register((instance, opts, done) => {
  console.log(instance.util('that is ', 'awesome')) // This will throw an error

  done()
})
```
Inside the second register call `instance.util` will throw an error because
`util` exists only inside the first register context.

Let's step back for a moment and dig deeper into this: every time you use the
`register` API, a new context is created which avoids the negative situations
mentioned above.

Do note that encapsulation applies to the ancestors and siblings, but not the
children.
```js
fastify.register((instance, opts, done) => {
  instance.decorate('util', (a, b) => a + b)
  console.log(instance.util('that is ', 'awesome'))

  fastify.register((instance, opts, done) => {
    console.log(instance.util('that is ', 'awesome')) // This will not throw an error
    done()
  })

  done()
})

fastify.register((instance, opts, done) => {
  console.log(instance.util('that is ', 'awesome')) // This will throw an error

  done()
})
```
*Take home message: if you need a utility that is available in every part of
your application, take care that it is declared in the root scope of your
application. If that is not an option,  you can use the `fastify-plugin` utility
as described [here](#distribution).*

`decorate` is not the only API that you can use to extend the server
functionality, you can also use `decorateRequest` and `decorateReply`.

*`decorateRequest` and `decorateReply`? Why do we need them if we already have
`decorate`?*

Good question, we added them to make Fastify more developer-friendly. Let's see
an example:
```js
fastify.decorate('html', payload => {
  return generateHtml(payload)
})

fastify.get('/html', (request, reply) => {
  reply
    .type('text/html')
    .send(fastify.html({ hello: 'world' }))
})
```
It works, but it could be much better!
```js
fastify.decorateReply('html', function (payload) {
  this.type('text/html') // This is the 'Reply' object
  this.send(generateHtml(payload))
})

fastify.get('/html', (request, reply) => {
  reply.html({ hello: 'world' })
})
```
Reminder that the `this` keyword is not available on *arrow functions*,
so when passing functions in *`decorateReply`* and *`decorateRequest`* as
a utility that also needs access to the `request` and `reply` instance,
a function that is defined using the `function` keyword is needed instead
of an *arrow function expression*.

In the same way you can do this for the `request` object:
```js
fastify.decorate('getHeader', (req, header) => {
  return req.headers[header]
})

fastify.addHook('preHandler', (request, reply, done) => {
  request.isHappy = fastify.getHeader(request.raw, 'happy')
  done()
})

fastify.get('/happiness', (request, reply) => {
  reply.send({ happy: request.isHappy })
})
```
Again, it works, but it can be much better!
```js
fastify.decorateRequest('setHeader', function (header) {
  this.isHappy = this.headers[header]
})

fastify.decorateRequest('isHappy', false) // This will be added to the Request object prototype, yay speed!

fastify.addHook('preHandler', (request, reply, done) => {
  request.setHeader('happy')
  done()
})

fastify.get('/happiness', (request, reply) => {
  reply.send({ happy: request.isHappy })
})
```

We have seen how to extend server functionality and how to handle the
encapsulation system, but what if you need to add a function that must be
executed whenever the server "[emits](../Reference/Lifecycle.md)" an
event?

## Hooks
<a id="hooks"></a>

You just built an amazing utility, but now you need to execute that for every
request, this is what you will likely do:
```js
fastify.decorate('util', (request, key, value) => { request[key] = value })

fastify.get('/plugin1', (request, reply) => {
  fastify.util(request, 'timestamp', new Date())
  reply.send(request)
})

fastify.get('/plugin2', (request, reply) => {
  fastify.util(request, 'timestamp', new Date())
  reply.send(request)
})
```
I think we all agree that this is terrible. Repeated code, awful readability and
it cannot scale.

So what can you do to avoid this annoying issue? Yes, you are right, use a
[hook](../Reference/Hooks.md)!

```js
fastify.decorate('util', (request, key, value) => { request[key] = value })

fastify.addHook('preHandler', (request, reply, done) => {
  fastify.util(request, 'timestamp', new Date())
  done()
})

fastify.get('/plugin1', (request, reply) => {
  reply.send(request)
})

fastify.get('/plugin2', (request, reply) => {
  reply.send(request)
})
```
Now for every request, you will run your utility. You can register as many hooks
as you need.

Sometimes you want a hook that should be executed for just a subset of routes,
how can you do that? Yep, encapsulation!

```js
fastify.register((instance, opts, done) => {
  instance.decorate('util', (request, key, value) => { request[key] = value })

  instance.addHook('preHandler', (request, reply, done) => {
    instance.util(request, 'timestamp', new Date())
    done()
  })

  instance.get('/plugin1', (request, reply) => {
    reply.send(request)
  })

  done()
})

fastify.get('/plugin2', (request, reply) => {
  reply.send(request)
})
```
Now your hook will run just for the first route!

An alternative approach is to make use of the [onRoute hook](../Reference/Hooks.md#onroute)
to customize application routes dynamically from inside the plugin. Every time
a new route is registered, you can read and modify the route options. For example,
based on a [route config option](../Reference/Routes.md#routes-options):

```js
fastify.register((instance, opts, done) => {
  instance.decorate('util', (request, key, value) => { request[key] = value })
  
  function handler(request, reply, done) {
    instance.util(request, 'timestamp', new Date())
    done()
  }

  instance.addHook('onRoute', (routeOptions) => {
    if (routeOptions.config && routeOptions.config.useUtil === true) {
      // set or add our handler to the route preHandler hook
      if (!routeOptions.preHandler) {
        routeOptions.preHandler = [handler]
        return
      }
      if (Array.isArray(routeOptions.preHandler)) {
        routeOptions.preHandler.push(handler)
        return
      }
      routeOptions.preHandler = [routeOptions.preHandler, handler]
    }
  })

  fastify.get('/plugin1', {config: {useUtil: true}}, (request, reply) => {
    reply.send(request)
  })

  fastify.get('/plugin2', (request, reply) => {
    reply.send(request)
  })

  done()
})
```

This variant becomes extremely useful if you plan to distribute your plugin, as
described in the next section.

As you probably noticed by now, `request` and `reply` are not the standard
Node.js *request* and *response* objects, but Fastify's objects.


## How to handle encapsulation and distribution
<a id="distribution"></a>

Perfect, now you know (almost) all of the tools that you can use to extend
Fastify. Nevertheless, chances are that you came across one big issue: how is
distribution handled?

The preferred way to distribute a utility is to wrap all your code inside a
`register`. Using this, your plugin can support asynchronous bootstrapping
*(since `decorate` is a synchronous API)*, in the case of a database connection
for example.

*Wait, what? Didn't you tell me that `register` creates an encapsulation and
that the stuff I create inside will not be available outside?*

Yes, I said that. However, what I didn't tell you is that you can tell Fastify
to avoid this behavior with the
[`fastify-plugin`](https://github.com/fastify/fastify-plugin) module.
```js
const fp = require('fastify-plugin')
const dbClient = require('db-client')

function dbPlugin (fastify, opts, done) {
  dbClient.connect(opts.url, (err, conn) => {
    fastify.decorate('db', conn)
    done()
  })
}

module.exports = fp(dbPlugin)
```
You can also tell `fastify-plugin` to check the installed version of Fastify, in
case you need a specific API.

As we mentioned earlier, Fastify starts loading its plugins __after__
`.listen()`, `.inject()` or `.ready()` are called and as such, __after__ they
have been declared. This means that, even though the plugin may inject variables
to the external Fastify instance via [`decorate`](../Reference/Decorators.md),
the decorated variables will not be accessible before calling `.listen()`,
`.inject()` or `.ready()`.

In case you rely on a variable injected by a preceding plugin and want to pass
that in the `options` argument of `register`, you can do so by using a function
instead of an object:
```js
const fastify = require('fastify')()
const fp = require('fastify-plugin')
const dbClient = require('db-client')

function dbPlugin (fastify, opts, done) {
  dbClient.connect(opts.url, (err, conn) => {
    fastify.decorate('db', conn)
    done()
  })
}

fastify.register(fp(dbPlugin), { url: 'https://example.com' })
fastify.register(require('your-plugin'), parent => {
  return { connection: parent.db, otherOption: 'foo-bar' }
})
```
In the above example, the `parent` variable of the function passed in as the
second argument of `register` is a copy of the **external Fastify instance**
that the plugin was registered at. This means that we can access any
variables that were injected by preceding plugins in the order of declaration.

## ESM support
<a id="esm-support"></a>

ESM is supported as well from [Node.js
`v13.3.0`](https://nodejs.org/api/esm.html) and above! Just export your plugin
as an ESM module and you are good to go!

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

export default plugin
```

## Handle errors
<a id="handle-errors"></a>

One of your plugins may fail during startup. Maybe you expect it
and you have a custom logic that will be triggered in that case. How can you
implement this? The `after` API is what you need. `after` simply registers a
callback that will be executed just after a register, and it can take up to
three parameters.

The callback changes based on the parameters you are giving:

1. If no parameter is given to the callback and there is an error, that error
   will be passed to the next error handler.
1. If one parameter is given to the callback, that parameter will be the error
   object.
1. If two parameters are given to the callback, the first will be the error
   object; the second will be the done callback.
1. If three parameters are given to the callback, the first will be the error
   object, the second will be the top-level context unless you have specified
   both server and override, in that case, the context will be what the override
   returns, and the third the done callback.

Let's see how to use it:
```js
fastify
  .register(require('./database-connector'))
  .after(err => {
    if (err) throw err
  })
```

## Custom errors
<a id="custom-errors"></a>

If your plugin needs to expose custom errors, you can easily generate consistent
error objects across your codebase and plugins with the
[`@fastify/error`](https://github.com/fastify/fastify-error) module.

```js
const createError = require('@fastify/error')
const CustomError = createError('ERROR_CODE', 'message')
console.log(new CustomError())
```

## Emit Warnings
<a id="emit-warnings"></a>

If you want to deprecate an API, or you want to warn the user about a specific
use case, you can use the
[`process-warning`](https://github.com/fastify/process-warning) module.

```js
const warning = require('process-warning')()
warning.create('MyPluginWarning', 'MP_ERROR_CODE', 'message')
warning.emit('MP_ERROR_CODE')
```

## Let's start!
<a id="start"></a>

Awesome, now you know everything you need to know about Fastify and its plugin
system to start building your first plugin, and please if you do, tell us! We
will add it to the [*ecosystem*](https://github.com/fastify/fastify#ecosystem)
section of our documentation!

If you want to see some real-world examples, check out:
- [`@fastify/view`](https://github.com/fastify/point-of-view) Templates
  rendering (*ejs, pug, handlebars, marko*) plugin support for Fastify.
- [`@fastify/mongodb`](https://github.com/fastify/fastify-mongodb) Fastify
  MongoDB connection plugin, with this you can share the same MongoDB connection
  pool in every part of your server.
- [`@fastify/multipart`](https://github.com/fastify/fastify-multipart) Multipart
  support for Fastify
- [`@fastify/helmet`](https://github.com/fastify/fastify-helmet) Important
  security headers for Fastify


*Do you feel like something is missing here? Let us know! :)*