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

## Hooks

Hooks are registered with the `fastify.addHook` method and allow you to listen
to specific events in the application or request/response lifecycle. You have to
register a hook before the event is triggered, otherwise, the event is lost.

By using hooks you can interact directly with the lifecycle of Fastify. There
are Request/Reply hooks and application hooks:

- [Request/Reply Hooks](#requestreply-hooks)
  - [onRequest](#onrequest)
  - [preParsing](#preparsing)
  - [preValidation](#prevalidation)
  - [preHandler](#prehandler)
  - [preSerialization](#preserialization)
  - [onError](#onerror)
  - [onSend](#onsend)
  - [onResponse](#onresponse)
  - [onTimeout](#ontimeout)
  - [Manage Errors from a hook](#manage-errors-from-a-hook)
  - [Respond to a request from a hook](#respond-to-a-request-from-a-hook)
- [Application Hooks](#application-hooks)
  - [onReady](#onready)
  - [onClose](#onclose)
  - [onRoute](#onroute)
  - [onRegister](#onregister)
- [Scope](#scope)
- [Route level hooks](#route-level-hooks)
- [Diagnostics Channel Hooks](#diagnostics-channel-hooks)

**Notice:** the `done` callback is not available when using `async`/`await` or
returning a `Promise`. If you do invoke a `done` callback in this situation
unexpected behavior may occur, e.g. duplicate invocation of handlers.

## Request/Reply Hooks

[Request](./Request.md) and [Reply](./Reply.md) are the core Fastify objects.

`done` is the function to continue with the [lifecycle](./Lifecycle.md).

It is easy to understand where each hook is executed by looking at the
[lifecycle page](./Lifecycle.md).

Hooks are affected by Fastify's encapsulation, and can thus be applied to
selected routes. See the [Scopes](#scope) section for more information.

There are eight different hooks that you can use in Request/Reply *(in order of
execution)*:

### onRequest
```js
fastify.addHook('onRequest', (request, reply, done) => {
  // Some code
  done()
})
```
Or `async/await`:
```js
fastify.addHook('onRequest', async (request, reply) => {
  // Some code
  await asyncMethod()
})
```

**Notice:** in the [onRequest](#onrequest) hook, `request.body` will always be
`null`, because the body parsing happens before the
[preValidation](#prevalidation) hook.

### preParsing

If you are using the `preParsing` hook, you can transform the request payload
stream before it is parsed. It receives the request and reply objects as other
hooks, and a stream with the current request payload.

If it returns a value (via `return` or via the callback function), it must
return a stream.

For instance, you can uncompress the request body:

```js
fastify.addHook('preParsing', (request, reply, payload, done) => {
  // Some code
  done(null, newPayload)
})
```
Or `async/await`:
```js
fastify.addHook('preParsing', async (request, reply, payload) => {
  // Some code
  await asyncMethod()
  return newPayload
})
```

**Notice:** in the [preParsing](#preparsing) hook, `request.body` will always be
`null`, because the body parsing happens before the
[preValidation](#prevalidation) hook.

**Notice:** you should also add a `receivedEncodedLength` property to the
returned stream. This property is used to correctly match the request payload
with the `Content-Length` header value. Ideally, this property should be updated
on each received chunk.

**Notice**: The old syntaxes `function(request, reply, done)` and `async
function(request, reply)` for the parser are still supported but they are
deprecated.

### preValidation

If you are using the `preValidation` hook, you can change the payload before it
is validated. For example:

```js
fastify.addHook('preValidation', (request, reply, done) => {
  request.body = { ...request.body, importantKey: 'randomString' }
  done()
})
```
Or `async/await`:
```js
fastify.addHook('preValidation', async (request, reply) => {
  const importantKey = await generateRandomString()
  request.body = { ...request.body, importantKey }
})
```

### preHandler
```js
fastify.addHook('preHandler', (request, reply, done) => {
  // some code
  done()
})
```
Or `async/await`:
```js
fastify.addHook('preHandler', async (request, reply) => {
  // Some code
  await asyncMethod()
})
```
### preSerialization

If you are using the `preSerialization` hook, you can change (or replace) the
payload before it is serialized. For example:

```js
fastify.addHook('preSerialization', (request, reply, payload, done) => {
  const err = null
  const newPayload = { wrapped: payload }
  done(err, newPayload)
})
```
Or `async/await`:
```js
fastify.addHook('preSerialization', async (request, reply, payload) => {
  return { wrapped: payload }
})
```

Note: the hook is NOT called if the payload is a `string`, a `Buffer`, a
`stream`, or `null`.

### onError
```js
fastify.addHook('onError', (request, reply, error, done) => {
  // Some code
  done()
})
```
Or `async/await`:
```js
fastify.addHook('onError', async (request, reply, error) => {
  // Useful for custom error logging
  // You should not use this hook to update the error
})
```
This hook is useful if you need to do some custom error logging or add some
specific header in case of error.

It is not intended for changing the error, and calling `reply.send` will throw
an exception.

This hook will be executed only after the `customErrorHandler` has been
executed, and only if the `customErrorHandler` sends an error back to the user
*(Note that the default `customErrorHandler` always sends the error back to the
user)*.

**Notice:** unlike the other hooks, pass an error to the `done` function is not
supported.

### onSend
If you are using the `onSend` hook, you can change the payload. For example:

```js
fastify.addHook('onSend', (request, reply, payload, done) => {
  const err = null;
  const newPayload = payload.replace('some-text', 'some-new-text')
  done(err, newPayload)
})
```
Or `async/await`:
```js
fastify.addHook('onSend', async (request, reply, payload) => {
  const newPayload = payload.replace('some-text', 'some-new-text')
  return newPayload
})
```

You can also clear the payload to send a response with an empty body by
replacing the payload with `null`:

```js
fastify.addHook('onSend', (request, reply, payload, done) => {
  reply.code(304)
  const newPayload = null
  done(null, newPayload)
})
```

> You can also send an empty body by replacing the payload with the empty string
> `''`, but be aware that this will cause the `Content-Length` header to be set
> to `0`, whereas the `Content-Length` header will not be set if the payload is
> `null`.

Note: If you change the payload, you may only change it to a `string`, a
`Buffer`, a `stream`, or `null`.


### onResponse
```js
fastify.addHook('onResponse', (request, reply, done) => {
  // Some code
  done()
})
```
Or `async/await`:
```js
fastify.addHook('onResponse', async (request, reply) => {
  // Some code
  await asyncMethod()
})
```

The `onResponse` hook is executed when a response has been sent, so you will not
be able to send more data to the client. It can however be useful for sending
data to external services, for example, to gather statistics.

### onTimeout

```js
fastify.addHook('onTimeout', (request, reply, done) => {
  // Some code
  done()
})
```
Or `async/await`:
```js
fastify.addHook('onTimeout', async (request, reply) => {
  // Some code
  await asyncMethod()
})
```
`onTimeout` is useful if you need to monitor the request timed out in your
service (if the `connectionTimeout` property is set on the Fastify instance).
The `onTimeout` hook is executed when a request is timed out and the HTTP socket
has been hanged up. Therefore, you will not be able to send data to the client.


### Manage Errors from a hook
If you get an error during the execution of your hook, just pass it to `done()`
and Fastify will automatically close the request and send the appropriate error
code to the user.

```js
fastify.addHook('onRequest', (request, reply, done) => {
  done(new Error('Some error'))
})
```

If you want to pass a custom error code to the user, just use `reply.code()`:
```js
fastify.addHook('preHandler', (request, reply, done) => {
  reply.code(400)
  done(new Error('Some error'))
})
```
*The error will be handled by [`Reply`](./Reply.md#errors).*

Or if you're using `async/await` you can just throw an error:
```js
fastify.addHook('onResponse', async (request, reply) => {
  throw new Error('Some error')
})
```

### Respond to a request from a hook

If needed, you can respond to a request before you reach the route handler, for
example when implementing an authentication hook. Replying from a hook implies
that the hook chain is __stopped__ and the rest of the hooks and handlers are
not executed. If the hook is using the callback approach, i.e. it is not an
`async` function or it returns a `Promise`, it is as simple as calling
`reply.send()` and avoiding calling the callback. If the hook is `async`,
`reply.send()` __must__ be called _before_ the function returns or the promise
resolves, otherwise, the request will proceed. When `reply.send()` is called
outside of the promise chain, it is important to `return reply` otherwise the
request will be executed twice.

It is important to __not mix callbacks and `async`/`Promise`__, otherwise the
hook chain will be executed twice.

If you are using `onRequest` or `preHandler` use `reply.send`.

```js
fastify.addHook('onRequest', (request, reply, done) => {
  reply.send('Early response')
})

// Works with async functions too
fastify.addHook('preHandler', async (request, reply) => {
  await something()
  reply.send({ hello: 'world' })
  return reply // optional in this case, but it is a good practice
})
```

If you want to respond with a stream, you should avoid using an `async` function
for the hook. If you must use an `async` function, your code will need to follow
the pattern in
[test/hooks-async.js](https://github.com/fastify/fastify/blob/94ea67ef2d8dce8a955d510cd9081aabd036fa85/test/hooks-async.js#L269-L275).

```js
fastify.addHook('onRequest', (request, reply, done) => {
  const stream = fs.createReadStream('some-file', 'utf8')
  reply.send(stream)
})
```

If you are sending a response without `await` on it, make sure to always `return
reply`:

```js
fastify.addHook('preHandler', async (request, reply) => {
  setImmediate(() => { reply.send('hello') })

  // This is needed to signal the handler to wait for a response
  // to be sent outside of the promise chain
  return reply
})

fastify.addHook('preHandler', async (request, reply) => {
  // the @fastify/static plugin will send a file asynchronously,
  // so we should return reply
  reply.sendFile('myfile')
  return reply
})
```

## Application Hooks

You can hook into the application-lifecycle as well.

- [onReady](#onready)
- [onClose](#onclose)
- [onRoute](#onroute)
- [onRegister](#onregister)

### onReady
Triggered before the server starts listening for requests and when `.ready()` is
invoked. It cannot change the routes or add new hooks. Registered hook functions
are executed serially. Only after all `onReady` hook functions have completed
will the server start listening for requests. Hook functions accept one
argument: a callback, `done`, to be invoked after the hook function is complete.
Hook functions are invoked with `this` bound to the associated Fastify instance.

```js
// callback style
fastify.addHook('onReady', function (done) {
  // Some code
  const err = null;
  done(err)
})

// or async/await style
fastify.addHook('onReady', async function () {
  // Some async code
  await loadCacheFromDatabase()
})
```

### onClose
<a id="on-close"></a>

Triggered when `fastify.close()` is invoked to stop the server. It is useful
when [plugins](./Plugins.md) need a "shutdown" event, for example, to close an
open connection to a database.

The hook function takes the Fastify instance as a first argument, 
and a `done` callback for synchronous hook functions.
```js
// callback style
fastify.addHook('onClose', (instance, done) => {
  // Some code
  done()
})

// or async/await style
fastify.addHook('onClose', async (instance) => {
  // Some async code
  await closeDatabaseConnections()
})
```

### onRoute
<a id="on-route"></a>

Triggered when a new route is registered. Listeners are passed a `routeOptions`
object as the sole parameter. The interface is synchronous, and, as such, the
listeners are not passed a callback. This hook is encapsulated.
```js
fastify.addHook('onRoute', (routeOptions) => {
  //Some code
  routeOptions.method
  routeOptions.schema
  routeOptions.url // the complete URL of the route, it will include the prefix if any
  routeOptions.path // `url` alias
  routeOptions.routePath // the URL of the route without the prefix
  routeOptions.bodyLimit
  routeOptions.logLevel
  routeOptions.logSerializers
  routeOptions.prefix
})
```

If you are authoring a plugin and you need to customize application routes, like
modifying the options or adding new route hooks, this is the right place.

```js
fastify.addHook('onRoute', (routeOptions) => {
  function onPreSerialization(request, reply, payload, done) {
    // Your code
    done(null, payload)
  }
  // preSerialization can be an array or undefined
  routeOptions.preSerialization = [...(routeOptions.preSerialization || []), onPreSerialization]
})
```

### onRegister
<a id="on-register"></a>

Triggered when a new plugin is registered and a new encapsulation context is
created. The hook will be executed **before** the registered code.

This hook can be useful if you are developing a plugin that needs to know when a
plugin context is formed, and you want to operate in that specific context, thus
this hook is encapsulated.

**Note:** This hook will not be called if a plugin is wrapped inside
[`fastify-plugin`](https://github.com/fastify/fastify-plugin).
```js
fastify.decorate('data', [])

fastify.register(async (instance, opts) => {
  instance.data.push('hello')
  console.log(instance.data) // ['hello']

  instance.register(async (instance, opts) => {
    instance.data.push('world')
    console.log(instance.data) // ['hello', 'world']
  }, { prefix: '/hola' })
}, { prefix: '/ciao' })

fastify.register(async (instance, opts) => {
  console.log(instance.data) // []
}, { prefix: '/hello' })

fastify.addHook('onRegister', (instance, opts) => {
  // Create a new array from the old one
  // but without keeping the reference
  // allowing the user to have encapsulated
  // instances of the `data` property
  instance.data = instance.data.slice()

  // the options of the new registered instance
  console.log(opts.prefix)
})
```

## Scope
<a id="scope"></a>

Except for [onClose](#onclose), all hooks are encapsulated. This means that you
can decide where your hooks should run by using `register` as explained in the
[plugins guide](../Guides/Plugins-Guide.md). If you pass a function, that
function is bound to the right Fastify context and from there you have full
access to the Fastify API.

```js
fastify.addHook('onRequest', function (request, reply, done) {
  const self = this // Fastify context
  done()
})
```

Note that the Fastify context in each hook is the same as the plugin where the
route was registered, for example:

```js
fastify.addHook('onRequest', async function (req, reply) {
  if (req.raw.url === '/nested') {
    assert.strictEqual(this.foo, 'bar')
  } else {
    assert.strictEqual(this.foo, undefined)
  }
})

fastify.get('/', async function (req, reply) {
  assert.strictEqual(this.foo, undefined)
  return { hello: 'world' }
})

fastify.register(async function plugin (fastify, opts) {
  fastify.decorate('foo', 'bar')

  fastify.get('/nested', async function (req, reply) {
    assert.strictEqual(this.foo, 'bar')
    return { hello: 'world' }
  })
})
```

Warn: if you declare the function with an [arrow
function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions),
the `this` will not be Fastify, but the one of the current scope.


## Route level hooks
<a id="route-hooks"></a>

You can declare one or more custom lifecycle hooks ([onRequest](#onrequest),
[onResponse](#onresponse), [preParsing](#preparsing),
[preValidation](#prevalidation), [preHandler](#prehandler),
[preSerialization](#preserialization), [onSend](#onsend),
[onTimeout](#ontimeout), and [onError](#onerror)) hook(s) that will be
**unique** for the route. If you do so, those hooks are always executed as the
last hook in their category.

This can be useful if you need to implement authentication, where the
[preParsing](#preparsing) or [preValidation](#prevalidation) hooks are exactly
what you need. Multiple route-level hooks can also be specified as an array.

```js
fastify.addHook('onRequest', (request, reply, done) => {
  // Your code
  done()
})

fastify.addHook('onResponse', (request, reply, done) => {
  // your code
  done()
})

fastify.addHook('preParsing', (request, reply, done) => {
  // Your code
  done()
})

fastify.addHook('preValidation', (request, reply, done) => {
  // Your code
  done()
})

fastify.addHook('preHandler', (request, reply, done) => {
  // Your code
  done()
})

fastify.addHook('preSerialization', (request, reply, payload, done) => {
  // Your code
  done(null, payload)
})

fastify.addHook('onSend', (request, reply, payload, done) => {
  // Your code
  done(null, payload)
})

fastify.addHook('onTimeout', (request, reply, done) => {
  // Your code
  done()
})

fastify.addHook('onError', (request, reply, error, done) => {
  // Your code
  done()
})

fastify.route({
  method: 'GET',
  url: '/',
  schema: { ... },
  onRequest: function (request, reply, done) {
    // This hook will always be executed after the shared `onRequest` hooks
    done()
  },
  onResponse: function (request, reply, done) {
    // this hook will always be executed after the shared `onResponse` hooks
    done()
  },
  preParsing: function (request, reply, done) {
    // This hook will always be executed after the shared `preParsing` hooks
    done()
  },
  preValidation: function (request, reply, done) {
    // This hook will always be executed after the shared `preValidation` hooks
    done()
  },
  preHandler: function (request, reply, done) {
    // This hook will always be executed after the shared `preHandler` hooks
    done()
  },
  // // Example with an array. All hooks support this syntax.
  //
  // preHandler: [function (request, reply, done) {
  //   // This hook will always be executed after the shared `preHandler` hooks
  //   done()
  // }],
  preSerialization: (request, reply, payload, done) => {
    // This hook will always be executed after the shared `preSerialization` hooks
    done(null, payload)
  },
  onSend: (request, reply, payload, done) => {
    // This hook will always be executed after the shared `onSend` hooks
    done(null, payload)
  },
  onTimeout: (request, reply, done) => {
    // This hook will always be executed after the shared `onTimeout` hooks
    done()
  },
  onError: (request, reply, error, done) => {
    // This hook will always be executed after the shared `onError` hooks
    done()
  },
  handler: function (request, reply) {
    reply.send({ hello: 'world' })
  }
})
```

**Note**: both options also accept an array of functions.

## Diagnostics Channel Hooks

> **Note:** The `diagnostics_channel` is currently experimental on Node.js, so
> its API is subject to change even in semver-patch releases of Node.js. For
> versions of Node.js supported by Fastify where `diagnostics_channel` is
> unavailable, the hook will use the
> [polyfill](https://www.npmjs.com/package/diagnostics_channel) if it is
> available. Otherwise this feature will not be present.

Currently, one
[`diagnostics_channel`](https://nodejs.org/api/diagnostics_channel.html) publish
event, `'fastify.initialization'`, happens at initialization time. The Fastify
instance is passed into the hook as a property of the object passed in. At this
point, the instance can be interacted with to add hooks, plugins, routes or any
other sort of modification.

For example, a tracing package might do something like the following (which is,
of course, a simplification). This would be in a file loaded in the
initialization of the tracking package, in the typical "require instrumentation
tools first" fashion.

```js
const tracer = /* retrieved from elsehwere in the package */
const dc = require('diagnostics_channel')
const channel = dc.channel('fastify.initialization')
const spans = new WeakMap()

channel.subscribe(function ({ fastify }) {
  fastify.addHook('onRequest', (request, reply, done) => {
    const span = tracer.startSpan('fastify.request')
    spans.set(request, span)
    done()
  })

  fastify.addHook('onResponse', (request, reply, done) => {
    const span = spans.get(request)
    span.finish()
    done()
  })
})
```
