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Hooks

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:

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 and Reply are the core Fastify objects.

done is the function to continue with the lifecycle.

It is easy to understand where each hook is executed by looking at the lifecycle page.

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

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

onRequest​

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

Or async/await:

fastify.addHook('onRequest', async (request, reply) => {
  // Some code
  await asyncMethod()
})

Notice: in the onRequest hook, request.body will always be undefined, because the body parsing happens before the 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 decompress the request body:

fastify.addHook('preParsing', (request, reply, payload, done) => {
  // Some code
  done(null, newPayload)
})

Or async/await:

fastify.addHook('preParsing', async (request, reply, payload) => {
  // Some code
  await asyncMethod()
  return newPayload
})

Notice: in the preParsing hook, request.body will always be undefined, because the body parsing happens before the 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 size of the returned stream is checked to not exceed the limit set in bodyLimit option.

preValidation​

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

fastify.addHook('preValidation', (request, reply, done) => {
  request.body = { ...request.body, importantKey: 'randomString' }
  done()
})

Or async/await:

fastify.addHook('preValidation', async (request, reply) => {
  const importantKey = await generateRandomString()
  request.body = { ...request.body, importantKey }
})

preHandler​

The preHandler hook allows you to specify a function that is executed before a routes's handler.

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

Or async/await:

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:

fastify.addHook('preSerialization', (request, reply, payload, done) => {
  const err = null
  const newPayload = { wrapped: payload }
  done(err, newPayload)
})

Or async/await:

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​

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

Or async/await:

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 Custom Error Handler set by setErrorHandler has been executed, and only if the custom error handler sends an error back to the user (Note that the default error handler always sends the error back to the user).

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

onSend​

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

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:

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:

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, a ReadableStream, a Response, or null.

onResponse​

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

Or async/await:

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.

Note: setting disableRequestLogging to true will disable any error log inside the onResponse hook. In this case use try - catch to log errors.

onTimeout​

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

Or async/await:

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 hung up. Therefore, you will not be able to send data to the client.

onRequestAbort​

fastify.addHook('onRequestAbort', (request, done) => {
  // Some code
  done()
})

Or async/await:

fastify.addHook('onRequestAbort', async (request) => {
  // Some code
  await asyncMethod()
})

The onRequestAbort hook is executed when a client closes the connection before the entire request has been processed. Therefore, you will not be able to send data to the client.

Notice: client abort detection is not completely reliable. See: Detecting-When-Clients-Abort.md

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.

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():

fastify.addHook('preHandler', (request, reply, done) => {
  reply.code(400)
  done(new Error('Some error'))
})

The error will be handled by Reply.

Or if you're using async/await you can just throw an error:

fastify.addHook('onRequest', 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.

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

// Works with async functions too
fastify.addHook('preHandler', async (request, reply) => {
  setTimeout(() => {
    reply.send({ hello: 'from prehandler' })
  })
  return reply // mandatory, so the request is not executed further
// Commenting the line above will allow the hooks to continue and fail with FST_ERR_REP_ALREADY_SENT
})

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.

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:

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​

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.

// 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()
})

onListen​

Triggered when the server starts listening for requests. The hooks run one after another. If a hook function causes an error, it is logged and ignored, allowing the queue of hooks to continue. 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.

This is an alternative to fastify.server.on('listening', () => {}).

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

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

Note
This hook will not run when the server is started using fastify.inject() or fastify.ready()

onClose​

Triggered when fastify.close() is invoked to stop the server, after all in-flight HTTP requests have been completed. It is useful when plugins 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.

// callback style
fastify.addHook('onClose', (instance, done) => {
  // Some code
  done()
})

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

preClose​

Triggered when fastify.close() is invoked to stop the server, before all in-flight HTTP requests have been completed. It is useful when plugins have set up some state attached to the HTTP server that would prevent the server to close. It is unlikely you will need to use this hook, use the onClose for the most common case.

// callback style
fastify.addHook('preClose', (done) => {
  // Some code
  done()
})

// or async/await style
fastify.addHook('preClose', async () => {
  // Some async code
  await removeSomeServerState()
})

onRoute​

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.

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.

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]
})

To add more routes within an onRoute hook, the routes must be tagged correctly. The hook will run into an infinite loop if not tagged. The recommended approach is shown below.

const kRouteAlreadyProcessed = Symbol('route-already-processed')

fastify.addHook('onRoute', function (routeOptions) {
  const { url, method } = routeOptions

  const isAlreadyProcessed = (routeOptions.custom && routeOptions.custom[kRouteAlreadyProcessed]) || false

  if (!isAlreadyProcessed) {
    this.route({
      url,
      method,
      custom: {
        [kRouteAlreadyProcessed]: true
      },
      handler: () => {}
    })
  }
})

For more details, see this issue.

onRegister​

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.

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​

Except for 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. 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.

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:

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, the this will not be Fastify, but the one of the current scope.

Route level hooks​

You can declare one or more custom lifecycle hooks (onRequest, onResponse, preParsing, preValidation, preHandler, preSerialization, onSend, onTimeout, and 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 or preValidation hooks are exactly what you need. Multiple route-level hooks can also be specified as an array.

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()
  },
  // // Example with an async hook. All hooks support this syntax
  //
  // onRequest: async function (request, reply) {
  //  // This hook will always be executed after the shared `onRequest` hooks
  //  await ...
  // }
  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.

Using Hooks to Inject Custom Properties​

You can use a hook to inject custom properties into incoming requests. This is useful for reusing processed data from hooks in controllers.

A very common use case is, for example, checking user authentication based on their token and then storing their recovered data into the Request instance. This way, your controllers can read it easily with request.authenticatedUser or whatever you want to call it. That's how it might look like:

fastify.addHook('preParsing', async (request) => {
  request.authenticatedUser = {
    id: 42,
    name: 'Jane Doe',
    role: 'admin'
  }
})

fastify.get('/me/is-admin', async function (req, reply) {
  return { isAdmin: req.authenticatedUser?.role === 'admin' || false }
})

Note that .authenticatedUser could actually be any property name chosen by yourself. Using your own custom property prevents you from mutating existing properties, which would be a dangerous and destructive operation. So be careful and make sure your property is entirely new, also using this approach only for very specific and small cases like this example.

Regarding TypeScript in this example, you'd need to update the FastifyRequest core interface to include your new property typing (for more about it, see TypeScript page), like:

interface AuthenticatedUser { /* ... */ }

declare module 'fastify' {
  export interface FastifyRequest {
    authenticatedUser?: AuthenticatedUser;
  }
}

Although this is a very pragmatic approach, if you're trying to do something more complex that changes these core objects, then consider creating a custom Plugin instead.

Diagnostics Channel Hooks​

One diagnostics_channel 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.

const tracer = /* retrieved from elsewhere in the package */
const dc = require('node: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.handler')
    spans.set(request, span)
    done()
  })

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

Note: The TracingChannel class API is currently experimental and may undergo breaking changes even in semver-patch releases of Node.js.

Five other events are published on a per-request basis following the Tracing Channel nomenclature. The list of the channel names and the event they receive is:

  • tracing:fastify.request.handler:start: Always fires
    • { request: Request, reply: Reply, route: { url, method } }
  • tracing:fastify.request.handler:end: Always fires
    • { request: Request, reply: Reply, route: { url, method }, async: Bool }
  • tracing:fastify.request.handler:asyncStart: Fires for promise/async handlers
    • { request: Request, reply: Reply, route: { url, method } }
  • tracing:fastify.request.handler:asyncEnd: Fires for promise/async handlers
    • { request: Request, reply: Reply, route: { url, method } }
  • tracing:fastify.request.handler:error: Fires when an error occurs
    • { request: Request, reply: Reply, route: { url, method }, error: Error }

The object instance remains the same for all events associated with a given request. All payloads include a request and reply property which are an instance of Fastify's Request and Reply instances. They also include a route property which is an object with the matched url pattern (e.g. /collection/:id) and the method HTTP method (e.g. GET). The :start and :end events always fire for requests. If a request handler is an async function or one that returns a Promise then the :asyncStart and :asyncEnd events also fire. Finally, the :error event contains an error property associated with the request's failure.

These events can be received like so:

const dc = require('node:diagnostics_channel')
const channel = dc.channel('tracing:fastify.request.handler:start')
channel.subscribe((msg) => {
  console.log(msg.request, msg.reply)
})