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:

• Request/Reply Hooks
  • onRequest
  • preParsing
  • preValidation
  • preHandler
  • preSerialization
  • onError
  • onSend
  • onResponse
  • onTimeout
  • Manage Errors from a hook
  • Respond to a request from a hook
• Application Hooks
  • onReady
  • onClose
  • onRoute
  • onRegister
• Scope
• Route level 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 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 null, 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 uncompress 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 null, 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 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:

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

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 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:

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, 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.

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 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.

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('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.

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.

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
• onClose
• onRoute
• 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.

// 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

Triggered when fastify.close() is invoked to stop the server. 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()
})

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]
})

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()
  },
  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 if it is available. Otherwise this feature will not be present.

Currently, 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 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()
  })
})