Documentation

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.

Request/Response Hooks

By using the hooks you can interact directly inside the lifecycle of Fastify. There are four different Hooks that you can use (in order of execution):

  • 'onRequest'
  • 'preValidation'
  • 'preHandler'
  • 'onError'
  • 'onSend'
  • 'onResponse'

Example:

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

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

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

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

fastify.addHook('onSend', (request, reply, payload, next) => {
  // some code
  next()
})

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

Or async/await

fastify.addHook('onRequest', async (request, reply) => {
  // some code
  await asyncMethod()
  // error occurred
  if (err) {
    throw new Error('some errors occurred.')
  }
  return
})

fastify.addHook('preValidation', async (request, reply) => {
  // some code
  await asyncMethod()
  // error occurred
  if (err) {
    throw new Error('some errors occurred.')
  }
  return
})

fastify.addHook('preHandler', async (request, reply) => {
  // some code
  await asyncMethod()
  // error occurred
  if (err) {
    throw new Error('some errors occurred.')
  }
  return
})

fastify.addHook('onError', async (request, reply, error) => {
  // useful for custom error logging
  // you should not use this hook to update the error
})

fastify.addHook('onSend', async (request, reply, payload) => {
  // some code
  await asyncMethod()
  // error occurred
  if (err) {
    throw new Error('some errors occurred.')
  }
  return
})

fastify.addHook('onResponse', async (request, reply) => {
  // some code
  await asyncMethod()
  // error occurred
  if (err) {
    throw new Error('some errors occurred.')
  }
  return
})

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

Notice: in the onRequest and preValidation hooks, request.body will always be null, because the body parsing happens before the preHandler hook.

Request and Reply are the core Fastify objects.
next if the function to continue with the lifecycle.

It is pretty 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.

If you get an error during the execution of your hook, just pass it to next() and Fastify will automatically close the request and send the appropriate error code to the user.

fastify.addHook('onRequest', (request, reply, next) => {
  next(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, next) => {
  reply.code(400)
  next(new Error('some error'))
})

The error will be handled by Reply.

The onError Hook

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 back and error to the user (Note that the default customErrorHandler always send back the error to the user).
Notice: unlike the other hooks, pass an error to the next function is not supported.

fastify.addHook('onError', (request, reply, error, next) => {
  // apm stands for Application Performance Monitoring
  apm.sendError(error)
  next()
})

// Or async
fastify.addHook('onError', async (request, reply, error) => {
  // apm stands for Application Performance Monitoring
  apm.sendError(error)
})

The onSend Hook

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

fastify.addHook('onSend', (request, reply, payload, next) => {
  var err = null;
  var newPayload = payload.replace('some-text', 'some-new-text')
  next(err, newPayload)
})

// Or async
fastify.addHook('onSend', async (request, reply, payload) => {
  var 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, next) => {
  reply.code(304)
  const newPayload = null
  next(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.

The onResponse Hook

The onResponse hook is executed when a response has been sent, so you will not be able to send more data to the client, however you can use this hook to send some data to an external service or elaborate some statistics.

Respond to a request from a hook

If needed, you can respond to a request before you reach the route handler. An example could be an authentication hook. If you are using onRequest or preHandler use reply.send; if you are using a middleware, res.end.

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

// Works with async functions too
fastify.addHook('preHandler', async (request, reply) => {
  reply.send({ hello: 'world' })
})

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, next) => {
  const stream = fs.createReadStream('some-file', 'utf8')
  reply.send(stream)
})

Application Hooks

You are able to hook into the application-lifecycle as well. It's important to note that these hooks aren't fully encapsulated. The this inside the hooks are encapsulated but the handlers can respond to an event outside the encapsulation boundaries.

  • 'onClose'
  • 'onRoute'

'onClose'
Triggered when fastify.close() is invoked to stop the server. It is useful when plugins need a "shutdown" event, such as a connection to a database.
The first argument is the Fastify instance, the second one the done callback.

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

'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 do not get passed a callback.

fastify.addHook('onRoute', (routeOptions) => {
  // some code
  routeOptions.method
  routeOptions.schema
  routeOptions.url
  routeOptions.bodyLimit
  routeOptions.logLevel
  routeOptions.prefix
})

Scope

Except for Application Hooks, 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, next) {
  const self = this // Fastify context
  next()
})

Note: using an arrow function will break the binding of this to the Fastify instance.

Route level hooks

You can declare one or more custom preValidation and preHandler hook(s) that will be unique for the route. If you do so, those hooks always be executed as last hook in their category.
This can be useful if you need to run the authentication, and the preValidation hooks is exactly what you need for doing that. Let's make an example:

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

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

fastify.route({
  method: 'GET',
  url: '/',
  schema: { ... },
  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()
  },
  handler: function (request, reply) {
    reply.send({ hello: 'world' })
  }
})

Note: both options also accept an array of functions.