Reply

The second parameter of the handler function is Reply. Reply is a core Fastify object that exposes the following functions:

.code(statusCode) - Sets the status code.
.header(name, value) - Sets the headers.
.type(value) - Sets the header Content-Type.
.redirect([code,] url) - Redirect to the specified url, the status code is optional (default to 302).
.serialize(payload) - Serializes the specified payload using the default json serializer and returns the serialized payload.
.serializer(function) - Sets a custom serializer for the payload.
.notFound() - Invokes the 404 handler.
.send(payload) - Sends the payload to the user, could be a plain text, a buffer, JSON, stream, or an Error object.
.sent - A boolean value that you can use if you need to know it send has already been called.

fastify.get('/', options, function (request, reply) {
  // Your code
  reply
    .code(200)
    .header('Content-Type', 'application/json')
    .send({ hello: 'world' })
})

Additionally, Reply provides access to the context of the request:

fastify.get('/', {config: {foo: 'bar'}}, function (request, reply) {
  reply.send('handler config.foo = ' + reply.context.config.foo)
})

Code

If not set via reply.code, the resulting statusCode will be 200.

Sets a custom header to the response. If you not set a 'Content-Type' header, Fastify assumes that you are using 'application/json', unless you are send a stream, in that cases Fastify recognize it and sets the 'Content-Type' at 'application/octet-stream'.

Note that if you are using a custom serializer that does not serialize to JSON, you must set a custom 'Content-Type' header.

Redirect

Redirects a request to the specified url, the status code is optional, default to 302.

reply.redirect('/home')

Type

Sets the content type for the response. This is a shortcut for reply.header('Content-Type', 'the/type').

reply.type('text/html')

Serializer

Fastify was born as a full JSON compatible server, so out of the box will serialize your payload that you put in the .send() function using the internal serializers: fast-json-stringify if you set an output schema, otherwise JSON.stringify().

If you need to use a custom serializer, such as msgpack5 or protobuf, you can use the .serializer() utility. As noted above, if you are using a custom serializer that does not serialize to JSON, you must set a custom 'Content-Type' header.

reply
  .header('Content-Type', 'application/x-protobuf')
  .serializer(protoBuf.serialize)

Note that if a buffer is passed to reply.send it is expected to already be serialized and skip the serialization step.

Take a look here to understand how serialization is done.

NotFound

Invokes the 404 handler. This is useful inside handlers for routes with a wildcard * where you may want to forward the request to the 404 handler if some condition is not met.

A custom 404 handler can be set with fastify.setNotFoundHandler().

fastify.get('/*', options, function (request, reply) {
  if (!someCondition) {
    reply.notFound()
    return
  }
  // Handle the request
})

Send

As the name suggests, .send() is the function that sends the payload to the end user.

Objects

As noted above, if you are sending JSON objects, send will serialize the object with fast-json-stringify if you set an output schema, otherwise JSON.stringify() will be used.

fastify.get('/json', options, function (request, reply) {
  reply.send({ hello: 'world' })
})

Strings

If you pass a string to send without a Content-Type, it will be sent as plain text. If you set the Content-Type header and pass a string to send, it will be serialized with the custom serializer if one is set, otherwise it will be sent unmodified (unless the Content-Type header is set to application/json, in which case it will be JSON-serialized like an object — see the section above).

fastify.get('/json', options, function (request, reply) {
  reply.send('plain string')
})

Async-Await and Promises

Fastify natively handles promises and supports async-await.
Note that in the following examples we are not using reply.send.

fastify.get('/promises', options, function (request, reply) {
  const promise = new Promise(function (resolve) {
    setTimeout(resolve, 200, { hello: 'world' })
  })
  return promise
})

fastify.get('/async-await', options, async function (request, reply) {
  var res = await new Promise(function (resolve) {
    setTimeout(resolve, 200, { hello: 'world' })
  })
  return res
})

Rejected promises default to a 500 HTTP status code. Reject the promise, or throw in an async function, with an object that has statusCode (or status) and message properties to modify the reply.

fastify.get('/teapot', async function (request, reply) => {
  const err = new Error()
  err.statusCode = 418
  err.message = 'short and stout'
  throw err
})

If you want to know more please review Routes#async-await!

Streams

send can also handle streams out of the box, internally uses pump to avoid leaks of file descriptors. If you are sending a stream and you have not set a 'Content-Type' header, send will set it at 'application/octet-stream'.

fastify.get('/streams', function (request, reply) {
  const fs = require('fs')
  const stream = fs.createReadStream('some-file', 'utf8')
  reply.send(stream)
})

Errors

If you pass to send an object that is an instance of Error, Fastify will automatically create an error structured as the following:

{
  error: String        // the http error message
  message: String      // the user error message
  statusCode: Number   // the http status code
}

You can add some custom property to the Error object, such as code and headers, that will be used to enhance the http response.
Note: If you are passing an error to send and the statusCode is less than 400, Fastify will automatically set it at 500.

Tip: you can simplify errors by using the http-errors module to generate errors:

fastify.get('/', function (request, reply) {
  reply.send(httpErrors.Gone())
})

If you want to completely customize the error response, checkout setErrorHandler API.

Type of the final payload

It is crucial that the sent payload (if not undefined) is a string or a Buffer, otherwise send will throw at runtime.

Documentation