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Version: v1.14.x



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

  • .code(statusCode) - Sets the status code.
  • .status(statusCode) - An alias for .code(statusCode).
  • .header(name, value) - Sets a response header.
  • .getHeader(name) - Retrieve value of already set header.
  • .hasHeader(name) - Determine if a header has been set.
  • .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.
  • .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 if send has already been called.
  • .res - The http.ServerResponse from Node core.
fastify.get('/', options, function (request, reply) {
// Your code
.header('Content-Type', 'application/json; charset=utf-8')
.send({ hello: 'world' })

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

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


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

.header(key, value)

Sets a response header. If the value is omitted or undefined it is coerced to ''.

For more information, see http.ServerResponse#setHeader.


Retrieves the value of a previously set header.

reply.header('x-foo', 'foo')
reply.getHeader('x-foo') // 'foo'


Removed the value of a previously set header.

reply.header('x-foo', 'foo')
reply.getHeader('x-foo') // undefined


Returns a boolean indicating if the specified header has been set.


Redirects a request to the specified url, the status code is optional, default to 302 (if status code is not already set by calling code).



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



.send() will by default JSON-serialize any value that is not one of: Buffer, stream, string, undefined, Error. If you need to replace the default serializer with a custom serializer for a particular request, you can do so with the .serializer() utility. Be aware that if you are using a custom serializer, you must set a custom 'Content-Type' header.

.header('Content-Type', 'application/x-protobuf')

Note that you don't need to use this utility inside a handler because Buffers, streams, and strings (unless a serializer is set) are considered to already be serialized.

.header('Content-Type', 'application/x-protobuf')

See .send() for more information on sending different types of values.


As the name suggests, .sent is a property to indicate if a response has been sent via reply.send().

In case a route handler is defined as an async function or it returns a promise, it is possible to set reply.sent = true to indicate that the automatic invocation of reply.send() once the handler promise resolve should be skipped. By setting reply.sent = true, an application claims full responsibility of the low-level request and response. Moreover, hooks will not be invoked.

As an example:

app.get('/', (req, reply) => {
reply.sent = true
reply.res.end('hello world')

return Promise.resolve('this will be skipped')

If the handler rejects, the error will be logged.


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


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' })


If you pass a string to send without a Content-Type, it will be sent as text/plain; charset=utf-8. 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; charset=utf-8, 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')


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')


If you are sending a buffer and you have not set a 'Content-Type' header, send will set it to 'application/octet-stream'.

const fs = require('fs')
fastify.get('/streams', function (request, reply) {
fs.readFile('some-file', (err, fileBuffer) => {
reply.send(err || fileBuffer)


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) {

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

Errors with a status or statusCode property equal to 404 will be routed to the not found handler. See server.setNotFoundHandler API to learn more about handling such cases:

fastify.setNotFoundHandler(function (request, reply) {
reply.type('text/plain').send('a custom not found')

fastify.get('/', function (request, reply) {
reply.send(new httpErrors.NotFound())

Type of the final payload

The type of the sent payload (after serialization and going through any onSend hooks) must be one of the following types, otherwise an error will be thrown:

  • string
  • Buffer
  • stream
  • undefined
  • null

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) {
return new Promise(function (resolve) {
setTimeout(resolve, 200, { hello: 'world' })

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.