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Routes.md

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Fastify

Routes

The route methods configure the endpoints of the application. Routes can be declared using the shorthand method or the full declaration.

Full declaration

fastify.route(options)

Routes options

  • method: currently it supports GET, HEAD, TRACE, DELETE, OPTIONS, PATCH, PUT and POST. To accept more methods, the addHttpMethod must be used. It could also be an array of methods.

  • url: the path of the URL to match this route (alias: path).

  • schema: an object containing the schemas for the request and response. They need to be in JSON Schema format, check here for more info.

    • body: validates the body of the request if it is a POST, PUT, PATCH, TRACE, SEARCH, PROPFIND, PROPPATCH or LOCK method.
    • querystring or query: validates the querystring. This can be a complete JSON Schema object, with the property type of object and properties object of parameters, or simply the values of what would be contained in the properties object as shown below.
    • params: validates the params.
    • response: filter and generate a schema for the response, setting a schema allows us to have 10-20% more throughput.

  • exposeHeadRoute: creates a sibling HEAD route for any GET routes. Defaults to the value of exposeHeadRoutes instance option. If you want a custom HEAD handler without disabling this option, make sure to define it before the GET route.

  • attachValidation: attach validationError to request, if there is a schema validation error, instead of sending the error to the error handler. The default error format is the Ajv one.

  • onRequest(request, reply, done): a function called as soon as a request is received, it could also be an array of functions.

  • preParsing(request, reply, done): a function called before parsing the request, it could also be an array of functions.

  • preValidation(request, reply, done): a function called after the shared preValidation hooks, useful if you need to perform authentication at route level for example, it could also be an array of functions.

  • preHandler(request, reply, done): a function called just before the request handler, it could also be an array of functions.

  • preSerialization(request, reply, payload, done): a function called just before the serialization, it could also be an array of functions.

  • onSend(request, reply, payload, done): a function called right before a response is sent, it could also be an array of functions.

  • onResponse(request, reply, done): a function called when a response has been sent, so you will not be able to send more data to the client. It could also be an array of functions.

  • onTimeout(request, reply, done): a function called when a request is timed out and the HTTP socket has been hung up.

  • onError(request, reply, error, done): a function called when an Error is thrown or sent to the client by the route handler.

  • handler(request, reply): the function that will handle this request. The Fastify server will be bound to this when the handler is called. Note: using an arrow function will break the binding of this.

  • errorHandler(error, request, reply): a custom error handler for the scope of the request. Overrides the default error global handler, and anything set by setErrorHandler, for requests to the route. To access the default handler, you can access instance.errorHandler. Note that this will point to fastify's default errorHandler only if a plugin hasn't overridden it already.

  • childLoggerFactory(logger, binding, opts, rawReq): a custom factory function that will be called to produce a child logger instance for every request. See childLoggerFactory for more info. Overrides the default logger factory, and anything set by setChildLoggerFactory, for requests to the route. To access the default factory, you can access instance.childLoggerFactory. Note that this will point to Fastify's default childLoggerFactory only if a plugin hasn't overridden it already.

  • validatorCompiler({ schema, method, url, httpPart }): function that builds schemas for request validations. See the Validation and Serialization documentation.

  • serializerCompiler({ { schema, method, url, httpStatus, contentType } }): function that builds schemas for response serialization. See the Validation and Serialization documentation.

  • schemaErrorFormatter(errors, dataVar): function that formats the errors from the validation compiler. See the Validation and Serialization documentation. Overrides the global schema error formatter handler, and anything set by setSchemaErrorFormatter, for requests to the route.

  • bodyLimit: prevents the default JSON body parser from parsing request bodies larger than this number of bytes. Must be an integer. You may also set this option globally when first creating the Fastify instance with fastify(options). Defaults to 1048576 (1 MiB).

  • logLevel: set log level for this route. See below.

  • logSerializers: set serializers to log for this route.

  • config: object used to store custom configuration.

  • version: a semver compatible string that defined the version of the endpoint. Example.

  • constraints: defines route restrictions based on request properties or values, enabling customized matching using find-my-way constraints. Includes built-in version and host constraints, with support for custom constraint strategies.

  • prefixTrailingSlash: string used to determine how to handle passing / as a route with a prefix.

    • both (default): Will register both /prefix and /prefix/.
    • slash: Will register only /prefix/.
    • no-slash: Will register only /prefix.

    Note: this option does not override ignoreTrailingSlash in Server configuration.

  • request is defined in Request.

  • reply is defined in Reply.

🛈 Note: The documentation for onRequest, preParsing, preValidation, preHandler, preSerialization, onSend, and onResponse is detailed in Hooks. To send a response before the request is handled by the handler, see Respond to a request from a hook.

Example:

fastify.route({
  method: 'GET',
  url: '/',
  schema: {
    querystring: {
      type: 'object',
      properties: {
        name: { type: 'string' },
        excitement: { type: 'integer' }
      }
    },
    response: {
      200: {
        type: 'object',
        properties: {
          hello: { type: 'string' }
        }
      }
    }
  },
  handler: function (request, reply) {
    reply.send({ hello: 'world' })
  }
})

Shorthand declaration

The above route declaration is more Hapi-like, but if you prefer an Express/Restify approach, we support it as well:

fastify.get(path, [options], handler)

fastify.head(path, [options], handler)

fastify.post(path, [options], handler)

fastify.put(path, [options], handler)

fastify.delete(path, [options], handler)

fastify.options(path, [options], handler)

fastify.patch(path, [options], handler)

Example:

const opts = {
  schema: {
    response: {
      200: {
        type: 'object',
        properties: {
          hello: { type: 'string' }
        }
      }
    }
  }
}
fastify.get('/', opts, (request, reply) => {
  reply.send({ hello: 'world' })
})

fastify.all(path, [options], handler) will add the same handler to all the supported methods.

The handler may also be supplied via the options object:

const opts = {
  schema: {
    response: {
      200: {
        type: 'object',
        properties: {
          hello: { type: 'string' }
        }
      }
    }
  },
  handler: function (request, reply) {
    reply.send({ hello: 'world' })
  }
}
fastify.get('/', opts)

🛈 Note: Specifying the handler in both options and as the third parameter to the shortcut method throws a duplicate handler error.

Url building

Fastify supports both static and dynamic URLs.

To register a parametric path, use a colon before the parameter name. For wildcard, use a star. Static routes are always checked before parametric and wildcard routes.

// parametric
fastify.get('/example/:userId', function (request, reply) {
  // curl ${app-url}/example/12345
  // userId === '12345'
  const { userId } = request.params;
  // your code here
})
fastify.get('/example/:userId/:secretToken', function (request, reply) {
  // curl ${app-url}/example/12345/abc.zHi
  // userId === '12345'
  // secretToken === 'abc.zHi'
  const { userId, secretToken } = request.params;
  // your code here
})

// wildcard
fastify.get('/example/*', function (request, reply) {})

Regular expression routes are supported, but slashes must be escaped. Take note that RegExp is also very expensive in terms of performance!

// parametric with regexp
fastify.get('/example/:file(^\\d+).png', function (request, reply) {
  // curl ${app-url}/example/12345.png
  // file === '12345'
  const { file } = request.params;
  // your code here
})

It is possible to define more than one parameter within the same couple of slash ("/"). Such as:

fastify.get('/example/near/:lat-:lng/radius/:r', function (request, reply) {
  // curl ${app-url}/example/near/15°N-30°E/radius/20
  // lat === "15°N"
  // lng === "30°E"
  // r ==="20"
  const { lat, lng, r } = request.params;
  // your code here
})

Remember in this case to use the dash ("-") as parameters separator.

Finally, it is possible to have multiple parameters with RegExp:

fastify.get('/example/at/:hour(^\\d{2})h:minute(^\\d{2})m', function (request, reply) {
  // curl ${app-url}/example/at/08h24m
  // hour === "08"
  // minute === "24"
  const { hour, minute } = request.params;
  // your code here
})

In this case as parameter separator it is possible to use whatever character is not matched by the regular expression.

The last parameter can be made optional by adding a question mark ("?") to the end of the parameter name.

fastify.get('/example/posts/:id?', function (request, reply) {
  const { id } = request.params;
  // your code here
})

In this case, /example/posts and /example/posts/1 are both valid. The optional param will be undefined if not specified.

Having a route with multiple parameters may negatively affect performance. Prefer a single parameter approach, especially on routes that are on the hot path of your application. For more details, see find-my-way.

To include a colon in a path without declaring a parameter, use a double colon. For example:

fastify.post('/name::verb') // will be interpreted as /name:verb

Async Await

Are you an async/await user? We have you covered!

fastify.get('/', options, async function (request, reply) {
  const data = await getData()
  const processed = await processData(data)
  return processed
})

As shown, reply.send is not called to send data back to the user. Simply return the body and you are done!

If needed, you can also send data back with reply.send. In this case, do not forget to return reply or await reply in your async handler to avoid race conditions.

fastify.get('/', options, async function (request, reply) {
  const data = await getData()
  const processed = await processData(data)
  return reply.send(processed)
})

If the route is wrapping a callback-based API that will call reply.send() outside of the promise chain, it is possible to await reply:

fastify.get('/', options, async function (request, reply) {
  setImmediate(() => {
    reply.send({ hello: 'world' })
  })
  await reply
})

Returning reply also works:

fastify.get('/', options, async function (request, reply) {
  setImmediate(() => {
    reply.send({ hello: 'world' })
  })
  return reply
})

⚠ Warning:

  • When using both return value and reply.send(value), the first one takes precedence, the second is discarded, and a warn log is emitted.
  • Calling reply.send() outside of the promise is possible but requires special attention. See promise-resolution.
  • undefined cannot be returned. See promise-resolution.

Promise resolution

If the handler is an async function or returns a promise, be aware of the special behavior to support callback and promise control-flow. When the handler's promise resolves, the reply is automatically sent with its value unless you explicitly await or return reply in the handler.

  1. If using async/await or promises but responding with reply.send:
    • Do return reply / await reply.
    • Do not forget to call reply.send.
  2. If using async/await or promises:
    • Do not use reply.send.
    • Do return the value to send.

This approach supports both callback-style and async-await with minimal trade-off. However, it is recommended to use only one style for consistent error handling within your application.

🛈 Note: Every async function returns a promise by itself.

Route Prefixing

Sometimes maintaining multiple versions of the same API is necessary. A common approach is to prefix routes with the API version number, e.g., /v1/user. Fastify offers a fast and smart way to create different versions of the same API without changing all the route names by hand, called route prefixing. Here is how it works:

// server.js
const fastify = require('fastify')()

fastify.register(require('./routes/v1/users'), { prefix: '/v1' })
fastify.register(require('./routes/v2/users'), { prefix: '/v2' })

fastify.listen({ port: 3000 })
// routes/v1/users.js
module.exports = function (fastify, opts, done) {
  fastify.get('/user', handler_v1)
  done()
}
// routes/v2/users.js
module.exports = function (fastify, opts, done) {
  fastify.get('/user', handler_v2)
  done()
}

Fastify will not complain about using the same name for two different routes because it handles the prefix automatically at compilation time. This ensures performance is not affected.

Now clients will have access to the following routes:

  • /v1/user
  • /v2/user

This can be done multiple times and works for nested register. Route parameters are also supported.

To use a prefix for all routes, place them inside a plugin:

const fastify = require('fastify')()

const route = {
    method: 'POST',
    url: '/login',
    handler: () => {},
    schema: {},
}

fastify.register(function (app, _, done) {
  app.get('/users', () => {})
  app.route(route)

  done()
}, { prefix: '/v1' }) // global route prefix

await fastify.listen({ port: 3000 })

Route Prefixing and fastify-plugin

If using fastify-plugin to wrap routes, this option will not work. To make it work, wrap a plugin in a plugin:

const fp = require('fastify-plugin')
const routes = require('./lib/routes')

module.exports = fp(async function (app, opts) {
  app.register(routes, {
    prefix: '/v1',
  })
}, {
  name: 'my-routes'
})

Handling of / route inside prefixed plugins

The / route behaves differently based on whether the prefix ends with /. For example, with a prefix /something/, adding a / route matches only /something/. With a prefix /something, adding a / route matches both /something and /something/.

See the prefixTrailingSlash route option above to change this behavior.

Custom Log Level

Different log levels can be set for routes in Fastify by passing the logLevel option to the plugin or route with the desired value.

Be aware that setting logLevel at the plugin level also affects setNotFoundHandler and setErrorHandler.

// server.js
const fastify = require('fastify')({ logger: true })

fastify.register(require('./routes/user'), { logLevel: 'warn' })
fastify.register(require('./routes/events'), { logLevel: 'debug' })

fastify.listen({ port: 3000 })

Or pass it directly to a route:

fastify.get('/', { logLevel: 'warn' }, (request, reply) => {
  reply.send({ hello: 'world' })
})

Remember that the custom log level applies only to routes, not to the global Fastify Logger, accessible with fastify.log.

Custom Log Serializer

In some contexts, logging a large object may waste resources. Define custom serializers and attach them in the appropriate context.

const fastify = require('fastify')({ logger: true })

fastify.register(require('./routes/user'), {
  logSerializers: {
    user: (value) => `My serializer one - ${value.name}`
  }
})
fastify.register(require('./routes/events'), {
  logSerializers: {
    user: (value) => `My serializer two - ${value.name} ${value.surname}`
  }
})

fastify.listen({ port: 3000 })

Serializers can be inherited by context:

const fastify = Fastify({
  logger: {
    level: 'info',
    serializers: {
      user (req) {
        return {
          method: req.method,
          url: req.url,
          headers: req.headers,
          host: req.host,
          remoteAddress: req.ip,
          remotePort: req.socket.remotePort
        }
      }
    }
  }
})

fastify.register(context1, {
  logSerializers: {
    user: value => `My serializer father - ${value}`
  }
})

async function context1 (fastify, opts) {
  fastify.get('/', (req, reply) => {
    req.log.info({ user: 'call father serializer', key: 'another key' })
    // shows: { user: 'My serializer father - call father  serializer', key: 'another key' }
    reply.send({})
  })
}

fastify.listen({ port: 3000 })

Config

Registering a new handler, you can pass a configuration object to it and retrieve it in the handler.

// server.js
const fastify = require('fastify')()

function handler (req, reply) {
  reply.send(reply.routeOptions.config.output)
}

fastify.get('/en', { config: { output: 'hello world!' } }, handler)
fastify.get('/it', { config: { output: 'ciao mondo!' } }, handler)

fastify.listen({ port: 3000 })

Constraints

Fastify supports constraining routes to match certain requests based on properties like the Host header or any other value via find-my-way constraints. Constraints are specified in the constraints property of the route options. Fastify has two built-in constraints: version and host. Custom constraint strategies can be added to inspect other parts of a request to decide if a route should be executed.

Version Constraints

You can provide a version key in the constraints option to a route. Versioned routes allows multiple handlers to be declared for the same HTTP route path, matched according to the request's Accept-Version header. The Accept-Version header value should follow the semver specification, and routes should be declared with exact semver versions for matching.

Fastify will require a request Accept-Version header to be set if the route has a version set, and will prefer a versioned route to a non-versioned route for the same path. Advanced version ranges and pre-releases currently are not supported.

Be aware that using this feature will cause a degradation of the overall performances of the router.

fastify.route({
  method: 'GET',
  url: '/',
  constraints: { version: '1.2.0' },
  handler: function (request, reply) {
    reply.send({ hello: 'world' })
  }
})

fastify.inject({
  method: 'GET',
  url: '/',
  headers: {
    'Accept-Version': '1.x' // it could also be '1.2.0' or '1.2.x'
  }
}, (err, res) => {
  // { hello: 'world' }
})

⚠ Warning: Set a Vary header in responses with the value used for versioning (e.g., 'Accept-Version') to prevent cache poisoning attacks. This can also be configured in a Proxy/CDN.

const append = require('vary').append
fastify.addHook('onSend', (req, reply, payload, done) => {
  if (req.headers['accept-version']) { // or the custom header being used
    let value = reply.getHeader('Vary') || ''
    const header = Array.isArray(value) ? value.join(', ') : String(value)
    if ((value = append(header, 'Accept-Version'))) { // or the custom header being used
      reply.header('Vary', value)
    }
  }
 done()
})

If multiple versions with the same major or minor are declared, Fastify will always choose the highest compatible with the Accept-Version header value.

If the request lacks an Accept-Version header, a 404 error will be returned.

Custom version matching logic can be defined through the constraints configuration when creating a Fastify server instance.

Host Constraints

Provide a host key in the constraints route option to limit the route to certain values of the request Host header. host constraint values can be specified as strings for exact matches or RegExps for arbitrary host matching.

fastify.route({
  method: 'GET',
  url: '/',
  constraints: { host: 'auth.fastify.dev' },
  handler: function (request, reply) {
    reply.send('hello world from auth.fastify.dev')
  }
})

fastify.inject({
  method: 'GET',
  url: '/',
  headers: {
    'Host': 'example.com'
  }
}, (err, res) => {
  // 404 because the host doesn't match the constraint
})

fastify.inject({
  method: 'GET',
  url: '/',
  headers: {
    'Host': 'auth.fastify.dev'
  }
}, (err, res) => {
  // => 'hello world from auth.fastify.dev'
})

RegExp host constraints can also be specified allowing constraining to hosts matching wildcard subdomains (or any other pattern):

fastify.route({
  method: 'GET',
  url: '/',
  constraints: { host: /.*\.fastify\.dev/ }, // will match any subdomain of fastify.dev
  handler: function (request, reply) {
    reply.send('hello world from ' + request.headers.host)
  }
})

Asynchronous Custom Constraints

Custom constraints can be provided, and the constraint criteria can be fetched from another source such as a database. Use asynchronous custom constraints as a last resort, as they impact router performance.

function databaseOperation(field, done) {
  done(null, field)
}

const secret = {
  // strategy name for referencing in the route handler `constraints` options
  name: 'secret',
  // storage factory for storing routes in the find-my-way route tree
  storage: function () {
    let handlers = {}
    return {
      get: (type) => { return handlers[type] || null },
      set: (type, store) => { handlers[type] = store }
    }
  },
  // function to get the value of the constraint from each incoming request
  deriveConstraint: (req, ctx, done) => {
    databaseOperation(req.headers['secret'], done)
  },
  // optional flag marking if handlers without constraints can match requests that have a value for this constraint
  mustMatchWhenDerived: true
}

⚠ Warning: When using asynchronous constraints, avoid returning errors inside the callback. If errors are unavoidable, provide a custom frameworkErrors handler to manage them. Otherwise, route selection may break or expose sensitive information.

const Fastify = require('fastify')

const fastify = Fastify({
  frameworkErrors: function (err, res, res) {
    if (err instanceof Fastify.errorCodes.FST_ERR_ASYNC_CONSTRAINT) {
      res.code(400)
      return res.send("Invalid header provided")
    } else {
      res.send(err)
    }
  }
})