Starting with Fastify v3.0.0, middleware is not supported out of the box and requires an external plugin such as @fastify/express or @fastify/middie.

An example of registering the @fastify/express plugin to use Express middleware:

await fastify.register(require('@fastify/express'))
fastify.use(require('cors')())
fastify.use(require('dns-prefetch-control')())
fastify.use(require('frameguard')())
fastify.use(require('hsts')())
fastify.use(require('ienoopen')())
fastify.use(require('x-xss-protection')())

You can also use @fastify/middie, which provides support for simple Express-style middleware but with improved performance:

await fastify.register(require('@fastify/middie'))
fastify.use(require('cors')())

Remember that middleware can be encapsulated; this means that you can decide where your middleware should run by using register as explained in the plugins guide.

Fastify middleware does not expose the send method or other methods specific to the Fastify Reply instance. This is because Fastify wraps the incoming req and res Node instances using the Request and Reply objects internally, but this is done after the middleware phase. If you need to create middleware, you have to use the Node req and res instances. Otherwise, you can use the preHandler hook that already has the Request and Reply Fastify instances. For more information, see Hooks.

If you need to only run middleware under certain paths, just pass the path as the first parameter to use and you are done!

Note that this does not support routes with parameters, (e.g. /user/:id/comments) and wildcards are not supported in multiple paths.

const path = require('node:path')
const serveStatic = require('serve-static')

// Single path
fastify.use('/css', serveStatic(path.join(__dirname, '/assets')))

// Wildcard path
fastify.use('/css/(.*)', serveStatic(path.join(__dirname, '/assets')))

// Multiple paths
fastify.use(['/css', '/js'], serveStatic(path.join(__dirname, '/assets')))

Fastify offers some alternatives to the most commonly used middleware, such as @fastify/helmet in case of helmet, @fastify/cors for cors, and @fastify/static for serve-static.