Fastify out of the box provides an asynchronous middleware engine compatible with Express and Restify middlewares.

If you need a visual feedback to understand when the middlewares are executed take a look to the lifecycle page.

Fastify middlewares don't support the full syntax middleware(err, req, res, next), because error handling is done inside Fastify. Furthermore methods added by Express and Restify to the enhanced versions of req and res are not supported in Fastify middlewares.

Also, if you are using a middleware that bundles different, smaller middlewares, such as helmet, we recommend to use the single modules to get better performances.


or, in the specific case of helmet, you can use the fastify-helmet plugin, which is an optimized helmet integration for fastify:

const fastify = require('fastify')()
const helmet = require('fastify-helmet')


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

Fastify middlewares also do 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 middlewares phase. If you need to create a middleware you have to use the Node req and res instances. Otherwise, you can use the preHandler hook that has the Request and Reply Fastify instances. For more information, see Hooks.

Restrict middleware execution to a certain path(s)

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

Note that this does not support routes with parameters, (eg: /user/:id/comments) and wildcard is not supported in multiple paths.

const path = require('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')))