Middleware

Middleware works after/before Handler. We can get Request before dispatching or manipulate Response after dispatching.

Definition of Middleware

• Handler - should return Response object. Only one handler will be called.
• Middleware - should return nothing, will be proceeded to next middleware with await next()

The user can register middleware using app.use or using app.HTTP_METHOD as well as the handlers. For this feature, it's easy to specify the path and the method.

// match any method, all routes
app.use(logger())

// specify path
app.use('/posts/*', cors())

// specify method and path
app.post('/posts/*', basicAuth())

If the handler returns Response, it will be used for the end-user, and stopping the processing.

app.post('/posts', (c) => c.text('Created!', 201))

In this case, four middleware are processed before dispatching like this:

logger() -> cors() -> basicAuth() -> *handler*

Execution order

The order in which Middleware is executed is determined by the order in which it is registered. The process before the next of the first registered Middleware is executed first, and the process after the next is executed last. See below.

app.use(async (_, next) => {
  console.log('middleware 1 start')
  await next()
  console.log('middleware 1 end')
})
app.use(async (_, next) => {
  console.log('middleware 2 start')
  await next()
  console.log('middleware 2 end')
})
app.use(async (_, next) => {
  console.log('middleware 3 start')
  await next()
  console.log('middleware 3 end')
})

app.get('/', (c) => {
  console.log('handler')
  return c.text('Hello!')
})

Result is the following.

middleware 1 start
  middleware 2 start
    middleware 3 start
      handler
    middleware 3 end
  middleware 2 end
middleware 1 end

Built-in Middleware

Hono has built-in middleware.

import { Hono } from 'hono'
import { poweredBy } from 'hono/powered-by'
import { logger } from 'hono/logger'
import { basicAuth } from 'hono/basic-auth'

const app = new Hono()

app.use(poweredBy())
app.use(logger())

app.use(
  '/auth/*',
  basicAuth({
    username: 'hono',
    password: 'acoolproject',
  })
)

WARNING

In Deno, it is possible to use a different version of middleware than the Hono version, but this can lead to bugs. For example, this code is not working because the version is different.

import { Hono } from 'jsr:@hono/hono@4.4.0'
import { upgradeWebSocket } from 'jsr:@hono/hono@4.4.5/deno'

const app = new Hono()

app.get('/ws', upgradeWebSocket(() => ({
  // ...
})))

Custom Middleware

You can write your own middleware directly inside app.use():

// Custom logger
app.use(async (c, next) => {
  console.log(`[${c.req.method}] ${c.req.url}`)
  await next()
})

// Add a custom header
app.use('/message/*', async (c, next) => {
  await next()
  c.header('x-message', 'This is middleware!')
})

app.get('/message/hello', (c) => c.text('Hello Middleware!'))

However, embedding middleware directly within app.use() can limit its reusability. Therefore, we can separate our middleware into different files.

To ensure we don't lose type definitions for context and next, when separating middleware, we can use createMiddleware() from Hono's factory.

import { createMiddleware } from 'hono/factory'

const logger = createMiddleware(async (c, next) => {
  console.log(`[${c.req.method}] ${c.req.url}`)
  await next()
})

INFO

Type generics can be used with createMiddleware:

createMiddleware<{Bindings: Bindings}>(async (c, next) =>

Modify the Response After Next

Additionally, middleware can be designed to modify responses if necessary:

const stripRes = createMiddleware(async (c, next) => {
  await next()
  c.res = undefined
  c.res = new Response('New Response')
})

Third-party Middleware

Built-in middleware does not depend on external modules, but third-party middleware can depend on third-party libraries. So with them, we may make a more complex application.

For example, we have GraphQL Server Middleware, Sentry Middleware, Firebase Auth Middleware, and others.