# Mount Middleware

The Mount Middleware lets you mount an application built with another framework into your Hono application.

## Import

```ts twoslash
import { Hono } from 'hono'
import { mount } from 'hono/mount'
```

## Usage

Register the middleware with `app.all()` on a path that ends with `/*`:

```ts
import { Router as IttyRouter } from 'itty-router'

// Create itty-router application
const ittyRouter = IttyRouter()

// Handle `GET /itty-router/hello`
ittyRouter.get('/hello', () => new Response('Hello from itty-router'))

const app = new Hono()

app.all('/itty-router/*', mount(ittyRouter.handle))
```

By default, the mounted application receives a new `Request` with the matched path prefix removed from its URL. A request to `/itty-router/hello` reaches itty-router as `/hello`.

The mounted application is called as `handler(request, c.env, c.executionCtx)`, the same arguments as a Cloudflare Workers `fetch` handler. Use `optionHandler` to pass different arguments.

## Options

You can pass a function or an object as the second argument.

### <Badge type="info" text="optional" /> optionHandler: `(c: Context) => unknown`

Returns the arguments passed to the mounted application after the `Request`. An array is spread into multiple arguments. You can pass the function directly as the second argument:

```ts twoslash
import { Hono } from 'hono'
import { mount } from 'hono/mount'
const app = new Hono()
const anotherApp = (request: Request, ...args: unknown[]) =>
  new Response(request.url)
// ---cut---
// Call `anotherApp(request, c.env)`
app.all(
  '/app/*',
  mount(anotherApp, (c) => c.env)
)

// Call `anotherApp(request, c.env, c.executionCtx)`
app.all(
  '/app/*',
  mount(anotherApp, {
    optionHandler: (c) => [c.env, c.executionCtx],
  })
)
```

### <Badge type="info" text="optional" /> replaceRequest: `((originalRequest: Request) => Request) | false`

Controls which `Request` is passed to the mounted application:

```ts twoslash
import { Hono } from 'hono'
import { mount } from 'hono/mount'
const app = new Hono()
const anotherApp = (request: Request, ...args: unknown[]) =>
  new Response(request.url)
// ---cut---
// `/legacy/users` reaches the mounted application as `/api/users`
app.all(
  '/legacy/*',
  mount(anotherApp, {
    replaceRequest: (req) => {
      const url = new URL(req.url)
      url.pathname = url.pathname.replace(/^\/legacy/, '/api')
      return new Request(url, req)
    },
  })
)
```

To pass the original `Request` unchanged, set it to `false` as a shorthand for `(req) => req`:

```ts twoslash
import { Hono } from 'hono'
import { mount } from 'hono/mount'
const app = new Hono()
const anotherApp = (request: Request, ...args: unknown[]) =>
  new Response(request.url)
// ---cut---
app.all(
  '/app/*',
  mount(anotherApp, {
    replaceRequest: false,
  })
)
```