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outline: deep
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# Koa
[](https://www.npmjs.com/package/@feathersjs/koa)
[](https://github.com/feathersjs/feathers/blob/dove/packages/koa/CHANGELOG.md)
```
npm install @feathersjs/koa --save
```
The `@feathersjs/koa` module contains the [KoaJS](https://koajs.com/) framework integrations for Feathers. It will turn the Feathers app into a fully compatible KoaJS application.
## koa(app)
`koa(app) -> app` is a function that turns a [Feathers application](./application.md) into a fully KoaJS compatible application that additionally to Feathers functionality also lets you use the [KoaJS API](https://koajs.com/).
```ts
import { feathers } from '@feathersjs/feathers'
import { koa, errorHandler, bodyParser, rest } from '@feathersjs/koa'
const app = koa(feathers())
app.use(errorHandler())
app.use(authentication())
app.use(bodyParser())
app.configure(rest())
```
Also see the [additional middleware](#middleware) that `@feathersjs/koa` exposes.
## koa(app, koaApp)
`koa(app, koaApp) -> app` allows to extend an existing Koa application with the Feathers application `app`.
## koa()
If no Feathers application is passed, `koa() -> app` returns a plain Koa application (`new Koa()`).
## app.use(location|mw[, service])
`app.use(location|mw[, service]) -> app` registers either a [service object](./services.md), or a Koa middleware. If a path and [service object](./services.md) is passed it will use Feathers registration mechanism, for a middleware function Koa.
## app.listen(port)
`app.listen(port) -> HttpServer` will first call Koa [app.listen](https://github.com/koajs/koa/blob/master/docs/api/index.md#applisten) and then internally also call the [Feathers app.setup(server)](./application.md#setupserver).
```js
// Listen on port 3030
const server = await app.listen(3030)
```
## app.setup(server)
`app.setup(server) -> app` is usually called internally by `app.listen` but in the cases described below needs to be called explicitly.
### HTTPS
HTTPS requires creating a separate server in which case `app.setup(server)` also has to be called explicitly. In a generated application `src/index.js` should look like this:
```ts
import https from 'https'
import { app } from './app'
const port = app.get('port')
const server = https
.createServer(
{
key: fs.readFileSync('privatekey.pem'),
cert: fs.readFileSync('certificate.pem')
},
app.callback()
)
.listen(443)
// Call app.setup to initialize all services and SocketIO
app.setup(server)
server.on('listening', () => logger.info('Feathers application started'))
```
## params
In a Koa middleware, `ctx.feathers` is an object which will be extended as `params` in a service method call.
```ts
import { rest } from '@feathersjs/koa'
import type { NextFunction } from '@feathersjs/koa'
import type { Id, Params } from '@feathersjs/feathers'
class TodoService {
async get(id: Id, params: Params & { fromMiddleware: string }) {
const { fromMiddleware } = params
return { id, fromMiddleware }
}
}
// Register Koa middleware
app.use(async (ctx: any, next: NextFunction) => {
ctx.feathers = {
...ctx.feathers,
fromMiddleware: 'Hello from Koa middleware'
}
await next()
})
app.configure(rest())
// Register a service
app.use('todos', new TodoService())
```
Note that `app.configure(rest())` has to happen **after** any custom middleware.
### params.query
`params.query` will contain the URL query parameters sent from the client parsed using [koa-qs](https://github.com/koajs/qs).
Only `params.query` is passed between the server and the client, other parts of `params` are not. This is for security reasons so that a client can't set things like `params.user` or the database options. You can always map from `params.query` to other `params` properties in a [hook](./hooks.md).
To increase the array limit in query strings, `koa-qs` can be reinitalized with the options for the [qs](https://www.npmjs.com/package/qs) module:
```ts
// app.ts
import koaQs from 'koa-qs'
// ...
koaQs(app, 'extended', {
arrayLimit: 200
})
```
### params.provider
For any [service method call](./services.md) made through REST `params.provider` will be set to `rest`.
### params.route
Route placeholders in a service URL will be added to the services `params.route`. See the [FAQ entry on nested routes](../help/faq.md#how-do-i-do-nested-or-custom-routes) for more details on when and when not to use nested routes.
```ts
import { feathers } from '@feathersjs/feathers'
import { koa, errorHandler, bodyParser, rest } from '@feathersjs/koa'
const app = koa(feathers())
app.use('users/:userId/messages', {
async get(id, params) {
console.log(params.query) // -> ?query
console.log(params.provider) // -> 'rest'
console.log(params.fromMiddleware) // -> 'Hello world'
console.log(params.route) // will be `{ userId: '1' }` for GET /users/1/messages
return {
id,
params,
read: false,
text: `Feathers is great!`,
createdAt: new Date().getTime()
}
}
})
app.listen(3030)
```
## Service middleware
When registering a service, it is also possible to pass custom Koa middleware that should run `before` the specific service method in the `koa` [service option](./application.md#usepath-service--options):
```ts
app.use('/todos', new TodoService(), {
koa: {
before: [
async (ctx, next) => {
ctx.feathers // data that will be merged into sevice `params`
// This will run all subsequent middleware and the service call
await next()
// Then we have additional properties available on the context
ctx.hook // the hook context from the method call
ctx.body // the return value
}
]
}
})
```
Note that the order of middleware will be `[...before, serviceMethod]`.
## Middleware
### rest
```ts
import { rest } from '@feathersjs/koa'
app.configure(rest())
```
Configures the middleware for handling service calls via HTTP. It will also register authentication header parsing. The following (optional) options are available:
- `formatter` - A middleware that formats the response body
- `authentication` - The authentication `service` and `strategies` to use for parsing authentication information
### errorHandler
```ts
import { errorHandler } from '@feathersjs/koa'
app.use(errorHandler())
```
A middleware that formats errors as a Feathers error and sets the proper status code. Needs to be the first middleware registered in order to catch all other errors.
### authenticate
A middleware that allows to authenticate a user (or other authentication entity) using the [authentication service](./authentication/service.md) setting `ctx.feathers.user`. Not necessary for use with services but can be used in custom middleware.
```ts
import { authenticate } from '@feathersjs/koa'
// Authenticate other middleware with the JWT strategy
app.use(authenticate('jwt'))
// Authenticate a non default service
app.use(
authenticate({
service: 'api/v1',
strategies: ['jwt']
})
)
```
### parseAuthentication
The `parseAuthentication` middleware is registered automatically and will use the strategies of the default [authentication service](./authentication/service.md) to parse headers for authentication information. If you want to additionally parse authentication with a different authentication service this middleware can be registered again with that service configured.
```ts
import { parseAuthentication } from '@feathersjs/koa'
app.use(
parseAuthentication({
service: 'api/v1/authentication',
strategies: ['jwt', 'local']
})
)
```
### bodyParser
A reference to the [koa-body](https://github.com/koajs/koa-body) module.
### cors
A reference to the [@koa/cors](https://github.com/koajs/cors) module.
### serveStatic
A reference to the [koa-static](https://github.com/koajs/static) module.