npm package: @nestpro/platform-koa
Description: Koa HTTP adapter for Nest.js
Author: Martin Seidel
Latest version: 10.0.0

# Nest Koa HTTP Adapter

This package allows to use Koa and Koa router together with Nest.js framework.

It consists of `KoaAdapter` which is basically just mapping between Nest server and Koa.
`NestKoaApplication` is an interface of application created by `NestFactory`.
`NestKoaMiddleware` and `NestKoaFunctionalMiddleware` are interfaces for writing middleware for Nest together with Koa.
`koaToNestMiddleware` is an utility function which can convert your old Koa middleware so it can be used in Nest.

## How to use

#### Create application

```typescript
NestFactory.create<NestKoaApplication>(AppModule, new KoaAdapter());

// You can also pass your own instance of Koa app to the adapter
const koa = new Koa();
NestFactory.create<NestKoaApplication>(AppModule, new KoaAdapter(koa));
```

#### Middleware

You can still use your old middleware by converting by using `koaToNestMiddleware` function.

```typescript
const koaMiddleware = (ctx, next) => {
  ...
}

@Module({
  controllers: [HelloWorldController],
})
class TestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(koaToNestMiddleware(koaMiddleware))
      .forRoutes('*');
  }
}
```

Or you can implement class middleware by implementing `NestKoaMiddleware` interface.

```typescript
class Middleware implements NestKoaMiddleware {
  use(req: Koa.Request, res: Koa.Response, next: Koa.Next) {
    ...
  }
}

@Module({
  controllers: [HelloWorldController],
})
class TestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(Middleware)
      .forRoutes('*');
  }
}
```

#### CORS

To use CORS you'll have to install [`@koa/cors`](https://github.com/koajs/cors) package.

```
npm install @koa/cors
```

After installation is done you can set CORS same way as in normal NEST application.

> The `enableCors` method accepts options same as normal Nest application. The only difference is in `origin` property which should not be function.

```typescript
const app = NestFactory.create<NestKoaApplication>(AppModule, new KoaAdapter());

app.enableCors({});

await app.init();
```

#### Static assets

To use static assets you have to install [`koa-static`](https://github.com/koajs/static) package.

```
npm install koa-static
```

Once you have the dependency installed you can set you static assets folder.

```typescript
const app = NestFactory.create<NestKoaApplication>(AppModule, new KoaAdapter());

app.useStaticAssets(path.join(__dirname, 'static'));

// Or with options
app.useStaticAssets(path.join(__dirname, 'static'), options);

await app.init();
```

> The `useStaticAssets` method also accepts [options](https://github.com/koajs/static#options) which are exactly same as those from `koa-static`.

#### Views engine

To use MVC pattern you'll have to install [`koa-views`](https://www.npmjs.com/package/koa-views) package. This package allows you to use your favourite templating system.

```
npm install koa-views
```

To setup the view engine you have to specify your views folder and other options from [`koa-views`](https://github.com/queckezz/koa-views#api) or [`consolidate`](https://github.com/tj/consolidate.js) which is used under the hood.

```typescript
const app = NestFactory.create<NestKoaApplication>(AppModule, new KoaAdapter());

app.setViewEngine({
  viewsDir: path.join(__dirname, 'views'),
  map: {
    html: 'lodash',
  },
});

await app.init();
```

## Caveats

[Versioning feature](https://docs.nestjs.com/techniques/versioning#versioning) is not yet supported.

Nest components which operates with Koa response like exception filters needs to use the `koaReply` utility function from
this package because the implementation if the reply in adapter doesn't allow to use standard way of setting
`body` and `status` properties.

Another option is to inject the `HttpAdapterHost` dependency and use reply `reply` method from `httpAdapter` object.
But this is basically the same as using the `koaReply`.

#### Example

```typescript
@Catch()
export class ErrorFilter implements ExceptionFilter {
  public catch(error: any, host: ArgumentsHost): void {
    const httpArguments = host.switchToHttp().getResponse();

    // Your exception handling logic

    const reply = {};
    const statusCode = 500;

    koaReply(httpArguments.getResponse(), reply, statusCode);
  }
}
```