// application.ts
class App extends Preon.Application {
    test: string
}

// typings/index.d.ts
declare module 'preon' {
    interface Context {
        app: Application
    }
}

// controller/system.ts
export default class System extends Preon.Controller {
    test(ctx: Preon.Context) {
        ctx.app.test // TS2339: Property 'test' does not exist on type 'Application'.
    }
}