NestJS tip: how to attach decorators to all controllers without at once

What

In the official NestJS integration with OpenAPI (formerly Swagger) @nestjs/swagger we have a bunch of @Api* decorator factories such as @ApiBearerAuth().

Due to how TypeScript decorators works, you cannot bind those class decorators globally -- ie., for all the controllers that your entry module has discovered -- at once.

But using the built-in provider DiscoveryService (from @nestjs/core package exported by the DiscoveryModule NestJS module) we can do that!

DISCLAIMER: I'm not saying that this is good. I don't like the ideia of not having those decorators close to the controller.

How

For this demo, our controller will be as simple as this:

• app.controller.ts

import { Controller, Get } from '@nestjs/common'

@Controller()
export class AppController { // No Api* decorators here!
  @Get()
  hello() {
    return 'Hello, World!'
  }
}

Our entry module would look like this:

• app.module.ts

import { Module } from '@nestjs/common'
import { DiscoveryModule } from '@nestjs/core'
import { AppController } from './app.controller'

@Module({
  imports: [
    DiscoveryModule, // We'll use DiscoveryService later
  ],
  controllers: [
    AppController,
  ],
})
export class AppModule {}

And our entry file would look like this:

• main.ts

import { DiscoveryService, NestFactory } from '@nestjs/core'
import { AppModule } from './app.module'
import { ApiBearerAuth, DocumentBuilder, SwaggerModule } from '@nestjs/swagger'

async function bootstrap() {
  const app = await NestFactory.create(AppModule)

  const makeDocument = () => {
    const discoveryService = app.get(DiscoveryService)
    const controllers = discoveryService.getControllers()
    for (const controller of controllers) {
      // All the ApiX decorators that you want to bind globally
      ApiBearerAuth()(controller.metatype) // (LA)
    }

    const config = new DocumentBuilder()
      .setTitle('Cats example')
      .build()
    return SwaggerModule.createDocument(app, config) // (LB)
  }

  SwaggerModule.setup('/api', app, makeDocument) // (LC)

  await app.listen(3000) // (LD)
}
bootstrap()

Why this works

The SwaggerModule.createDocument is responsible to compose the OpenAPI spec. and will be invoked by SwaggerModule.setup when this object is needed. In our case we are supplying a factory that returns the document, not the document itself. This is important!

Also, we must call SwaggerModule.setup before app.listen/app.init because this one is responsible to add a new route (the /api in this example) that exposes the OpenAPI spec. and the Swagger UI.

In order to enhance all the controllers with @Api* decorators, we must run the code at (LA) line before (LB).

Due to the lazy loading mode of SwaggerModule.setup when calling our makeDocument factory, everything will work as we want:

• SwaggerModule.setup is being called before app.listen
• SwaggerModule.createDocument is being called after our custom enhancement of all the controllers found from AppModule (including child modules); thus, after line (LA)

At http://localhost:3000/api we could see this:

To be fair, we don't need to rely on the lazy loading mode of SwaggerModule.setup as long as we call (LA) before SwaggerModule.createDocument, but I prefer this approach to avoid impacting the bootstrapping time.

---

Of course that you can find alternatives approaches such as relying on the on module init lifecycle hook instead of having to use the app inside the makeDocument factory function.