Kepiye NestJS nggunakake decorators?

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Accepted Answer

NestJS dibangun sakelilingi **TypeScript decorators** — anotasi khusus sing diawali karo `@` sing nempel **metadata** menyang kelas, metode, lan parameter. NestJS maca metadata iki (ngliwati reflection) kanggo nyusun routing, dependency injection, validasi, lan liyane kanthi cara deklaratif. Decorators ana ing mana-mana ing Nest.

## Decorators kelas — nemtokake peran saka kelas

```ts
@Controller("users")     // this class is a CONTROLLER, prefix /users
export class UsersController {}

@Injectable()            // this class is a PROVIDER (injectable)
export class UsersService {}

@Module({ ... })         // this class is a MODULE
export class UsersModule {}
```

Decorators kelas nyatakake apa sing *ana* ing kelas ing arsitektur NestJS (controller, provider, modul) lan ngonfigurasi.

## Decorators metode — pemetaan rute lan nerapake perilaku

```ts
@Get()                   // GET request handler
@Post()                  // POST handler
@Get(":id")              // GET /users/:id
@HttpCode(204)           // set the response status code
@UseGuards(AuthGuard)    // apply a guard to this route
@UseInterceptors(LoggingInterceptor)
findAll() {}
```

## Decorators parameter — ngekstrak data saka request

```ts
findOne(
  @Param("id") id: string,       // route parameter
  @Query("sort") sort: string,   // query string
  @Body() dto: CreateUserDto,    // request body
  @Headers("auth") auth: string, // header
) {}
```

Decorators parameter kanthi deklaratif nggawe data sing persis dibutuhake saka request — tanpa parsing manual `req`.

## Decorators properti/validasi (ing DTOs)

```ts
export class CreateUserDto {
  @IsEmail()                     // validation rules (class-validator)
  email: string;

@IsString()
  @MinLength(8)
  password: string;
}
```

## Apa sebabe decorators? Metadata deklaratif

```text
Decorators attach METADATA that NestJS reads at runtime (via reflection) to:
  ✓ Register routes (@Get, @Controller)
  ✓ Wire dependency injection (@Injectable, @Inject)
  ✓ Apply guards/interceptors/pipes (@UseGuards, etc.)
  ✓ Validate input (@IsEmail, @IsString)
→ Behavior is declared WHERE it applies, concisely and readably.
```

## Decorators khusus

```ts
// you can create your own — e.g. extract the current user from the request
export const CurrentUser = createParamDecorator((data, ctx) =>
  ctx.switchToHttp().getRequest().user
);
// usage: findProfile(@CurrentUser() user: User) {}
```

## Mengapa iku penting

Decorators minangka fitur sintaksis sing nemtokake NestJS — sakabehe framework diatur sacedhaking decorators, dadi pangerten cara kerjane penting kanggo maca lan nulis kode NestJS apa wae.

Sakedhapeyan pendekatan **deklaratif**, mbyar-mbyar metadata: tinimbang konfigurasi imperatif, sampeyan nganotasi kelas, metode, lan parameter kanggo nyatakake routing (`@Controller`, `@Get`), dependency injection (`@Injectable`), perilaku lintas-cutting (`@UseGuards`, `@UseInterceptors`), lan validasi (`@IsEmail`) tepat ing ngendhi padha diterapake — ringkas lan mudah dibaca.

Mekanisme metadata-via-reflection iki (idea sing padha ing mburi Angular lan anotasi Java/Spring) yaiku sing ngaktifake gaya sing bersih lan terstruktur saka NestJS.

Mangerteni kategorine (kelas, metode, parameter, properti decorators), ngerti sing umum, lan bisa nggawe decorators khusus minangka fluency NestJS fundamental — decorators yaiku cara sing meh kabeh fitur diekspresikake, dadi siji saka nang awal lan paling penting kanggo pangerten ing framework.