Apa iku decorators lan kepriye carane digunakake?

Question

Accepted Answer

A **decorator** minangka deklarasi khusus (`@name`) sing nambahi perilaku utawa metadata kanggo kelas, metode, properti, utawa parameter. Iki minangka fungsi sing runtime nepike karo target sing didekorate, saengga sampeyan bisa mbungkus utawa anotasi kanthi deklaratif.

```ts
// A simple method decorator that logs calls
function log(target: any, key: string, desc: PropertyDescriptor) {
  const original = desc.value;
  desc.value = function (...args: any[]) {     // wrap the original method
    console.log(`calling ${key} with`, args);
    return original.apply(this, args);
  };
}

class Service {
  @log
  fetch(id: number) { return { id }; }
}
new Service().fetch(1); // logs "calling fetch with [1]" then runs
```

Decorator nampa deskriptor metode lan ngganti kanthi wrapper — perilaku cross-cutting (logging, caching, timing) tanpa ngubah awak metode.

## Panggonan sampeyan paling mirengake: frameworks

```ts
@Controller("users")          // NestJS — declares a route controller
export class UsersController {
  @Get(":id")                 // maps GET /users/:id
  findOne(@Param("id") id: string) { return id; } // param decorator injects the route param
}

@Entity()                     // TypeORM — maps a class to a DB table
class User { @Column() name: string; }
```

NestJS, Angular, TypeORM, lan class-validator dibangun ing saubering decorators kanggo DI, routing, ORM mapping, lan validasi.

## Loro rasa (penting)

```json
{ "experimentalDecorators": true, "emitDecoratorMetadata": true }
```

**Decorators legacy (eksperimental)** ndhuwur minangka apa sing Nest/Angular/TypeORM gumantung saiki. Ana uga **TC39 Stage-3 decorators** sing luwih anyar (TS 5+) karo signature beda, sing minangka standar masa depan — nanging umume framework sing ana isih nggunakake bentuk legacy.

## Kenapa iki penting

Decorators ngaktifake meta-programming sing resik lan deklaratif — tulang punggung server frameworks lan ORMs.

Ngandelake mekanisme (fungsi sing mbungkus target) lan pembelahan legacy-versus-standard iku penting kanggo gawa ing ekosistem NestJS/Angular.