Apa iku declaration merging?

Question

IT Interview Practice · Accepted Answer

**Declaration merging** yaiku TypeScript nggabung pirang-pirang deklarasi kanthi **jeneng padha** dadi siji definisi. Interfaces, namespaces, lan sawetara konstruk liyane nggabung kanthi otomatis.

## Interface merging

```ts
interface Box { width: number; }
interface Box { height: number; }
// Box is now { width: number; height: number } — both merged
```

## Use case pembunuh: nambahi tipe pihak katelu / global

Iki carane nambahi tipe menyang library utawa global sing ora sampeyan duwe:

```ts
// Add a custom property to Express's Request
declare global {
  namespace Express {
    interface Request { user?: { id: string }; } // merges into Express.Request
  }
}
request.user; // ✅ now typed everywhere

// Augment a module
declare module "some-lib" {
  interface Options { newOption: boolean; }
}
```

Kamit `Request`/`Options` ing library yaiku `interface`, deklarasi sampeyan *nggabung* menyang iku tinimbang bentur — ngidini sampeyan nambahi tipe kanthi aman sing ora bisa sampeyan sunting.

## Namespace + function merging

```ts
function greet() {}
namespace greet { export const version = "1.0"; }
greet.version; // "1.0" — namespace merged onto the function
```

## Kenapa `type` ora bisa gawe iki

```ts
type A = { x: number };
type A = { y: number }; // ❌ Error: duplicate identifier
```

Type aliases unik; mung interfaces (lan namespaces) nggabung — alasan kunci nggunakake `interface` kanggo API shapes sing bisa dikembangke.

## Kenapa iku penting

Declaration merging yaiku mekanisme ing mburi **module augmentation** — nambahi Express requests, nambahi `window`, nggawe library types khusus, sistim tema.

Iku pengetahuan penting kanggo typing app nyata ing ngendi sampeyan kudu gawe tipe sing ora sampeyan kontrol, tanpa nggandol.

Interface merging

Use case pembunuh: nambahi tipe pihak katelu / global

Namespace + function merging

Kenapa type ora bisa gawe iki

Kenapa iku penting

Pitakon sing gegandhengan

Kenapa `type` ora bisa gawe iki