Apa iku type narrowing lan paano compiler nggawe?

Question

Accepted Answer

**Narrowing** yaiku cara TypeScript nyempitne tipe sing lebar (kaya union) menyang tipe sing luwih spesifik ing cabang kode, adhedhasar runtime checks. Compiler nglacak kontrol alur lan nganyari tipe kanthi cocok.

```ts
function process(x: string | number) {
  if (typeof x === "string") {
    x.toUpperCase(); // ✅ here x is narrowed to string
  } else {
    x.toFixed(2);    // ✅ here x must be number
  }
}
```

## Alat-alat narrowing

```ts
typeof x === "string"          // typeof guard — for primitives
x instanceof Date              // instanceof — for classes
"role" in obj                  // in operator — property existence
Array.isArray(x)               // built-in guards
if (x) { ... }                  // truthiness narrows out null/undefined/0/""
if (x === null) { ... }         // equality narrowing
```

## Discriminated unions nyempitne kanthi bersih

```ts
type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; side: number };

function area(s: Shape) {
  switch (s.kind) {            // the discriminant
    case "circle": return Math.PI * s.r ** 2;  // s is the circle variant
    case "square": return s.side ** 2;          // s is the square variant
  }
}
```

Mriksa field literal sing dibagiake (`kind`) ngrembug menyang compiler persis varian apa sing ana, mbukak properti varian kasebut.

## Custom guards ngluas narrowing

```ts
function isUser(x: unknown): x is User { return !!x && typeof (x as any).name === "string"; }
```

## Mengapa iki penting

Narrowing yaiku apa sing ndadekne union types *bisa digunakake* — iki ngowahi sampeyan ngakses anggota jenis-spesifik mung sawise mbuktekne tipene, ngowahi runtime checks menyang jaminan compile-time.

Iki mekanisme saben dina ing urip `string | number`, `T | null`, lan tagged state.