Menene narewa nau'i kuma yadda compiler ake yin haka?

Question

Accepted Answer

**Narewa** shi ne yadda TypeScript ke canza nau'i mai faɗi (kamar hada kai) zuwa nau'i mafi takaici a cikin reshe na code, bisa ga kimantawa lokacin jajiya. Compiler yana bibiyar sadarar iko kuma yana sabunta nau'in bi da hankali.

```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
  }
}
```

## Kayan narewa

```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
```

## Haɗin gaida hadaɗɗiya na cikin halitta suna narawa kyau

```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
  }
}
```

Duba filin haraffi mai kama (`kind`) yana gaya wa compiler daidai wane nau'i ne kake da shi, yana bubuɗe kayan wannan nau'in.

## Garden na aiki halayen suna faɗaɗa narewa

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

## Me ya sa haka ya mahimmanci

Narewa shine abin da ya ba nau'oi hadaɗɗiya *abubuwan da ake iya shiga su* — yana ba ka damar tsarrafan kulawar nau'oi da abin sa kawai bayan cewa ka tabbata nau'in, yana canza bincike lokacin jajiya zuwa tabbacin lokacin shigar.

Shi ne abubuwan da ake buƙata yau-yau a bayan mudnin `string | number`, `T | null`, da jimlolo da aka lakabi.