Kuzamia ni nini na kompaila inafanyaje?

Question

Accepted Answer

**Kuzamia** ni jinsi TypeScript inarafiki aina pana (kama muunganisho) kuwa aina maalum zaidi ndani ya tawi la msimbo, kulingana na ukaguzi wa wakati wa uendeshaji. Kompaila inafuatilia mtiririko wa kudhibiti na inarafiki aina ipasavyo.

```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
  }
}
```

## Zana za kuzamia

```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
```

## Muunganisho umlaumulaum unakuzamia kwa uwazi

```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
  }
}
```

Kukagua sehemu ya mkakati ya herufi iliyoshirikiwa (`kind`) inasema kompaila kwa usahihi ni aina gani unayonayo, inafungua sifa za aina hizo.

## Walinzi maalum wanapanua kuzamia

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

## Kwa nini kiashiria

Kuzamia ndicho kinachofanya aina za muunganisho *kutumiwa* — inakuruhusu kufikia wanachama mahususi wa aina kwa usalama tu baada ya kuthibitisha aina, na kubadilisha ukaguzi wa wakati wa uendeshaji kuwa dhamana za muda wa kukamatia.

Ni utaratibu wa kila siku wa kushughulikia `string | number`, `T | null`, na hali iliyoashiria.