Miundo ya union na intersection ni nini?

Question

Accepted Answer

**Union** (`A | B`) inamaanisha "A **au** B." **Intersection** (`A & B`) inamaanisha "A **na** B kwa wakati mmoja."

```ts
// Union — value is one of several types
type Status = "loading" | "success" | "error"; // string literal union
let s: Status = "loading"; // only these three allowed

function id(x: string | number) { /* x is string OR number */ }
```

## Kufanya kazi na unions: unaweza kutumia tu kile kinachofanana

```ts
function format(x: string | number) {
  // x.toFixed(2); // ❌ toFixed doesn't exist on string
  if (typeof x === "number") return x.toFixed(2); // ✅ narrowed to number
  return x.toUpperCase();                          // ✅ here it's a string
}
```

Hadi utakapokosa union, unaweza tu kufikia wanachama walio nao kila mwanachama wa union — mkusanyaji anakukamatia.

## Intersection — kuchanganya maumbo

```ts
type Name = { name: string };
type Age = { age: number };
type Person = Name & Age; // must have BOTH name and age

const p: Person = { name: "Ann", age: 30 }; // both required
```

Intersections ni nzuri kwa kutunga/kuchanganya miundo ya kitu (mfano kuongeza props kwa aina iliyopo).

## Kwa nini ni muhimu

Unions huigiza "moja kati ya uwezekano kadhaa" — msingi wa uundaji salama wa hali (mfano unions zilizotofautiana kwa hali ya kupunguza), kuzamia kwa chaguo na aina zenye thamani tupu (`T | null`).

Intersections hutunga aina pamoja.

Kwa pamoja, wanafanya mfumo wa aina wa TypeScript uwe na kujieleza kiasi cha kuakisi data halisi kwa usahihi.