Ni nini kinachofanya vipatanishi vya jina (?) na readonly?

Question

IT Interview Practice · Accepted Answer

Vipatanishi hivi vya sifa vinadhibiti kama sifa ni **lazima** na kama inaweza **kukamatia tena**.

## Optional (`?`)

```ts
interface User {
  name: string;
  email?: string; // optional — may be absent
}

const a: User = { name: "Ann" };               // ✅ email omitted
const b: User = { name: "Bo", email: "b@x" };  // ✅ included

function f(u: User) {
  u.email.length;   // ❌ Error: email is possibly 'undefined'
  u.email?.length;  // ✅ optional chaining handles the maybe-undefined
}
```

Sifa ya jina maalum ina aina `T | undefined`, kwa hiyo mkusanyaji unakuzuia kukamatia kesi iliyoondolewa — kuepuka makosa ya "cannot read property of undefined".

## readonly

```ts
interface Point { readonly x: number; readonly y: number; }
const p: Point = { x: 1, y: 2 };
p.x = 5; // ❌ Error: cannot assign to readonly property

// arrays too
const nums: readonly number[] = [1, 2, 3];
nums.push(4); // ❌ push doesn't exist on readonly array
```

`readonly` ni wakati wa kukamatia tu (hakuna utekelezaji wa wakati wa uendeshaji), lakini inakagiza na kulazimisha kutobadilika katika mfumo wa aina, na kukamatia mabadiliko yasioyakutaka.

## Pamoja na kwa kiwango kikubwa

```ts
interface Config { readonly id: string; tags?: readonly string[]; }
type Frozen = Readonly<User>; // utility type makes ALL props readonly
```

## Kwa nini ni muhimu

`?` inaiga data halisi ya jina maalum na kulazimisha usalama wa null; `readonly` inaeleza na kulazimisha kutobadilika (nzuri kwa usanidi, accessories, na kuepuka mabadiliko yasioyakutaka ya hali).

Vote viwili vinasogeza ukaguzi wa usahihi ndani ya mkusanyaji.

Optional (?)

readonly

Pamoja na kwa kiwango kikubwa

Kwa nini ni muhimu

Maswali yanayohusiana

Optional (`?`)