Jinsi gani type inference inavyofanya kazi?

Question

Accepted Answer

**Type inference** ni TypeScript inayofanya kazi ya kubaini aina zisizobainishwa kutoka katika muktadha, ili usihitaji kuandika kila kitu. Inabainisha kutoka kwa maadili ya awali, kauli za return, na matumizi.

```ts
let x = 10;          // inferred: number
let s = "hi";        // inferred: string
const arr = [1, 2];  // inferred: number[]
const obj = { a: 1, b: "x" }; // inferred: { a: number; b: string }
```

## Inference katika kazi na callbacks

```ts
function double(n: number) { return n * 2; } // return inferred as number

[1, 2, 3].map(n => n * 2); // `n` inferred as number from the array — no annotation needed
```

Hii **contextual typing** ndiyo sababu callbacks kawaida hazihitaji aweza za parametri — TypeScript inajua kwamba `map` kwenye `number[]` inapitisha `number`.

## let vs const widening

```ts
let a = "hello";     // widened to string (let can be reassigned)
const b = "hello";   // narrowed to the literal type "hello"
```

`const` inabainisha aina ya *literal* kwa sababu haiwezi kubadilika; `let` inapanua hadi kwa aina ya jumla. Hii ni muhimu kwa unions:

```ts
const dir = "up";              // type "up"
let dir2 = "up";               // type string
```

## Wakati wa kuandika aweza kwa njia nyingine

- Parametri za kazi (haziwezi kubainiwa).
- Wakati inference ni zaidi sana/kidogo (mfano unataka `string` si literal).
- Vidonge tupu: `const xs = []` inabainisha `any[]` — andika kama `number[]`.

## Kwa nini ni muhimu

Inference inaifanya code ya TypeScript karibu na concise kama JavaScript lakini ikabaki fully typed.

Kuelewa *lini* inakali (na wakati inapanua) inakuwezesha kuandika aweza tu mahali inaposaidia kwa kweli.