Kuna tofauti gani kati ya var, let na const?

Question

IT Interview Practice · Accepted Answer

Wanatofautiana kwa njia tatu: **scope**, **hoisting behavior** na **reassignment**.

```js
function demo() {
  if (true) {
    var a = 1;    // function-scoped — leaks out of the if-block
    let b = 2;    // block-scoped — only exists inside { }
    const c = 3;  // block-scoped, can't be reassigned
  }
  console.log(a); // 1  (var escaped the block)
  console.log(b); // ReferenceError (let stayed in the block)
}
```

## Tofauti muhimu

- **`var`** ni **function-scoped**, hoisted na initialized kwa `undefined`, na inaweza kutengenezwa tena. Hii husababisha mabovu yanayojulika — kuzuia.
- **`let`** ni **block-scoped** (`{ }`), inaweza kuwekwa upya, na iko katika *temporal dead zone* hadi mstari wake wa mgao (kuipata mapema kutaeneza).
- **`const`** ni block-scoped na **haiwezi kuwekwa upya** — lakini kumbuka kwamba binding ni constant, *si* thamani:

```js
const user = { name: "Ann" };
user.name = "Bob"; // ✅ allowed — mutating the object, not reassigning
user = {};          // ❌ TypeError — can't rebind `user`
```

## Kanuni ya kidole

Kubadilisha kwa default kwa **`const`**; badili kwa **`let`** tu wakati lazima kuweka tena; **kamwe usitumie `var`** katika kanuni mpya. Hii inaifanya nia kuwa wazi ("hii haitabadilika") na kuepuka mabovu ya scope na hoisting confusion ambayo `var` inaleta.

Tofauti muhimu

Kanuni ya kidole

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