Ni nini tofauti kati ya null na undefined?

Question

Accepted Answer

Zote zinawakilisha "hakuna thamani", lakini kwa **maksudi na asili** tofauti.

- **`undefined`**: *kutokuwepo* kwa thamani, kawaida inawekwa na injini ya JavaScript — variable iliyotangazwa lakini haina thamani, mali ya kitu iliyokosekana, kigezo cha chaguo la kukokotoza usichokipitisha, au chaguo la kukokotoza bila `return`.
- **`null`**: thamani ya *kusadikiwa* "tupu" ambayo **wewe** unaitumia kusema "hakuna kitu hapa kwa maksudi".

```js
let a;                 // undefined — declared, not assigned
const obj = {};
obj.missing;           // undefined — property doesn't exist
function f() {}
f();                   // undefined — no return

const b = null;        // null — you chose to empty it
```

## Ulinganisho

```js
null == undefined;   // true  — loose equality treats them as equal
null === undefined;  // false — different types
typeof undefined;    // "undefined"
typeof null;         // "object"  (historical bug)
```

## Mbinu ya vitendo

```js
const value = input ?? "default"; // defaults on EITHER null or undefined
user?.address?.city;               // optional chaining — undefined if any link is null/undefined
```

## Kwa nini ni muhimu

Tumia **`null`** kusafisha thamani kwa maksudi (k.m. "hakuna mtumiaji aliyechaguliwa"), na tojifie **`undefined`** kama "haijawekwa bado". Waendeshaji `??` (nullish coalescing) na `?.` (optional chaining) hutunza zote kwa wakati mmoja, ndiyo sababu zinatumika sana.