What is the difference between null and undefined?

Question

IT Interview Practice · Accepted Answer

Both represent "no value," but with different **intent and origin**.

- **`undefined`**: the *absence* of a value, usually set by the JavaScript engine — a declared-but-unassigned variable, a missing object property, a function parameter you didn't pass, or a function with no `return`.
- **`null`**: an *intentional* "empty" value that **you** assign to say "deliberately nothing here."

```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
```

## Comparisons

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

## Practical handling

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

## Why it matters

Use **`null`** to deliberately clear a value (e.g. "no selected user"), and treat **`undefined`** as "not set yet." The operators `??` (nullish coalescing) and `?.` (optional chaining) handle both at once, which is why they're so widely used.

Comparisons

Practical handling

Why it matters

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