undefined vs null in JavaScript

undefined vs null in JavaScript

undefined and null both represent "no value," but they have different meanings and origins.

undefined: The Engine's Default

undefined is what the engine assigns when a variable has no value:

• Declared but unassigned variable.
• Missing function parameter.
• Function with no return.
• Missing object property.

let x;
console.log(x); // undefined

function foo(a) { return a; }
console.log(foo()); // undefined

const obj = {};
console.log(obj.missing); // undefined

null: The Developer's Intent

null is what you assign when you intentionally want to say "no value":

let user = null; // no user yet
``
Use `null` to explicitly clear a value or signal "intentionally empty."

### Key Differences

| Feature | `undefined` | `null` |
|---|---|---|
| Origin | Engine default | Developer assignment |
| `typeof` | `"undefined"` | `"object"` (historical bug) |
| Loose equality (`==`) | equal to `null` | equal to `undefined` |
| Strict equality (`===`) | not equal to `null` | not equal to `undefined` |
| In JSON | `JSON.stringify` omits it | `JSON.stringify` includes `null` |
| Numeric coercion | `NaN` | `0` |

### Numeric Coercion Surprise

```js
Number(undefined); // NaN
Number(null);      // 0
``
This means `null + 5` is `5`, but `undefined + 5` is `NaN`. This is a real source of bugs.

### JSON Behavior

```js
JSON.stringify({ a: undefined, b: null });
// '{"b":null}'
``
`undefined` properties are omitted from JSON. `null` is preserved. This matters for API payloads.

### When to Use Which

- Use `undefined` for "not yet set" (let the engine assign it; check with `=== undefined`).
- Use `null` for "intentionally no value" (assign it yourself).
- Use `== null` to catch both in one check.

### The Takeaway

`undefined` is the engine's default for missing values. `null` is the developer's explicit "no value." `typeof null` is `"object"` (a bug). `null` coerces to `0`, `undefined` to `NaN`. JSON omits `undefined` but keeps `null`. Use `null` for intentional emptiness.