# Null vs Undefined

In JavaScript, `null` and `undefined` both represent the absence of a value, but they are used in slightly different contexts and convey different meanings.

## 1. Undefined

`undefined` means a variable has been declared but has not yet been assigned a value. It is the default value of uninitialized variables, function arguments that were not provided, and missing properties of objects.

*   **Type:** `undefined`
*   **Meaning:** "Value is not assigned yet" or "Value does not exist".
*   **Origin:** Usually set by the JavaScript engine automatically.

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

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

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

## 2. Null

`null` is an assignment value. It can be assigned to a variable as a representation of no value.

*   **Type:** `object` (This is a well-known bug in JavaScript).
*   **Meaning:** "Intentionally empty" or "No object".
*   **Origin:** Usually set by the programmer explicitly.

```javascript
let y = null;
console.log(y); // null
```

## 3. Key Differences

| Feature | `undefined` | `null` |
| :--- | :--- | :--- |
| **Definition** | Variable declared but not assigned. | Assignment value representing "no value". |
| **Type (`typeof`)** | `"undefined"` | `"object"` |
| **Arithmetic** | `1 + undefined` = `NaN` | `1 + null` = `1` (converts to 0) |
| **JSON** | Omitted from JSON strings. | Preserved as `null`. |

## 4. Equality Checks

*   **Loose Equality (`==`):** They are equal.
*   **Strict Equality (`===`):** They are NOT equal.

```javascript
console.log(null == undefined);  // true
console.log(null === undefined); // false
```

## 5. Best Practices

1.  **Let JS handle `undefined`:** Don't explicitly assign `undefined` to a variable. If you want to clear a variable, use `null`.
2.  **Check for both:** If you want to check if a variable has *any* value, use loose equality against `null` (which catches both).

```javascript
if (value == null) {
  // value is either null or undefined
}
```

[[programming/javascript/vanilla/javascript]]
[[programming/javascript/vanilla/data-types]]