# JavaScript Data Types

JavaScript is a **dynamically typed** language, meaning variables are not bound to a specific data type. You can assign a number to a variable and later assign a string to the same variable.

There are 8 basic data types in JavaScript, divided into **Primitives** and **Reference Types**.

## 1. Primitive Types

Primitives are immutable (cannot be changed) and stored directly by value in the stack.

1.  **String**: Represents textual data.
    ```javascript
    let name = "Alice";
    ```
2.  **Number**: Represents both integers and floating-point numbers.
    ```javascript
    let n = 123;
    let f = 12.345;
    ```
3.  **BigInt**: For integers larger than `2^53 - 1`.
    ```javascript
    let big = 9007199254740991n;
    ```
4.  **Boolean**: Logical `true` or `false`.
    ```javascript
    let isActive = true;
    ```
5.  **Undefined**: A variable that has not been assigned a value.
    ```javascript
    let x; // undefined
    ```
6.  **Null**: Represents an intentional absence of any object value.
    ```javascript
    let y = null;
    ```
7.  **Symbol** (ES6): A unique and immutable identifier, often used for object property keys.
    ```javascript
    let sym = Symbol("id");
    ```

## 2. Reference Types (Objects)

Reference types are mutable and stored in the heap, with a reference (pointer) stored in the stack.

*   **Object**: A collection of key-value pairs.
*   **Array**: An ordered list of values (technically a special type of object).
*   **Function**: A callable object.

## 3. The `typeof` Operator

The `typeof` operator returns a string indicating the type of the operand.

```javascript
typeof undefined     // "undefined"
typeof 0             // "number"
typeof 10n           // "bigint"
typeof true          // "boolean"
typeof "foo"         // "string"
typeof Symbol("id")  // "symbol"
typeof Math          // "object"
typeof null          // "object" (This is a historical bug in JS)
typeof alert         // "function"
```

## 4. `null` vs `undefined`

*   **`undefined`**: "I don't know what this is yet." It is the default value of uninitialized variables.
*   **`null`**: "This is explicitly nothing." It is a value assigned by a programmer to indicate "no value".

[[programming/javascript/vanilla/javascript]]