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en/tutorials/js/1js-variables

# JavaScript Variables & Data Types

Variables store data values. JavaScript has multiple ways to declare variables and many data types.

## Declaring Variables

### `var` — Old Way (Avoid)

```javascript
var name = "John";
var age = 25;
```

Function-scoped, can be redeclared, and hoisted. Use `let` or `const` instead.

### `let` — When Reassigning

```javascript
let name = "John";
name = "Jane"; // OK
```

Block-scoped and can be reassigned.

### `const` — Default Choice

```javascript
const PI = 3.14159;
// PI = 3; // Error!
```

Block-scoped, cannot be reassigned. Must initialize immediately.

**Rule:** Use `const` by default. Use `let` only when reassignment is needed.

## Naming Rules

```javascript
let userName = "John";  // camelCase
let _private = true;    // underscore OK
let $element = "div";   // dollar sign OK

// let 123abc = "bad";  // Error: can't start with number
// let my-name = "bad"; // Error: no hyphens
```

**Convention:** Use camelCase for variables and functions.

## Primitive Data Types

```javascript
let name = "John";          // String
let age = 25;               // Number
let isActive = true;        // Boolean
let x;                      // undefined
let data = null;            // null
let id = Symbol('id');      // Symbol
let big = 9007199254740991n; // BigInt
```

**Remember:** `null` is intentional emptiness. `undefined` means no value assigned.

## Checking Types

```javascript
typeof "John"     // "string"
typeof 25         // "number"
typeof true       // "boolean"
typeof undefined  // "undefined"
typeof null       // "object" (known JS bug)
typeof {}         // "object"
typeof []         // "object"
```

## Type Conversion

```javascript
// String → Number
Number("123")      // 123
parseInt("123abc") // 123
+"123"             // 123

// Number → String
String(123)        // "123"
(123).toString()   // "123"
`${123}`           // "123"

// To Boolean
Boolean(0)         // false
Boolean("")        // false
Boolean(null)      // false
Boolean("hello")   // true
Boolean({})        // true
Boolean([])        // true
```

## Best Practices

1. **Use `const` by default** — `let` only when needed
2. **Use descriptive names** — `userName` not `x`
3. **Use camelCase** — `myVariable` not `my_variable`
4. **Initialize variables** — Avoid `undefined`
5. **Use template literals** — Instead of string concatenation

## Common Mistakes

1. **Using `var`** — Use `let`/`const` instead
2. **Redeclaring with `let`** — Not allowed
3. **Confusing `null` and `undefined`** — Different meanings
4. **Using `typeof null`** — Returns `"object"` (bug in JS)
5. **Confusing `=` and `===`** — Assignment vs comparison

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