# Understanding Variables and Data Types in JavaScript

## Variables — Containers for Storing Data

A variable is a container where you can store data. In JavaScript, there are 3 ways to declare variables:

• var — Old method, function-scoped, can be overwritten anytime

• let — Modern method, block-scoped, value can be changed

• const — Block-scoped, value CANNOT be changed after assignment

Adds element at beginning.

```javascript
var score = 10; // old way
let name = "Shaaz"; // modern way
const pi = 3.14; // constant, never changes
```

**Tip:** *Use 'const' by default. Use 'let' only when the value needs to change. Avoid 'var' in modern code*.

**✅Remember:** *var is function-scoped. let and const are block-scoped. This difference matters inside loops and if-blocks*.

## Data Types

JavaScript has 7 main data types. Knowing these is very important for interviews:

**1\. Number — for all numeric values**

```javascript
let age = 25;
let price = 99.99;
```

**2\. String — for text**

```javascript
let name = "Shaaz";
let city = 'Delhi';
```

**3\. Boolean — true or false only**

```javascript
let isLoggedIn = true;
let isAdmin = false;
```

**4\. Null — intentionally empty**

Use null when you want to say 'this variable exists but has no value on purpose'.

```javascript
let emptyValue = null;
```

**5\. Undefined — not assigned yet**

When you declare a variable but do not give it a value, it becomes undefined automatically.

```javascript
let noValue;
console.log(noValue); // Output: undefined
```

**6\. Object — key-value pairs**

Objects store multiple related values together.

```javascript
let person = { name: "Shaaz", age: 25, city: "Delhi" };
console.log(person.name); // Output: Shaaz
```

**7\. Array — ordered list of values**

Arrays store multiple values in a sequence. Each value has an index starting from 0.

```javascript
Adds element at beginning.

const nums = [2, 3];

nums.unshift(1);

console.log(nums);
// [1, 2, 3]
```

**✅Remember:** *null means empty on purpose. undefined means not yet assigned. They are NOT the same!*

**🟢 Tip:** *Use typeof to check the data type of any* `variable: console.log(typeof 'hello'); // Output: string`