# JavaScript Series —  Phase 1
 day 3 — JavaScript Fundamentals

## 📅 Day 3 — Variables in JavaScript

## **1\. 📦 What is a Variable?**

A variable is a **named container used to store data**.

```plaintext
let name = "Tuhin";
let age = 20;

console.log(name);
console.log(age);
```

You can think of it as:

```plaintext
name → "Tuhin"
age  → 20
```

## **2\. 🔑** `var`**,** `let`**, and** `const`

JavaScript provides three keywords for declaring variables.

```plaintext
var city = "Kolkata";
let age = 20;
const country = "India";
```

`var`

*   Older way of declaring variables  
    
*   Function-scoped  
    
*   Can be redeclared and reassigned  
    

`let`

*   Modern way to declare a variable  
    
*   Block-scoped  
    
*   Can be reassigned  
    
*   Cannot be redeclared in the same scope  
    

`const`

*   Block-scoped  
    
*   Cannot be reassigned  
    
*   Cannot be redeclared  
    
*   Used when the variable reference should remain constant
    

## **3\. ✏️ Variable Naming Rules**

JavaScript variable names have some important rules:

```plaintext
let firstName = "Tuhin";
let user_age = 20;
let $price = 500;
let _value = 100;
```

Rules:

*   Can contain letters, numbers, `_`, and `$`  
    
*   Cannot start with a number  
    
*   Cannot contain spaces  
    
*   Cannot use reserved JavaScript keywords  
    
*   JavaScript is **case-sensitive**  
    

❌ Invalid:

```plaintext
let 1name = "Tuhin";
let user name = "Tuhin";
```

✅ Valid:

```plaintext
let name1 = "Tuhin";
let userName = "Tuhin";
```

A common convention is **camelCase**:

```plaintext
let firstName = "Tuhin";
let totalAmount = 5000;
```

## **4\. 🔄 Reassigning Variables**

Reassignment means giving an existing variable a new value.

With `let`:

```plaintext
let age = 20;

age = 21;

console.log(age);
```

Output:

```plaintext
21
```

With `const`:

```plaintext
const country = "India";

country = "Japan"; // Error
```

So:

```plaintext
let   → can be reassigned
const → cannot be reassigned
```

## **5\. ⚖️** `let` **vs** `const` **vs** `var`

| Feature | `var` | `let` | `const` |
| --- | --- | --- | --- |
| Reassign | ✅ | ✅ | ❌ |
| Redeclare | ✅ | ❌ | ❌ |
| Block scoped | ❌ | ✅ | ✅ |
| Modern usage | ⚠️ Avoid generally | ✅ | ✅ |
| Common use | Legacy code | Changing values | Fixed references |

### ⭐ Best Practice

For modern JavaScript:

```plaintext
const name = "Tuhin";
let age = 20;
```

Use `const` **by default**, and use `let` **when you know the value needs to change**.

Generally, avoid `var` in new code unless you specifically need its older scoping behavior.