JavaScript Variables

What's a Variable?

A variable is like a labeled box where you store something. You give it a name, put a value inside, and use that name later to get the value back.

Think of it like this:

┌─────────────────────┐
│   Your Computer     │
│   Memory            │
│                     │
│  ┌───────────────┐  │
│  │ name         │  │ ← Label
│  │ "John"       │  │ ← Value stored
│  └───────────────┘  │
│                     │
│  ┌───────────────┐  │
│  │ age           │  │ ← Label
│  │ 25            │  │ ← Value stored
│  └───────────────┘  │
└─────────────────────┘

When you write let name = 'John', you're telling JavaScript: "Create a box labeled 'name' and put 'John' inside it."

How to Create Variables

JavaScript gives you three ways to create variables, but you only need two of them:

1. const - For Things That Don't Change

const myName = 'John';
const birthYear = 1990;

Once you put a value in a const box, you can't change it. It stays the same.

When to use: Most of the time. If something shouldn't change, use const.

2. let - For Things That Change

let age = 25;
age = 26;  // This works! You can change it

With let, you can change the value later.

When to use: When the value needs to change, like a counter or a score.

3. var - Don't Use This

var is the old way. It causes problems. Just use const or let instead.

Step-by-Step: Creating Your First Variable

Let's create a variable step by step:

Step 1: Decide what you want to store

// I want to store my name

Step 2: Choose const or let

// My name won't change, so use const
const

Step 3: Give it a name

const myName

Step 4: Put the value in

const myName = 'John';

Step 5: Use it

console.log(myName);  // Shows: John

Where Variables Live (Scope)

Variables have a "home" - the place where they exist. This is called scope.

┌─────────────────────────────┐
│  function myFunction() {     │
│    ┌─────────────────────┐  │
│    │  if (true) {         │  │
│    │    let x = 10;       │  │ ← x lives here
│    │  }                   │  │
│    │  console.log(x);     │  │ ← ERROR! x doesn't exist here
│    └─────────────────────┘  │
│  }                           │
└─────────────────────────────┘

The variable x only exists inside the if block. Outside that block, it doesn't exist. This prevents bugs.

Real Example: Using Variables

Let's say you're building a simple calculator:

// Store the first number
const firstNumber = 10;

// Store the second number
const secondNumber = 5;

// Calculate the result
const result = firstNumber + secondNumber;

// Show the result
console.log(result);  // Shows: 15

See how we stored values, used them, and got a result? That's what variables do.

Changing Values

With let, you can change the value:

let score = 0;
console.log(score);  // 0

score = 10;
console.log(score);  // 10

score = score + 5;
console.log(score);  // 15

But with const, you can't:

const score = 0;
score = 10;  // ERROR! Can't change a const

Quick Rules

1. Use const most of the time - It's safer
2. Use let when you need to change the value - Like counters or scores
3. Never use var - It's outdated and causes problems
4. Give variables clear names - userName is better than u or x

That's it! Variables are just boxes with labels. You put stuff in, you get stuff out. Start with const, use let when you need to change things.