What Are Variables?

Variables store information in memory.

Real-life analogy:

Variable = labeled box storing data

Example:

• Name box → "Ali"

• Age box → 22

Why Variables Are Needed

Variables help store and reuse data.

Without variables:

console.log("Ali");
console.log("Ali");
console.log("Ali");

With variables:

const name = "Ali";

console.log(name);
console.log(name);

Declaring Variables

JavaScript provides:

• var

• let

• const

Using let

let allows changing values later.

Example

let age = 20;

console.log(age);

age = 21;

console.log(age);

Output

20
21

Using const

const creates variables whose value cannot be reassigned.

Example

const country = "India";

console.log(country);

Trying to Change const

const country = "India";

country = "USA";

Result

Error

Using var

var is the older way of declaring variables.

Example

var city = "Lucknow";

console.log(city);

Modern JavaScript usually prefers:

• let

• const

Difference Between var, let, and const

Primitive Data Types

Primitive data types are basic value types in JavaScript.

1. String

Used for text.

Example

const name = "Ali";

2. Number

Used for numeric values.

Example

const age = 22;

3. Boolean

Represents true or false.

Example

const isStudent = true;

4. Undefined

Variable declared but no value assigned.

Example

let phoneNumber;

console.log(phoneNumber);

Output

undefined

5. Null

Represents intentional empty value.

Example

const selectedUser = null;

Data Type Examples

Checking Data Types

Use:

typeof

Example

console.log(typeof "Ali");
console.log(typeof 25);
console.log(typeof true);

Output

string
number
boolean

What Is Scope?

Scope means:

Where a variable can be accessed

Simple Scope Example

let message = "Hello";

function test() {
  console.log(message);
}

test();

message is accessible inside the function.

Block Scope Example

{
  let age = 20;

  console.log(age);
}

Works inside the block.

Outside Block

{
  let age = 20;
}

console.log(age);

Result

Error

Scope Visualization

Practical Example

const name = "Sara";
let age = 19;
let isStudent = true;

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

Assignment

Step 1: Declare Variables

const name = "Ali";
let age = 22;
let isStudent = true;

Step 2: Print Values

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

Step 3: Change let

age = 23;

console.log(age);

Step 4: Try Changing const

name = "Ahmed";

Observe the error.

Key Takeaways

Final Notes

Variables and data types are the foundation of JavaScript.

Almost everything in programming depends on:

• Storing data

• Modifying data

• Accessing data correctly

Mastering these basics makes learning functions, arrays, objects, and APIs much easier.