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💻 JavaScript Learning Series, Day 2

Topic 2: Variables & Data Types in JavaScript

Today, we'll learn how to store information in JavaScript and understand the different types of data we can work with.

1. What Are Variables?

A variable is a named container used to store data in a program.

For example:

let name = "John";
let age = 25;
let salary = 45000;

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

Output:

John
25
45000

Here:

• name stores a string.
• age stores a number.
• salary stores a number.

2. How to Declare Variables

JavaScript provides three main ways to declare variables.

A. var

• Function-scoped.
• Can be redeclared.
• Can be reassigned.
• Generally avoided in modern JavaScript.

var city = "Mumbai";
var city = "Pune";

console.log(city);

B. let

• Block-scoped.
• Cannot be redeclared in the same scope.
• Can be reassigned.

let age = 25;
age = 30;

console.log(age);

Output: 30

C. const

• Block-scoped.
• Cannot be redeclared in the same scope.
• Cannot be reassigned.

const country = "India";

console.log(country);

Output: India

Remember: A const object or array can still have its contents modified. The variable binding itself cannot be reassigned.

3. Difference Between var, let & const

Feature: Scope
var: Function
let: Block
const: Block

Feature: Reassignment
var: Yes
let: Yes
const: No

Feature: Redeclaration
var: Yes
let: No
const: No

Feature: Hoisting
var: Yes
let: Yes
const: Yes

Feature: Initialization required
var: No
let: No
const: Yes

Best Practice: Use const by default and let when reassignment is required. Avoid var in modern JavaScript.

4. JavaScript Data Types

JavaScript has two broad categories of data types.

A. Primitive Data Types

There are seven primitive types.

1. String

Used to store text.

let name = "Rahul";

2. Number

Used for integers and decimal values.

let age = 25;
let price = 99.99;

3. Boolean

Stores either true or false.

let isActive = true;
let isCompleted = false;

4. Undefined

A variable that has been declared but not assigned a value.

let result;

console.log(result);

Output: undefined

5. Null

Represents an intentional absence of a value.

let selectedUser = null;

6. BigInt

Used for integers larger than the safe integer limit of the Number type.

let bigNumber = 12345678901234567890n;

7. Symbol

Creates unique identifiers.

let id = Symbol("user");

B. Non-Primitive Data Types

These are commonly represented using objects.

1. Object

Stores data in key-value pairs.

let student = {
    name: "Rahul",
    age: 22,
    city: "Delhi"
};

2. Array

Stores multiple values in an ordered collection.

let fruits = ["Apple", "Mango", "Banana"];

3. Function

A reusable block of code.

function greet() {
    console.log("Hello!");
}

5. Checking Data Types Using typeof

JavaScript provides the typeof operator to identify the type of a value.

console.log(typeof "Hello");
console.log(typeof 100);
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof 100n);
console.log(typeof Symbol("id"));
console.log(typeof {});

string
number
boolean
undefined
bigint
symbol
object

Important: typeof null also returns "object". This is a historical JavaScript behavior.

6. Dynamic Typing in JavaScript

JavaScript is dynamically typed. This means a variable can hold different types of values during execution.

let data = 100;

console.log(data);

data = "Hello JavaScript";

console.log(data);

100
Hello JavaScript

The same variable first stores a number and then stores a string.

7. Type Conversion

Type conversion means converting one data type into another.

String to Number

let value = "100";

let result = Number(value);

Output: 100

Number to String

let age = 25;

let result = String(age);

Output: 25 (as a string)

String to Boolean

let value = "Hello";
console.log(Boolean(value));

Output: true

8. Variable Naming Rules

Follow these rules when naming variables:

- Names can contain letters, numbers, _ and $.
- Names cannot start with a number.
- Variable names are case-sensitive.
- Reserved keywords cannot be used as variable names.
- Use meaningful names.

Good examples:

let firstName = "Rahul";
let totalPrice = 500;
let isLoggedIn = true;

Invalid examples:

let 1name = "Rahul";
let let = 100;

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