1.📝 String

A String is used to store text.

Strings can be written using:

Single quotes ' ' Double quotes " " Backticks

Example:

let name = "Tuhin"; let city = 'Kolkata'; let message = Hello JavaScript!;

You can also check the type using:

console.log(typeof name);

Output:

string

Where are Strings used?

Names

Messages

Email addresses

Addresses

Text content

User input

2. 🔢 Number

The Number data type is used for numerical values.

Example:

let age = 21;
let price = 99.99;
let temperature = -5;

JavaScript uses the same Number type for both integers and floating-point numbers.

console.log(typeof age);

Output:

number

You can perform mathematical operations:

let a = 10;
let b = 5;

console.log(a + b);
console.log(a - b);
console.log(a * b);
console.log(a / b);

3. ✅ Boolean

A Boolean can have only two values:

true
false

Example:

let isLoggedIn = true;
let isAdmin = false;

Boolean values are commonly used for conditions and decision-making.

For example:

let age = 20;

console.log(age >= 18);

Output:

true

Where are Booleans used?

• Login status

• User permissions

• Conditions

• Form validation

• On/off states

4. ❓ Undefined

A variable has the value undefined when it has been declared but no value has been assigned to it.

Example:

let result;

console.log(result);

Output:

undefined

You can check its type:

console.log(typeof result);

Output:

undefined

Important point

undefined usually means:

A value has not been assigned yet.

4. ❓ Undefined

A variable has the value undefined when it has been declared but no value has been assigned to it.

Example:

let result;

console.log(result);

Output:

undefined

You can check its type:

console.log(typeof result);

Output:

undefined

Important point

undefined usually means:

A value has not been assigned yet.

5. 🚫 Null

null represents the intentional absence of a value.

Example:

let selectedUser = null;

console.log(selectedUser);

Output:

null

The important difference is:

undefined → No value has been assigned
null      → We intentionally set the value to "nothing"

For example:

let user = null;

This can mean that there is currently no user selected.

A JavaScript quirk

typeof null

returns:

object

This is a well-known historical behavior in JavaScript

6. 🔢 BigInt

BigInt is used to represent integers larger than the maximum safe integer that JavaScript's Number type can reliably represent.

You create a BigInt by adding n at the end of an integer:

let bigNumber = 123456789012345678901234567890n;

console.log(bigNumber);

Check the type:

console.log(typeof bigNumber);

Output:

bigint

Important

You generally cannot directly mix BigInt and Number in arithmetic:

10n + 5

This causes an error.

Instead, use the same type:

10n + 5n

7. 🔑 Symbol

A Symbol creates a unique value.

Example:

let id = Symbol("id");

console.log(id);

Every Symbol is unique, even when the descriptions are the same:

let a = Symbol("id");
let b = Symbol("id");

console.log(a === b);

Output:

false

Where are Symbols useful?

They are commonly used as unique object property keys, especially when you want to avoid property-name collisions.

For beginners, remember:

Symbol represents a unique identifier.

8. 📦 Objects

An Object is used to store collections of related data.

Example:

let student = {
    name: "Tuhin",
    age: 21,
    course: "Computer Science"
};

You can access the values:

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

Output:

Tuhin
21

Objects can contain different types of values:

let user = {
    name: "Tuhin",
    age: 21,
    isLoggedIn: true
};

Here:

name       → String
age        → Number
isLoggedIn → Boolean

Objects are extremely important because they are used heavily in JavaScript applications, APIs, React, Node.js, and web development.

🧩 Primitive vs Non-Primitive Data Types

This is one of the most important concepts in Day 4.

Primitive Data Types

JavaScript has 7 primitive data types:

1. String

2. Number

3. BigInt

4. Boolean

5. Undefined

6. Null

7. Symbol

Primitive values represent individual values.

Example:

let name = "Tuhin"; let age = 21; let isStudent = true; Non-Primitive Data Types

The main non-primitive/reference type is:

Object

Objects can contain multiple values.

Example:

let student = { name: "Tuhin", age: 21, course: "Computer Science" };

Arrays and functions are also objects in JavaScript's type system.

Example:

let numbers = [10, 20, 30]; function greet() { console.log("Hello!"); } 📊 Primitive vs Non-Primitive Primitive Non-Primitive String Object Number Array Boolean Function Undefined Other object types Null BigInt Symbol Simple way to remember Primitive ↓ Single value

Non-Primitive ↓ Can represent complex/structured data

🔍 Checking Data Types with typeof

JavaScript provides the typeof operator to check a value's type.

console.log(typeof "Hello");
console.log(typeof 100);
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof 100n);

Output:

string
number
boolean
undefined
bigint

For objects:

let student = {
    name: "Tuhin"
};

console.log(typeof student);

Output:

object

Remember the special case:

console.log(typeof null);

Output:

object

This is a historical JavaScript quirk.