A data type defines the kind of value a variable can store.

Examples:

let name = "Samir";

let age = 21;

let isStudent = true;

"Samir" → String
21       → Number
true     → Boolean

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What are Data Types?

Data types tell JavaScript:

• What value is stored
• How it can be used
• What operations are allowed

Example:

let age = 21;

JavaScript knows that 21 is a Number, so mathematical operations can be performed on it.

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JavaScript Data Types

JavaScript has two categories of data types.

Primitive Types

• String
• Number
• Boolean
• Undefined
• Null
• BigInt
• Symbol

Non-Primitive Type

• Object

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Data Types Overview

Data Type	Example
String	"Hello"
Number	100
Boolean	true
Undefined	undefined
Null	null
BigInt	123n
Symbol	Symbol()
Object	{}

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typeof Operator

The typeof operator returns the type of a value.

Syntax:

typeof value

Example:

console.log(typeof "Hello");

Output:

string

---

String

Strings represent textual data.

Strings can be written using:

Double quotes:

let name = "Samir";

Single quotes:

let city = 'Kathmandu';

Template literals:

let country = `Nepal`;

Example:

let message = "Hello World";

console.log(message);

Output:

Hello World

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String Concatenation

Strings can be joined together using the + operator.

let firstName = "Samir";

let lastName = "Niroula";

let fullName =
firstName + " " + lastName;

console.log(fullName);

Output:

Samir Niroula

---

Template Literals

The modern way to combine strings is with template literals.

let name = "Samir";

console.log(`Hello ${name}`);

Output:

Hello Samir

---

typeof String

typeof "Hello";

Output:

string

---

Number

Numbers represent both whole numbers and decimal numbers.

Integer:

let age = 21;

Decimal:

let price = 99.99;

---

Mathematical Operations

let a = 10;

let b = 5;

console.log(a + b);

Output:

15

Other operations:

10 - 5;

10 * 5;

10 / 5;

10 % 3;

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Special Numbers

Infinity

console.log(1 / 0);

Output:

Infinity

NaN

console.log("Hello" * 5);

Output:

NaN

NaN stands for Not a Number.

---

typeof Number

typeof 100;

Output:

number

---

Boolean

A Boolean represents logical values.

A Boolean has only two possible values:

• true
• false

Example:

let isLoggedIn = true;

let isAdmin = false;

Example:

let isLoggedIn = true;

if (isLoggedIn) {

    console.log("Welcome");

}

Output:

Welcome

---

typeof Boolean

typeof true;

Output:

boolean

---

Undefined

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

Example:

let name;

console.log(name);

Output:

undefined

Visual:

Variable Exists

↓

Value Missing

---

typeof Undefined

typeof undefined;

Output:

undefined

---

Null

null represents an intentional absence of a value.

Example:

let user = null;

Meaning:

No User Exists Yet

Another example:

let selectedProduct = null;

---

Null vs Undefined

Undefined:

let name;

Meaning:

Value Not Assigned

Null:

let name = null;

Meaning:

Value Intentionally Empty

---

typeof Null

typeof null;

Output:

object

This is a historical bug in JavaScript that has been kept for compatibility.

---

BigInt

BigInt is used to store extremely large integers.

Example:

const big =
123456789012345678901234567890n;

Notice the n at the end.

---

Why BigInt?

Regular numbers have a maximum safe value.

Number.MAX_SAFE_INTEGER

Output:

9007199254740991

Values larger than this may lose precision.

Use BigInt instead.

Example:

const big =
999999999999999999999999999n;

console.log(big);

---

typeof BigInt

typeof 100n;

Output:

bigint

---

Symbol

A Symbol creates a unique value.

Every Symbol is unique.

Example:

const id1 =
Symbol();

const id2 =
Symbol();

console.log(id1 === id2);

Output:

false

Even though both Symbols are empty.

---

Symbol with Description

const userId =
Symbol("id");

Common uses:

• Unique IDs
• Object keys
• Library development

---

typeof Symbol

typeof Symbol();

Output:

symbol

---

Object

Objects store collections of related data.

Example:

const user = {

    name: "Samir",

    age: 21,

    isStudent: true

};

Visual:

user

├── name

├── age

└── isStudent

---

Accessing Properties

console.log(user.name);

Output:

Samir

Another example:

const car = {

    brand: "Toyota",

    model: "Corolla"

};

console.log(car.brand);

Output:

Toyota

---

Arrays are Objects

const fruits = [

    "Apple",

    "Mango"

];

typeof fruits;

Output:

object

---

Functions are Objects

function greet() {

}

typeof greet;

Output:

function

Functions are a special type of object.

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Primitive vs Object

Primitive values are stored directly.

let name = "Samir";

Objects are stored by reference.

const user = {

    name: "Samir"

};

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Complete Example

let name = "Samir";

let age = 21;

let isStudent = true;

let score;

let product = null;

let big =
999999999999999n;

let id = Symbol();

const user = {

    name: "Samir",

    age: 21

};

---

typeof Examples

typeof "Hello";

Output:

string

typeof 100;

Output:

number

typeof true;

Output:

boolean

typeof undefined;

Output:

undefined

typeof null;

Output:

object

typeof 100n;

Output:

bigint

typeof Symbol();

Output:

symbol

typeof {};

Output:

object

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Data Types Summary

Data Type	Example	typeof Result
String	"Hello"	"string"
Number	100	"number"
Boolean	true	"boolean"
Undefined	undefined	"undefined"
Null	null	"object"
BigInt	100n	"bigint"
Symbol	Symbol()	"symbol"
Object	{}	"object"

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Memory Trick

Primitive Types:

String

Number

Boolean

Undefined

Null

BigInt

Symbol

↓

SNBUNBS

Everything else is an Object.

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Key Takeaway

JavaScript has 7 primitive data types and 1 major non-primitive data type.

Primitive Types:

• String
• Number
• Boolean
• Undefined
• Null
• BigInt
• Symbol

Non-Primitive Type:

• Object

Understanding data types is important because every variable in JavaScript stores one of these types, and the available operations depend on the type of value being stored.