JavaScript Data Types

1. What are Data Types?

A data type defines what kind of value a variable can hold. Every value in JavaScript has a type.

There are two main categories:

1. Primitive (basic) types

2. Non-primitive (reference) types

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2. Primitive Data Types

These are simple, store single values, and are immutable (cannot be changed).

a) Number

Represents numbers (integers or decimals).

let age = 23;
let price = 19.99;

b) String

Represents text. Can use ' ', " ", or ` `.

let name = "Adib";
let message = `Hi, my name is ${name}`;

c) Boolean

Represents true or false.

let isStudent = true;

d) Undefined

Variable is declared but not assigned.

let x;
console.log(x); // undefined

e) Null

Represents no value.

let y = null;

f) Symbol (ES6+)

Represents a unique value.

let id = Symbol('id');

g) BigInt (ES2020+)

Represents very large numbers.

let bigNumber = 12345678901234567890n;

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3. Non-Primitive (Reference) Types

These can store multiple values and are mutable (can be changed).

a) Object

Stores key-value pairs.

let person = { name: "Adib", age: 23 };

b) Array

Stores lists of values.

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

c) Function

Stores reusable blocks of code.

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

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4. Checking Data Types

Use typeof to check the type of a variable:

console.log(typeof 25);         // "number"
console.log(typeof "Adib");     // "string"
console.log(typeof true);       // "boolean"
console.log(typeof undefined);  // "undefined"
console.log(typeof null);       // "object" (JavaScript quirk)
console.log(typeof Symbol());   // "symbol"
console.log(typeof 10n);        // "bigint"
console.log(typeof {});         // "object"
console.log(typeof []);         // "object" (array is an object)
console.log(typeof function(){}); // "function"

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5. Quick Summary Table

Data Type	Example	Notes
Number	23, 19.99	Integers & decimals
String	"Hello", 'Hi'	Text
Boolean	true, false	True/False values
Undefined	let x;	Not assigned
Null	let x = null;	Empty value
Symbol	Symbol('id')	Unique value
BigInt	12345678901234n	Very large numbers
Object	{name: "Adib"}	Key-value pairs
Array	[1,2,3]	List of values
Function	function(){}	Reusable code block