Data Types

JavaScript has 8 basic data types, divided into primitive and reference (non-primitive) types.

1. Primitive Types (Immutable)

Stored directly in the variable.

a) String

Text data.

let name = "Henry";

b) Number

Both integers and floats.

let age = 15;
let pi = 3.14;

c) Boolean

Logical values.

let isOnline = true;

d) undefined

A variable declared but not assigned.

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

e) null

Intentional absence of value.

let data = null;

f) Symbol (ES6)

Unique and immutable identifier.

let sym = Symbol("id");

g) BigInt (ES2020)

Handles large integers beyond Number.MAX_SAFE_INTEGER.

let big = 1234567890123456789012345678901234567890n;

2. Reference Types (Objects)

Stored as references in memory.

a) Object

Collection of key-value pairs.

let user = {
  name: "Aman",
  age: 15
};

b) Array

Indexed list of values.

let scores = [90, 85, 100];

c) Function

Functions are objects with callable behavior.

function greet() {
  console.log("Hi");
}

3. Type Checking

Use typeof to check types:

typeof "hello";     // "string"
typeof 42;          // "number"
typeof null;        // "object" (bug in JS)
typeof [];          // "object"
typeof undefined;   // "undefined"

To check arrays:

Array.isArray([1, 2, 3]); // true

4. Dynamic Typing

JavaScript is dynamically typed, so variables can change type at runtime.

let x = "hello";
x = 123; // valid