A Data Type defines the kind of data a variable can hold, such as numbers, strings, booleans, or complex objects.

JavaScript is a dynamically typed language, which means variables can hold any type of data and types are determined at runtime.

JavaScript data types are broadly classified into two categories:

These are immutable (cannot be altered) and hold single values.

Primitive types are:

const age = 25;
const pi = 3.14;
console.log(typeof age); // number

✅ Interview Tip: JavaScript has only one number type (both integers and floats).

const name = "Arun";
const message = `Welcome, ${name}`;
console.log(typeof message); // string

✅ Use backticks (`) for template literals to interpolate variables.

let isLoggedIn = true;
if (isLoggedIn) {
    console.log("Dashboard shown");
}

✅ Used heavily in conditional logic (if, while, for).

let address = null;
console.log(typeof address); // ⚠️ object (this is a JavaScript bug/quirk)

✅ null represents an intentional "no value".

let phone;
console.log(typeof phone); // undefined

✅ A variable declared but not initialized is automatically undefined.

const id = Symbol('userId');
const anotherId = Symbol('userId');
console.log(id === anotherId); // false

✅ Each Symbol() call returns a unique value, used as hidden keys in objects.

const big = 1234567890123456789012345678901234567890n;
console.log(typeof big); // bigint

✅ Used when numbers exceed Number.MAX_SAFE_INTEGER.

These store multiple values or collections and are mutable.

These are stored by reference in memory.

const fruits = ["Apple", "Banana", "Cherry"];
fruits.push("Mango");
console.log(fruits); // ["Apple", "Banana", "Cherry", "Mango"]

✅ Arrays can hold mixed types, are zero-indexed, and have many useful methods (push, map, filter, etc.).

const user = {
    name: "Arun",
    age: 24,
    isVerified: true,
    skills: ["JS", "React"]
};
console.log(user.name); // Arun

✅ Objects are the backbone of JavaScript — everything is either a primitive or object.

function greet(name) {
    return `Hello, ${name}!`;
}
console.log(greet("Arun")); // Hello, Arun!

✅ Functions are first-class citizens in JS. They can be stored in variables, passed as arguments, and returned from other functions.

1. Primitive values are compared by value.
2. Objects/arrays/functions are compared by reference.
3. typeof null === "object" is a JS bug (well known).
4. Array and Function are special types of objects.
5. Use Array.isArray() to check for arrays (not typeof).