JavaScript Data Types Explained

In JavaScript, data types define the kind of data a variable can hold. Understanding data types helps you write better, bug-free code. Let's explore the two main categories: Primitive and Reference types.

1. Primitive Data Types

Primitive types are basic, immutable values stored directly in memory. JavaScript has 7 of them:

• String
• Number
• Boolean
• Null
• Undefined
• Symbol (ES6)
• BigInt (ES11)

🔤 String

Used for textual data.

let name = "Alice";
let greeting = 'Hello!';
let message = `Hi, ${name}`;

🔢 Number

Used for both integers and decimals.

let age = 30;
let price = 99.99;

✅ Boolean

Represents either true or false.

let isLoggedIn = true;
let isAdmin = false;

❌ Null

Intentional absence of any value.

let user = null;

🌀 Undefined

A variable declared but not assigned a value.

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

🔣 Symbol

A unique and immutable primitive value, often used as object keys.

const id = Symbol("id");
const id2 = Symbol("id");
console.log(id === id2); // false

🔢 BigInt

Used for very large integers beyond the safe integer limit.

const bigNumber = 1234567890123456789012345678901234567890n;

2. Reference Data Types

Reference types are objects stored as references in memory.

• Object
• Array
• Function
• Date, RegExp, etc.

📦 Object

const user = {
  name: "Alice",
  age: 25
};

📚 Array

const colors = ["red", "green", "blue"];

🔧 Function

function greet(name) {
  return `Hello, ${name}`;
}

3. Checking Data Types

Use the typeof operator to check a variable's type.

typeof "hello"     // string
typeof 123         // number
typeof false       // boolean
typeof undefined   // undefined
typeof null        // object (quirk!)
typeof {}          // object
typeof []          // object
typeof function(){} // function

4. Summary Table