JavaScript Data Types

In JavaScript, data types define the type of value a variable can hold. JavaScript is dynamically typed, meaning a variable can hold different types of values at different times.

1️⃣ Primitive Data Types (Immutable & Stored by Value)

JavaScript has 7 primitive data types:

Data Type	Example	Description
Number	`let x = 10;`	Stores integers & floating-point numbers.
String	`let name = "Alice";`	Text enclosed in quotes (`" "` or `' '` or ```````).
Boolean	`let isReady = true;`	`true` or `false` values.
Undefined	`let y;`	A variable declared but not assigned a value.
Null	`let z = null;`	Represents "nothing" or an empty value.
BigInt	`let bigNum = 12345678901234567890n;`	Used for very large numbers.
Symbol	`let sym = Symbol("id");`	Unique identifiers (mostly for objects).

🔹 Example

let age = 25; // Number
let username = "John"; // String
let isLoggedIn = false; // Boolean
let notAssigned; // Undefined
let emptyValue = null; // Null
let bigNumber = 12345678901234567890n; // BigInt
let uniqueId = Symbol("id"); // Symbol

2️⃣ Non-Primitive (Reference) Data Types

These types store references to objects in memory.

Data Type	Example	Description
Object	`let person = {name: "John", age: 30};`	Key-value pairs.
Array	`let fruits = ["apple", "banana", "cherry"];`	Ordered list of values.
Function	`function greet() { return "Hello"; }`	A block of code that can be executed later.

🔹 Example

let person = { name: "Alice", age: 25 }; // Object
let numbers = [1, 2, 3, 4]; // Array
function sayHello() { console.log("Hello!"); } // Function

3️⃣ typeof Operator (Checking Data Types)

You can check a variable's data type using typeof.

console.log(typeof 42); // "number"
console.log(typeof "Hello"); // "string"
console.log(typeof true); // "boolean"
console.log(typeof undefined); // "undefined"
console.log(typeof null); // "object" (This is a known JavaScript bug!)
console.log(typeof Symbol("id")); // "symbol"
console.log(typeof { key: "value" }); // "object"
console.log(typeof [1, 2, 3]); // "object" (Arrays are objects)
console.log(typeof function() {}); // "function"

4️⃣ Special Cases

❗ null is an object (typeof null returns "object"). This is a long-standing JavaScript bug.
❗ Arrays are also objects, but they have additional properties.
❗ Functions are objects with executable code.

🎯 Summary

✅ Primitive types: Number, String, Boolean, Undefined, Null, BigInt, Symbol
✅ Non-primitive types: Object, Array, Function
✅ Use typeof to check a variable's type
✅ Objects and arrays are stored as references in memory

🚀 Now you're ready to handle data types like a pro! Let me know if you need more details. 😊

Souy Soeng

JavaScript Data Types