JavaScript

In JavaScript, data types represent the kinds of values we can work with and store in variables. Knowing them is essential to write bug-free code.

JavaScript has two main categories of data types:

1. Primitive Data Types

These are basic, immutable (unchangeable) data types.

Data Type	Description
String	Text data
Number	Any numeric value
Boolean	True or False
Undefined	Variable declared but not assigned a value
Null	Intentionally empty value
Symbol (ES6)	Unique and immutable value
BigInt (ES11)	Large integers beyond Number limit

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1. String – Text values

let name = "John";
let message = 'Hello';
let sentence = `Welcome, ${name}`;

• Strings can be in “double quotes”, ‘single quotes’, or template literals (backticks).
• Template literals support string interpolation using ${}.

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2. Number – Numeric values

let age = 25;
let price = 99.99;
let negative = -10;

• Includes both integers and floating-point numbers.
• JavaScript uses only one type for numbers: number.

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3. Boolean – Logical values

let isActive = true;
let isLoggedIn = false;

• Only two possible values: true or false.
• Common in conditions (if, while, etc.).

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4. Undefined – Uninitialized variable

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

• A variable declared but not assigned gets undefined.

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5. Null – Empty value

let data = null;

• Represents “no value” or “empty” on purpose.
• Useful when you want to clear a variable manually.

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6. Symbol – Unique identifiers (ES6)

let id = Symbol("userID");

• Used to create unique keys in objects, even if values are the same.
• Mostly used in advanced object handling and libraries.

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7. BigInt – Very large integers (ES11)

let bigNumber = 1234567890123456789012345678901234567890n;

• Can store numbers larger than 2^53 - 1.
• Add n at the end to declare a BigInt.

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2. Non-Primitive (Reference) Data Types

These are more complex and store references (not actual values).

Type	Description
Object	Collection of key-value pairs
Array	Ordered list of values
Function	Reusable block of code

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1. Object – Group of key-value pairs

let person = {
  name: "Alice",
  age: 30,
  isStudent: false
};

• Values are accessed using object.key or object["key"].

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2. Array – List of values

let colors = ["red", "green", "blue"];
console.log(colors[0]); // Output: red

• Arrays use indexes starting from 0.
• Can store mixed data types too: let mixed = [1, "text", true];

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3. Function – Block of reusable code

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

• Functions are also objects in JavaScript and can be passed as arguments.

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Checking Data Types

Use the typeof operator:

console.log(typeof "Hello");    // string
console.log(typeof 42);         // number
console.log(typeof true);       // boolean
console.log(typeof null);       // object (quirk in JavaScript)
console.log(typeof undefined);  // undefined
console.log(typeof []);         // object (Arrays are objects)
console.log(typeof {});         // object
console.log(typeof function(){}); // function

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Quick Summary

Data Type	Example
String	"Hello", 'Hi'
Number	123, 3.14, -99
Boolean	true, false
Undefined	let x;
Null	let x = null;
Symbol	Symbol("id")
BigInt	1234567890n
Object	{ name: "John" }
Array	[1, 2, 3]
Function	function greet() {}

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