JavaScript data types, JavaScript string number boolean object

Primitive Data Types

1. String – "Hello"
2. Number – 42, 3.14
3. Boolean – true or false
4. Undefined – declared but no value assigned
5. Null – intentional empty value
6. Symbol – unique identifiers
7. BigInt – for very large integers
8. (11) YouTube
9. Build a To-Do List App in Python – Console-Based Project for Beginners – 804

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Non-Primitive Data Types

• Object

javascriptCopyEditlet person = { name: "Ali", age: 25 };

• Array

javascriptCopyEditlet fruits = ["apple", "banana"];

• Function

javascriptCopyEditfunction greet() { console.log("Hi!"); }

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Type Conversion

Implicit:

javascriptCopyEditconsole.log("5" + 2); // "52"

Explicit:

javascriptCopyEditNumber("5"); // 5
String(123); // "123"

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typeof Operator

javascriptCopyEdittypeof "Hello"; // string
typeof 42; // number

🌐 JavaScript Data Types: Full Guide

JavaScript is a dynamically typed language, meaning you don’t need to specify variable types. The type is determined automatically at runtime.

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🔹 1. Primitive Data Types

Primitive types are the most basic data types in JavaScript. They are immutable and represent a single value.

🟡 a. Number

• Represents both integer and floating-point numbers.
• Includes special values: Infinity, -Infinity, and NaN (Not-a-Number).

let age = 25;
let price = 19.99;
let temperature = -5;
let notANumber = 0 / 0; // NaN

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🟡 b. String

• Represents a sequence of characters.
• Enclosed in single ('), double ("), or backticks (`\) for template literals.

let name = "Alice";
let greeting = 'Hello';
let message = `Welcome, ${name}!`; // Template literal

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🟡 c. Boolean

• Represents one of two values: true or false.

let isLoggedIn = true;
let hasPermission = false;

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🟡 d. Undefined

• A variable that has been declared but not assigned a value is undefined.

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

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🟡 e. Null

• Represents an intentional absence of any value.

let selectedItem = null;

Note: typeof null returns "object" – this is a well-known quirk in JavaScript.

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🟡 f. Symbol (ES6)

• Represents a unique and immutable identifier.
• Often used to create hidden object properties.

let sym1 = Symbol("id");
let sym2 = Symbol("id");
console.log(sym1 === sym2); // false

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🟡 g. BigInt (ES11)

• Used for arbitrarily large integers beyond the safe limit of the Number type.

let bigNumber = 9007199254740991n;
let anotherBig = BigInt("123456789012345678901234567890");

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🔸 2. Non-Primitive (Reference) Data Types

These types can store collections of values or more complex entities. They are mutable and stored by reference.

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🔷 a. Object

• The most important complex data type in JavaScript.
• Stores key-value pairs.

let person = {
  name: "John",
  age: 30,
  isEmployed: true
};

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🔷 b. Array

• A special kind of object used for storing ordered collections.

let colors = ["red", "green", "blue"];
let numbers = [1, 2, 3, 4, 5];

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🔷 c. Function

• A function is also an object in JavaScript.

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

Or as an expression:

let greet = function(name) {
  return `Hi, ${name}`;
};

Or with arrow syntax:

let greet = (name) => `Hey, ${name}`;

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🔷 d. Date, RegExp, Map, Set, WeakMap, WeakSet, etc.

These are built-in object types in JavaScript.

• Date: Work with dates and times.
• RegExp: Pattern matching with regular expressions.
• Map: Key-value pairs with any data type as keys.
• Set: Stores unique values.
• WeakMap and WeakSet: Similar to Map and Set but with weak references (used for memory management).

let now = new Date();
let pattern = /hello/i;
let map = new Map();
map.set("key1", "value1");

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🧠 Dynamic Typing Example

JavaScript allows you to change the type of a variable during runtime:

let data = 42;      // number
data = "hello";     // string
data = true;        // boolean

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🔍 typeof Operator

Use typeof to check the type of a variable:

console.log(typeof 123);        // "number"
console.log(typeof "abc");      // "string"
console.log(typeof true);       // "boolean"
console.log(typeof undefined);  // "undefined"
console.log(typeof null);       // "object" (quirk)
console.log(typeof {});         // "object"
console.log(typeof []);         // "object"
console.log(typeof function(){}); // "function"

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📌 Summary Table

Type	Category	Example	typeof result
Number	Primitive	42, 3.14, NaN	"number"
String	Primitive	"hello", 'world'	"string"
Boolean	Primitive	true, false	"boolean"
Undefined	Primitive	let x;	"undefined"
Null	Primitive	null	"object"
Symbol	Primitive	Symbol("id")	"symbol"
BigInt	Primitive	12345678901234567890n	"bigint"
Object	Reference	{ name: "Alice" }	"object"
Array	Reference	[1, 2, 3]	"object"
Function	Reference	function() {}	"function"

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✅ Best Practices

• Use const or let instead of var.
• Avoid comparing different data types with ==; use === (strict equality).
• Prefer using Array.isArray() instead of typeof for arrays.
• Be cautious with null and undefined.