JavaScript Keywords - Reserved Words List, Data Types & Examples

JavaScript Data Types

JavaScript is a dynamic or loosely typed language, means we don't need to specify type of the variable because it is dynamically specified by JavaScript engine based on the type of input. We just need to use var or let keyword to create variable. There are basically two types of data type in JavaScript, which are given below-

Primitive Data Types:- There are besically five types of primitive data type in JavaScript, which are given below-

Non-primitive or Reference Data Types There are besically two types of non-primitive data type in JavaScript, which are given below-

Data Types	Description
String	It represents any string (example "hello").
Number	It represents any numeric value (example 777).
Boolean	It represents boolean value (example true or false).
Undefined	It represents undefined value.
Null	It represents null value.

JavaScript Keywords

JavaScript keywords are reserved words, which we cannot use to name the variables or functions. There are total of 63 keywords which JavaSscript provides to the programmers.

Keywords, which are marked with * symbols are the new keywords in ECMAScript 5 and 6.

Keywords
abstract	arguments	boolean	break
byte	case	catch	char
const	continue	class*	debugger
default	delete	do	double
else	eval	enum*	export*
extends*	false	final	finally
float	for	function	goto
if	implements	in	instanceof
int	interface	import*	let
long	native	new	null
package	private	protected	public
return	short	static	switch
synchronized	super*	this	throw
throws	transient	true	try
typeof	var	void	volatile
while	with	yield	-

Type Checking with typeof and instanceof

Use typeof to check primitive types and instanceof to check object types.

Type Checking

// typeof - returns a string
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" (historical bug in JS!)
console.log(typeof {});           // "object"
console.log(typeof []);           // "object"
console.log(typeof function(){}); // "function"
console.log(typeof Symbol());     // "symbol"
console.log(typeof 42n);          // "bigint"

// instanceof - checks prototype chain
console.log([] instanceof Array);   // true
console.log({} instanceof Object);  // true
console.log([] instanceof Object);  // true (arrays are objects)

// Best way to check for array
console.log(Array.isArray([]));     // true
console.log(Array.isArray({}));     // false

// Null check
const val = null;
console.log(val === null);          // true (use strict equality)

ES6+ New Data Types - Symbol and BigInt

Symbol & BigInt

// Symbol - unique, immutable identifier (ES6)
const id1 = Symbol('id');
const id2 = Symbol('id');
console.log(id1 === id2); // false - every Symbol is unique

// Useful as unique object keys
const USER_ID = Symbol('userId');
const user = { [USER_ID]: 123, name: 'Alice' };
console.log(user[USER_ID]); // 123

// BigInt - integers larger than Number.MAX_SAFE_INTEGER (ES2020)
const bigNum = 9007199254740991n; // note the 'n' suffix
const bigger = bigNum + 1n;
console.log(bigger); // 9007199254740992n

// Cannot mix BigInt with regular numbers
// console.log(bigNum + 1); // TypeError
console.log(Number(bigNum) + 1); // convert first

// Practical: cryptography, database IDs, financial calculations
const MAX_SAFE = Number.MAX_SAFE_INTEGER;
console.log(MAX_SAFE + 1 === MAX_SAFE + 2); // true (precision lost!)
const bigSafe = BigInt(MAX_SAFE);
console.log(bigSafe + 1n === bigSafe + 2n);  // false (correct)

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