JavaScript - Reserved Keywords

Reserved Keywords in JavaScript

The reserved keywords in JavaScript are predefined keywords used to serve the built-in functionality of the programming language. For example, the var and let keywords are used to define variables, the function keyword is used to define the functions, etc. JavaScript contains more that fifty reserved keywords.

In simple terms, you can't use the reserved keywords as an identifier. If you do, you will get the conflicts, and the code will generate the wrong output or throw an error.

For example, the below code will throw an error as ‘function’ is used as an identifier.

var function = "Hello";

Reserved Keywords

Here is the list of reserved keywords; you can’t use them as an identifier −

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

Reserved Keywords added in ES5 and ES6

Some new keywords are added in the ES5 and ES6 versions of JavaScript. However, some are currently in use, and some keywords are reserved for future versions.

await	class	enum	export
extends	import	let	Super

Removed Reserved Keywords

Some reserved keywords are removed from JavaScript, which you can’t use to achieve a particular functionality. Still, you can’t use the keywords below as an identifier as many browsers don’t support them.

abstract	boolean	byte	char
double	final	float	goto
int	long	native	short
synchronized	throws	transient	volatile

JavaScript Objects, Properties, and Methods

You should not use the name of JavaScript built-in objects, properties, and methods names as an identifier.

JavaScript Built-in Objects

Array	ArrayBuffer	Boolean	DataView
Date	Error	eval	Float32Array
Float64Array	Function	Generator	GeneratorFunction
Int8Array	Int16Array	Int32Array	Intl
JSON	Map	Math	Number
Object	Promise	Proxy	RangeError
ReferenceError	Reflect	RegExp	Set
String	Symbol	SyntaxError	TypeError
Uint8Array	Uint8ClampedArray	Uint16Array	Uint32Array
URIError	WeakMap	WeakSet

JavaScript Built-in Properties

length

constructor

prototype

__proto__

caller

callee

JavaScript Methods

toString	shift	indexOf	split
toLocaleString	unshift	lastIndexOf	substr
valueOf	slice	includes	substring
toLocaleDateString	splice	isArray	toLowerCase
toLocaleTimeString	sort	from	toLocaleLowerCase
toLocaleString	forEach	of	toUpperCase
toFixed	map	charAt	toLocaleUpperCase
toExponential	filter	charCodeAt	trim
toPrecision	reduce	codePointAt	startsWith
concat	reduceRight	normalize	endsWith
join	every	repeat	match
pop	some	replace	test
push	find	search	reverse
findIndex	slice

However, you can explore more built-in JavaScript methods and avoid using them as an identifier.

Other Reserved Keywords

JavaScript can be used with other programming languages like HTML, Java, etc. So, you should also avoid keywords that are reserved in HTML, Java, etc.

Here is the list of other reserved keywords, and most of them are properties of the ‘window’ object.

alert	elements	frameRate	radio
all	embed	hidden	reset
anchor	embeds	history	screenX
anchors	encodeURI	image	screenY
area	encodeURIComponent	images	scroll
assign	escape	offscreenBuffering	secure
blur	event	open	select
button	fileUpload	opener	self
checkbox	focus	option	setInterval
clearInterval	form	outerHeight	setTimeout
clearTimeout	forms	outerWidth	status
clientInformation	frame	packages	submit
close	innerHeight	pageXOffset	taint
closed	innerWidth	pageYOffset	text
confirm	layer	parent	textarea
constructor	layers	parseFloat	top
crypto	link	parseInt	unescape
decodeURI	location	password	untaint
decodeURIComponent	mimeTypes	pkcs11	window
defaultStatus	navigate	plugin	document
navigator	prompt	element	frames
propertyIsEnum

HTML Event Handlers

You shouldn’t use the HTML even handlers as a variable name in JavaScript.

Here, we have listed some of the event handlers.

onclick	ondblclick	onmouseover	onmouseout
onmousemove	onkeydown	onkeyup	onkeypress
onfocus	onblur	onchange	onsubmit
onreset	onload	onunload	onresize
onscroll

In short, you should avoid using all the above keywords as a variable or function name.