JavaScript Reserved Words

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In JavaScript, some identifiers are reserved words and cannot be used as variables or function names.

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JavaScript Standards

All modern browsers fully support ECMAScript 3 (ES3, the third edition of JavaScript from 1999).

ECMAScript 4 (ES4) was never adopted.

ECMAScript 5 (ES5, released in 2009) is the latest official version of JavaScript.

Time passes, and we are now beginning to see complete support for ES5 in all modern browsers.

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JavaScript Reserved Words

In JavaScript you cannot use these reserved words as variables, labels, or function names:

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

Words marked with* are new in ECMAScript5

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JavaScript Objects, Properties, and Methods

You should also avoid using the name of JavaScript built-in objects, properties, and methods:

Array	Date	eval	function	hasOwnProperty
Infinity	isFinite	isNaN	isPrototypeOf	length
Math	NaN	name	Number	Object
prototype	String	toString	undefined	valueOf

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Java Reserved Words

JavaScript is often used together with Java. You should avoid using some Java objects and properties as JavaScript identifiers:

getClass

java

JavaArray

javaClass

JavaObject

JavaPackage

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Windows Reserved Words

JavaScript can be used outside HTML. It can be used as the programming language in many other applications.

In HTML you must (for portability you should) avoid using the name of HTML and Windows objects and properties:

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

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HTML Event Handlers

In addition you should avoid using the name of all HTML event handlers.

Examples:

onblur	onclick	onerror	onfocus
onkeydown	onkeypress	onkeyup	onmouseover
onload	onmouseup	onmousedown	onsubmit

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Nonstandard JavaScript

In addition to reserved words, there are also some nonstandard keywords used in some JavaScript implementations.

One example is the const keyword used to define variables. Some JavaScript engines will treat const as a synonym to var. Other engines will treat const as a definition for read-only variables.

Const is an extension to JavaScript. It is supported by the JavaScript engine used in Firefox and Chrome. But it is not a part of the JavaScript standards ES3 or ES5. Do not use it.