JavaScript reserved keywords

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

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JavaScript Standards

All modern browsers fully support ECMAScript 3 (ES3, the third version of JavaScript, starting in 1999).

ECMAScript 4 (ES4) failed.

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

Over time, we are starting to see that all modern browsers have full support for ES5.

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JavaScript reserved keywords

Javascript reserved keywords cannot be used as variable, label or function names. Some reserved keywords are used by Javascript for future extensions.

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

* The marked keyword is newly added in ECMAScript5.

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JavaScript Objects, Properties, and Methods

You should also avoid using the names of built-in JavaScript objects, properties, and methods as JavaScript variable or function names:

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 Keyword

JavaScript Regular Used with Java. There are some Java objects and properties that you should avoid using as JavaScript identifiers:

getClassjavaJavaArray javaClassJavaObjectJavaPackage

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Windows Reserved Keywords

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

In HTML, you must (and for portability, you should) avoid using the names of HTML and Windows objects and properties as Javascript variable and function names:

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 handler

In addition, you should also avoid using the name of the HTML event handler as the name of Javascript variables and functions .

Example:

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Non-standard JavaScript

In addition to reserved keywords, there are also some non-standard keywords in JavaScript implementation.

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

Const is a JavaScript extension. The JavaScript engine supports its use in Firefox and Chrome. But it's not the JavaScript standard ES3 or components of ES5. Recommendation: Do not use it.

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