JavaScript reserved Keywords

JavaScript reserved Keywords

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

All modern browsers fully support ECMAScript 3 (Es3,javascript's third edition, starting from 1999).

ECMAScript 4 (ES4) was not approved.

ECMAScript 5 (released in es5,2009) is the latest official version of JavaScript.

As time goes on, we begin to see that all modern browsers have fully supported ES5.

JavaScript reserved Keywords

A reserved keyword for Javascript may not be used as a variable, label, or function name. Some reserved keywords are used as Javascript for later expansion.

Abstract	Arguments	Boolean	Break	Byte
Case	Catch	Char	class*	Const
Continue	Debugger	Default	Delete	Todo
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 keyword for the tag is newly added in ECMAScript5. JavaScript objects, properties, and methods

You should also avoid using JavaScript-built names of objects, properties, and methods as a variable or function name for javascript:

Array	Date	Eval	function	hasOwnProperty
Infinity	Isfinite	isNaN	isPrototypeOf	Length
Math	NaN	Name	Number	Object
Prototype	String	Tostring	Undefined	valueof

Java reserved Keywords

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

GetClass

Java

Javaarray

Javaclass

Javaobject

Javapackage

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 (you should also do this for portability) avoid using the names of HTML and Windows objects and properties as Javascript variables 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

HTML Event Handle

In addition, you should avoid using the name of an HTML event handle as a Javascript variable and function name.

Instance:

Onblur	OnClick	OnError	onfocus
OnKeyDown	onkeypress	OnKeyUp	onmouseover
OnLoad	OnMouseUp	OnMouseDown	OnSubmit

non-standard JavaScript

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

One instance is the const keyword, which is used to define variables. Some JavaScript engines treat const as synonymous with Var. Other engines define the const as a read-only variable.

Const is an extension of JavaScript. The JavaScript engine supports it in Firefox and Chrome. But it is not part of JavaScript standard ES3 or ES5. Recommendation: Do not use it .