js-keywords and reserved words

ECMA-262 describes a set of keywords that have a specific purpose. These keywords can be used to represent the start or end of a control statement, or to perform a specific operation. By rule, keywords are also language-reserved and cannot be used as identifiers. Here are all the keywords for ECMAScript (the 5th version of the New keyword with *):

Break	Do	instanceof	typeof
Case	Else	New	Var
Catch	Finally	Return	void
Continue	For	Switch	While
debugger*	function	This	With
Default	If	Throw	Delete
Inch	Try

ECMA-262 also describes another set of reserved words that cannot be used as identifiers. Although reserved words do not have any specific purpose in this language. But they are likely to be used as keywords in the future. The following are all reserved words as defined in ECMA-262 version 3rd:

Abstract	Enum	Int	Short
Boolean	Export	Interface	Static
Byte	Extends	Long	Super
Char	Final	Native	Synchronized
Class	Float	Package	Throws
Const	Goto	Private	Transient
Debugger	Implements	Protected	Volatile
Double	Import	Public

The 5th edition reduces the reserved words when running in non-strict mode to the following:

Class	Enum	Extends	Super
Const	Export	Import

In strict mode, version 5th also imposes restrictions on the following reserved words:

Implements	Package	Public	Interface
Private	Static	Let	Protected
Yield

Note that let and yield are the new reserved words for the 5th edition, and the other reserved words are defined in version 3rd. To maximize compatibility, it is recommended that the reserved word defined in version 3rd be used as a reference when programming with let and yield.

Using keywords as identifiers in the JavaScript engine that implements ECMAScript 3 can cause "Identifier expected" errors. Using reserved words as identifiers may or may not cause the same error, depending on the particular engine.

The 5th edition made a few changes to the rules for using keywords and reserved words. Keywords and reserved words, although still not available as identifiers, can now be used as property names for objects. In general, it is best not to use keywords and reserved words as identifiers and property names for compatibility with future versions of ECMAScript.

In addition to the reserved words and keywords listed above, ECMA-262 version 5th also imposes restrictions on eval and arguments. In strict mode, these two names cannot be used as identifiers or property names, or an error is thrown.

js-keywords and reserved words