2.8 Reserved Words

There are a number of reserved words in JavaScript. These are words that you cannot use as identifiers (variable names, function names, and loop labels) in your JavaScript programs. Table 2-1 lists the keywords standardized by ECMAScript v3. These words have special meaning to JavaScript -- they are part of the language syntax itself.

Table 2-1. Reserved JavaScript keywords

break	do	if	switch	typeof
case	else	in	this	var
catch	false	instanceof	throw	void
continue	finally	new	true	while
default	for	null	try	with
delete	function	return

Table 2-2 lists other reserved keywords. These words are not currently used in JavaScript, but they are reserved by ECMAScript v3 as possible future extensions to the language.

Table 2-2. Words reserved for ECMA extensions

abstract	double	goto	native	static
boolean	enum	implements	package	super
byte	export	import	private	synchronized
char	extends	int	protected	throws
class	final	interface	public	transient
const	float	long	short	volatile
debugger

In addition to some of the formally reserved words just listed, current drafts of the ECMAScript v4 standard are contemplating the use of the keywords as, is, namespace, and use. Current JavaScript interpreters will not prevent you from using these four words as identifiers, but you should avoid them anyway.

You should also avoid using as identifiers the names of global variables and functions that are predefined by JavaScript. If you create variables or functions with these names, either you will get an error (if the property is read-only) or you will redefine the existing variable or function -- something you should not do unless you know exactly what you're doing. Table 2-3 lists global variables and functions defined by the ECMAScript v3 standard. Specific implementations may define other global properties, and each specific JavaScript embedding (client-side, server-side, etc.) will have its own extensive list of global properties.^[2]

^[2] See the Window object in the client-side reference section of this book for a list of the additional global variables and functions defined by client-side JavaScript.

Table 2-3. Other identifiers to avoid

arguments	encodeURI	Infinity	Object	String
Array	Error	isFinite	parseFloat	SyntaxError
Boolean	escape	isNaN	parseInt	TypeError
Date	eval	Math	RangeError	undefined
decodeURI	EvalError	NaN	ReferenceError	unescape
decodeURIComponent	Function	Number	RegExp	URIError