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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.[4]

[4]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



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