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JavaScript Reserved Words


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


JavaScript Standards

All modern browsers fully support ECMAScript 3 (ES3, the third edition of JavaScript from 1999).

ECMAScript 4 (ES4) was never adopted.

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

Time passes, and we are now beginning to see complete support for ES5 in all modern browsers.


JavaScript Reserved Words

In JavaScript you cannot use these reserved words as variables, labels, or function names:

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

Words marked with* are new in ECMAScript5


JavaScript Objects, Properties, and Methods

You should also avoid using the name of JavaScript built-in objects, properties, and methods:

Array Date eval function hasOwnProperty
Infinity isFinite isNaN isPrototypeOf length
Math NaN name Number Object
prototype String toString undefined valueOf


Java Reserved Words

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

getClass java JavaArray javaClass JavaObject JavaPackage


Windows Reserved Words

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

In HTML you must (for portability you should) avoid using the name of HTML and Windows objects and properties:

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 Handlers

In addition you should avoid using the name of all HTML event handlers.

Examples:

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


Nonstandard JavaScript

In addition to reserved words, there are also some nonstandard keywords used in some JavaScript implementations.

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

Const is an extension to JavaScript. It is supported by the JavaScript engine used in Firefox and Chrome. But it is not a part of the JavaScript standards ES3 or ES5. Do not use it.




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