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