Choosing the wrong JavaScript keywords might be a reason you encounter various bugs. There is a whole list of JavaScript reserved words that you should not use, as they are used to execute the code or name JavaScript built-in objects. Some should be avoided because they come from other languages or Windows operating system.
In this tutorial, you will learn all about JavaScript reserved words. You will find extensive lists of all the words you should avoid using when creating variables, functions, and declarations.
Contents
JavaScript Reserved Words: Main Tips
- There are some identifiers in JavaScript that are reserved, thus cannot be used as new variable or function names.
- There are also words that are not forbidden to use as custom identifiers, but should be avoided because they already belong to built-in objects in JavaScript.
ECMAScript Versions
Before looking into the JavaScript reserved words, let's have a look at the release timeline of different versions of the language itself:
- ECMAScript 3 - released in 1999, December.
- ECMAScript 4 - abandoned.
- ECMAScript 5 - released in 2009, December.
- ECMAScript 6 - released in 2015, June (the latest official version of JavaScript).
As time goes by, ECMAScript 5 and 6 are finally getting the necessary support in all modern browsers.
List Of Terms to Avoid
The words specified below cannot be used as JavaScript keywords, names of variables, functions, or labels, because they are already reserved for others. For example, JavaScript yield is a keyword used to pause and resume the work of a generator function. Therefore, you can't apply this name to a new function you create.
Review the table carefully, and memorize the JavaScript reserved words:
| arguments | abstract | break | byte | boolean |
| char | const | case | class* | catch |
| debugger | delete | default | do | continue |
| double | else | eval | export* | enum* |
| false | finally | final | float | extends* |
| function | if | goto | implements | for |
| in | interface | import* | int | instanceof |
| long | null | native | new | let |
| private | return | protected | public | package |
| static | synchronized | super* | switch | short |
| throw | true | throws | transient | this |
| typeof | volatile | var | void | try |
| yield | with | while |
Note: You can see * symbol next to some of the words (for example, JavaScript enum). It marks words that are new in ECMAScript 5 and 6: for example, JavaScript super wasn't around until 2015.
It is also advised to avoid using names of JavaScript built-in objects, methods, and properties for your declarations:
| Date | hasOwnProperty | eval | function | Array |
| isFinite | length | isNaN | isPrototypeOf | Infinity |
| NaN | Object | name | Number | Math |
| String | valueOf | toString | undefined | prototype |
Java Reserved Words
Even if you're a newbie in coding, we're sure you already know the difference between JavaScript and Java. However, these two languages do tend to go hand in hand and be used together. For that reason, it may be best to avoid using Java objects and properties names for your JavaScript keywords, too.
See the list below to see the Java reserved words. You'll see it's rather short and easy to remember:
| JavaArray | java | JavaPackage | JavaObject | getClass | javaClass |
Windows and HTML Terms
As you saw in the previous section, some word usage limitations also arise from using JavaScript with other languages and systems. JavaScript may also be used outside of HTML, on the system. Therefore, try not to use Windows objects and properties names for JavaScript variables and functions:
| all | area | anchor | anchors | alert |
| blur | clearInterval | button | checkbox | assign |
| clientInformation | confirm | close | closed | clearTimeout |
| crypto | defaultStatus | decodeURI | decodeURIComponent | constructor |
| element | embeds | elements | embed | document |
| encodeURIComponent | fileUpload | escape | event | encodeURI |
| form | innerHeight | forms | frame | focus |
| layer | location | layers | link | innerWidth |
| navigate | frameRate | navigator | frames | mimeTypes |
| history | offscreenBuffering | image | images | hidden |
| opener | outerWidth | option | outerHeight | open |
| pageXOffset | parseFloat | pageYOffset | parent | packages |
| password | prompt | pkcs11 | plugin | parseInt |
| radio | screenY | reset | screenX | propertyIsEnum |
| secure | setInterval | select | self | scroll |
| status | text | submit | taint | setTimeout |
| top | window | unescape | untaint | textarea |
It is also advised not to use event handler names for custom JavaScript keywords. This will let you avoid possible errors in your code:
| onclick | onfocus | onerror | onblur |
| onkeypress | onmouseover | onkeyup | onkeydown |
| onmouseup | onsubmit | onmousedown | onload |
JavaScript Reserved Words: Summary
- There are some words (JavaScript
enum,static,public, etc.) that are considered reserved and should be avoided when declaring variables, functions and other custom declarations. - Some reserved words (like
arrayorfunction) are part of JavaScript, including built-in object properties and HTML event listeners. - Some reserved words come from Java programming language, as well as Windows operating system.