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Reserved Keywords in JavaScript
The reserved keywords in JavaScript are predefined keywords used to serve the built-in functionality of the programming language. For example, the var and let keywords are used to define variables, the function keyword is used to define the functions, etc. JavaScript contains more that fifty reserved keywords.
In simple terms, you can”t use the reserved keywords as an identifier. If you do, you will get the conflicts, and the code will generate the wrong output or throw an error.
For example, the below code will throw an error as ‘function’ is used as an identifier.
var function = "Hello";
Reserved Keywords
Here is the list of reserved keywords; you can’t use them as an identifier −
| abstract | double | implements | return |
| arguments | else | in | switch |
| await | enum | instanceof | synchronized |
| boolean | eval | int | this |
| break | export | interface | throw |
| byte | extends | let | throws |
| case | false | long | transient |
| catch | final | native | true |
| char | finally | new | try |
| class | float | null | typeof |
| const | for | package | var |
| continue | function | private | void |
| debugger | goto | protected | volatile |
| default | if | public | yield |
| delete | implements | short | while |
| do | import | static | with |
| double | in | super |
Reserved Keywords added in ES5 and ES6
Some new keywords are added in the ES5 and ES6 versions of JavaScript. However, some are currently in use, and some keywords are reserved for future versions.
| await | class | enum | export |
| extends | import | let | Super |
Removed Reserved Keywords
Some reserved keywords are removed from JavaScript, which you can’t use to achieve a particular functionality. Still, you can’t use the keywords below as an identifier as many browsers don’t support them.
| abstract | boolean | byte | char |
| double | final | float | goto |
| int | long | native | short |
| synchronized | throws | transient | volatile |
JavaScript Objects, Properties, and Methods
You should not use the name of JavaScript built-in objects, properties, and methods names as an identifier.
JavaScript Built-in Objects
| Array | ArrayBuffer | Boolean | DataView |
| Date | Error | eval | Float32Array |
| Float64Array | Function | Generator | GeneratorFunction |
| Int8Array | Int16Array | Int32Array | Intl |
| JSON | Map | Math | Number |
| Object | Promise | Proxy | RangeError |
| ReferenceError | Reflect | RegExp | Set |
| String | Symbol | SyntaxError | TypeError |
| Uint8Array | Uint8ClampedArray | Uint16Array | Uint32Array |
| URIError | WeakMap | WeakSet |
JavaScript Built-in Properties
| length | constructor | prototype | __proto__ | caller | callee |
JavaScript Methods
| toString | shift | indexOf | split |
| toLocaleString | unshift | lastIndexOf | substr |
| valueOf | slice | includes | substring |
| toLocaleDateString | splice | isArray | toLowerCase |
| toLocaleTimeString | sort | from | toLocaleLowerCase |
| toLocaleString | forEach | of | toUpperCase |
| toFixed | map | charAt | toLocaleUpperCase |
| toExponential | filter | charCodeAt | trim |
| toPrecision | reduce | codePointAt | startsWith |
| concat | reduceRight | normalize | endsWith |
| join | every | repeat | match |
| pop | some | replace | test |
| push | find | search | reverse |
| findIndex | slice |
However, you can explore more built-in JavaScript methods and avoid using them as an identifier.
Other Reserved Keywords
JavaScript can be used with other programming languages like HTML, Java, etc. So, you should also avoid keywords that are reserved in HTML, Java, etc.
Here is the list of other reserved keywords, and most of them are properties of the ‘window’ object.
| alert | elements | frameRate | radio |
| all | embed | hidden | reset |
| anchor | embeds | history | screenX |
| anchors | encodeURI | image | screenY |
| area | encodeURIComponent | images | scroll |
| assign | escape | offscreenBuffering | secure |
| blur | event | open | select |
| button | fileUpload | opener | self |
| checkbox | focus | option | setInterval |
| clearInterval | form | outerHeight | setTimeout |
| clearTimeout | forms | outerWidth | status |
| clientInformation | frame | packages | submit |
| close | innerHeight | pageXOffset | taint |
| closed | innerWidth | pageYOffset | text |
| confirm | layer | parent | textarea |
| constructor | layers | parseFloat | top |
| crypto | link | parseInt | unescape |
| decodeURI | location | password | untaint |
| decodeURIComponent | mimeTypes | pkcs11 | window |
| defaultStatus | navigate | plugin | document |
| navigator | prompt | element | frames |
| propertyIsEnum |
HTML Event Handlers
You shouldn’t use the HTML even handlers as a variable name in JavaScript.
Here, we have listed some of the event handlers.
| onclick | ondblclick | onmouseover | onmouseout |
| onmousemove | onkeydown | onkeyup | onkeypress |
| onfocus | onblur | onchange | onsubmit |
| onreset | onload | onunload | onresize |
| onscroll |
In short, you should avoid using all the above keywords as a variable or function name.
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