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---
title: 'SyntaxError: "x" is a reserved identifier'
slug: Web/JavaScript/Reference/Errors/Reserved_identifier
page-type: javascript-error
---
{{jsSidebar("Errors")}}
The JavaScript exception "_variable_ is a reserved identifier" occurs
when [reserved keywords](/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#keywords) are used as identifiers.
## Message
```plain
SyntaxError: Unexpected reserved word (V8-based)
SyntaxError: implements is a reserved identifier (Firefox)
SyntaxError: Cannot use the reserved word 'implements' as a variable name. (Safari)
```
## Error type
{{jsxref("SyntaxError")}}
## What went wrong?
[Reserved keywords](/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#keywords) will throw in
if they are used as identifiers. These are reserved in
strict mode and sloppy mode:
- `enum`
The following are only reserved when they are found in strict mode code:
- `implements`
- `interface`
- {{jsxref("Statements/let", "let")}}
- `package`
- `private`
- `protected`
- `public`
- `static`
## Examples
### Strict and non-strict reserved keywords
The `enum` identifier is generally reserved.
```js-nolint example-bad
const enum = { RED: 0, GREEN: 1, BLUE: 2 };
// SyntaxError: enum is a reserved identifier
```
In strict mode code, more identifiers are reserved.
```js-nolint example-bad
"use strict";
const package = ["potatoes", "rice", "fries"];
// SyntaxError: package is a reserved identifier
```
You'll need to rename these variables.
```js example-good
const colorEnum = { RED: 0, GREEN: 1, BLUE: 2 };
const list = ["potatoes", "rice", "fries"];
```
### Update older browsers
If you are using an older browser that does not yet implement
[`let`](/en-US/docs/Web/JavaScript/Reference/Statements/let) or
[`class`](/en-US/docs/Web/JavaScript/Reference/Statements/class),
for example, you should update to a more recent browser version that does support these
new language features.
```js
"use strict";
class DocArchiver {}
// SyntaxError: class is a reserved identifier
// (throws in older browsers only, e.g. Firefox 44 and older)
```
## See also
- [Lexical grammar](/en-US/docs/Web/JavaScript/Reference/Lexical_grammar)