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SyntaxError: "x" is a reserved identifier

The JavaScript exception "variable is a reserved identifier" occurs when reserved keywords are used as identifiers.

Message​

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​

What went wrong?​

Reserved 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.

const enum = { RED: 0, GREEN: 1, BLUE: 2 };
// SyntaxError: enum is a reserved identifier

In strict mode code, more identifiers are reserved.

"use strict";
const package = ["potatoes", "rice", "fries"];
// SyntaxError: package is a reserved identifier

You'll need to rename these variables.

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 or class, for example, you should update to a more recent browser version that does support these new language features.

"use strict";
class DocArchiver {}

// SyntaxError: class is a reserved identifier
// (throws in older browsers only, e.g. Firefox 44 and older)

See also​