TypeScript · Const

unique

unique symbol gives each symbol a declaration-specific identity. It is useful for strongly typed tokens and symbol-keyed APIs where distinct symbols must remain incompatible.

Since: TypeScript 2.7

How it works

The unique symbol type gives a symbol a declaration-specific identity rather than treating it as interchangeable with every other symbol. It is restricted to const declarations and readonly static properties; use typeof the declaration when referring to that particular symbol type.

Syntax

const name: unique symbol = Symbol();

Key points

unique symbol represents a symbol as a distinct literal-like type.
Every unique symbol type is also a symbol type, so it can be used where symbol is expected.
Identity comes from the declaration, allowing separate constants to act as nominally distinct keys or tokens.
Values can originate from Symbol or Symbol.for calls, or from an explicit unique-symbol annotation.
A variable with this type must be immutable through a const declaration.

Common mistakes

A variable explicitly annotated as unique symbol must be const; the other supported declaration form is a readonly static property.
Different unique symbols cannot be assigned to one another or compared as equivalent types.
Use typeof on the symbol declaration to refer to its exact unique-symbol type.

Example: Distinct symbol identities

const RED: unique symbol = Symbol("red");
const BLUE: unique symbol = Symbol("blue");

type Color = typeof RED | typeof BLUE;

function nameOf(color: Color): string {
  return color === RED ? "red" : "blue";
}

console.log(nameOf(RED));

Each declared constant creates a distinct symbol type, and typeof retrieves that declaration-specific type for the union.

When to use it

Nominal tokens

Define private or collision-resistant capability tokens shared across modules.

Typed symbol keys

Model symbol-keyed properties with exact, declaration-specific keys.

Unique discriminants

Create discriminants that cannot accidentally overlap with other symbols.

symbol
typeof
readonly static properties

Source: Symbols. Checked 2026-10-15.