type literal - nberlette/is GitHub Wiki

@nick/is/type/literal

IsLiteral

Signature

export type IsLiteral<T, True = true, False = false> = [T] extends [never]
  ? False
  : [T] extends [null] ? True
  : [T] extends [undefined] ? True
  : [T] extends [boolean] ? IsBoolean<T, False, True>
  : [T] extends [number] ? number extends T ? False : True
  : [T] extends [string] ? string extends T ? False : True
  : [T] extends [bigint] ? bigint extends T ? False : True
  : [T] extends [symbol] ? symbol extends T ? False : True
  : [T] extends [Record<PropertyKey, any>]
    ? Record<PropertyKey, any> extends T ? False : True
  : T extends readonly [] | readonly [any, ...readonly any[]]
    ? number extends T["length"] ? False : True
  : [T] extends [object] ? object extends T ? False : True
  : False;

If the given type T is a literal value (meaning a string, number, boolean, bigint, symbol, object literal, or a tuple), this type will resolve to the True type parameter, which has a default value of true. Otherwise it resolves to the False type parameter, which has a default value of false.

Type Parameters
  • T
  • True (default: true)
  • False (default: false)
Category

Types

Examples

import type { IsLiteral } from "@nick/is/type";

type A1 = IsLiteral<"foo">; // true
type A2 = IsLiteral<string | 420>; // false
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