[Typescript] Medium 152. IsFixedStringLiteralType

ometimes you may want to determine whether a string literal is a definite type. For example, when you want to check whether the type specified as a class identifier is a fixed string literal type.

type Action<ID extends string> = { readonly id: ID };

 

Since it must be fixed, the following types must be determined as false.

• never type
• Union of string literal types
• Template literal types with embedded string, number, bigint, boolean

Determine whether the given type S is a definite string literal type.

 

Answer:

type IsFixedNonNumericString<S> = S extends ''
  ? true
  : S extends `${infer C}${infer T}`
    ? '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' extends C
      ? false
      : IsFixedNonNumericString<T>
    : false;

type IsFixedStringLiteralType<S extends string, T = S> = [S] extends [never]
  ? false
  : S extends unknown
    ? [T] extends [S]
      ? IsFixedNonNumericString<S>
      : false
    : false;

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type testcase =
  | Expect<Equal<IsFixedStringLiteralType<'ABC'>, true>>
  | Expect<Equal<IsFixedStringLiteralType<string>, false>>
  | Expect<Equal<IsFixedStringLiteralType<'ABC' | 'DEF'>, false>>
  | Expect<Equal<IsFixedStringLiteralType<never>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${string}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${string & {}}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${number}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${bigint}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${boolean}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${true}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${false}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${null}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${undefined}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${string}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${string & {}}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${number}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${bigint}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${boolean}`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${true}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${false}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${null}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${undefined}`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${string}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${string & {}}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${number}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${bigint}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${boolean}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`${true}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${false}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${null}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`${undefined}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${string}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${string & {}}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${number}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${bigint}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${boolean}DEF`>, false>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${true}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${false}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${null}DEF`>, true>>
  | Expect<Equal<IsFixedStringLiteralType<`ABC${undefined}DEF`>, true>>
  | true