feat(typescript-types): Add IsEmptyKey<S>

packages/typescript-types/README.md

@@ -14,9 +14,12 @@ The types included in this library are categorized by their purpose.
 | [`IsNever<T>`][]                        | `true` if `T` is `never`, `false` otherwise (`null`, `undefined`, `any` also yield `false`)               |
 | [`If<Test, TrueBranch, FalseBranch>`][] | Returns `TrueBranch` if `Test` is `true`, `FalseBranch` otherwise[^if_remark].                            |
 | [`IsKeyOf<T, K>`][]                     | `true` if `K` is a key of `T`, `false` otherwise. If `T` is `any`, any `K` but `never` will yield `true`. |
+| [`IsEmptyString<S>`][]                  | `true` if `S` is the empty string `''`, `false` otherwise.[^is-empty-string_remark]                       |
 
 [^if_remark]: If `boolean` is passed as `Test` the return value is a union of both branches, i.e. `TrueBranch | FalseBranch`.
 
+[^is-empty-string_remark]: If `T` is `any` will yield `true`, as it is taken to be `any` string.
+
 [`IsAny<T>`]: src/is-any.ts
 [`IsNever<T>`]: src/is-never.ts
 [`If<Test, TrueBranch, FalseBranch>`]: src/if.ts

packages/typescript-types/src/is-empty-string.ts

@@ -0,0 +1 @@
+export type IsEmptyString<T extends string> = '' extends T ? true : false

packages/typescript-types/test/is-empty-string.tst.ts

@@ -0,0 +1,8 @@
+import type { IsEmptyString } from '@/is-empty-string.js'
+import { expect } from 'tstyche'
+
+expect<IsEmptyString<''>>().type.toBe<true>()
+expect<IsEmptyString<any>>().type.toBe<true>()
+
+expect<IsEmptyString<'nonempty'>>().type.toBe<false>()
+expect<IsEmptyString<never>>().type.toBe<false>()