feat(typescript-types): Add isKeyOf<T,K>

packages/typescript-types/README.md

@@ -8,11 +8,12 @@ The types included in this library are categorized by their purpose.
 
 #### Test Types
 
-| Type                                    | Description                                                                                 |
-| --------------------------------------- | ------------------------------------------------------------------------------------------- |
-| [`IsAny<T>`][]                          | `true` if `T` is `any`, `false` otherwise (`null`, `undefined` also yield `false`)          |
-| [`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].              |
+| Type                                    | Description                                                                                               |
+| --------------------------------------- | --------------------------------------------------------------------------------------------------------- |
+| [`IsAny<T>`][]                          | `true` if `T` is `any`, `false` otherwise (`null`, `undefined` also yield `false`)                        |
+| [`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`. |
 
 [^if_remark]: If `boolean` is passed as `Test` the return value is a union of both branches, i.e. `TrueBranch | FalseBranch`.
 

packages/typescript-types/src/is-key-of.ts

@@ -0,0 +1,4 @@
+import type { If } from './if.js'
+import type { IsNever } from './is-never.js'
+
+export type IsKeyOf<T, K> = If<IsNever<T>, false, If<IsNever<K>, false, K extends keyof T ? true : false>>

packages/typescript-types/test/is-key-of.tst.ts

@@ -0,0 +1,32 @@
+import type { IsKeyOf } from '@/is-key-of.js'
+import { expect } from 'tstyche'
+
+interface Example1 {
+	requiredKey: number
+	optionalKey?: number
+	unknownNonOptionalKey: unknown
+	undefinedNonOptionalKey: undefined
+	neverKey: never
+}
+
+expect<IsKeyOf<Example1, 'requiredKey'>>().type.toBe<true>()
+expect<IsKeyOf<Example1, 'optionalKey'>>().type.toBe<true>()
+expect<IsKeyOf<Example1, 'unknownNonOptionalKey'>>().type.toBe<true>()
+expect<IsKeyOf<Example1, 'undefinedNonOptionalKey'>>().type.toBe<true>()
+expect<IsKeyOf<Example1, 'neverKey'>>().type.toBe<true>()
+expect<IsKeyOf<string, 'toString'>>().type.toBe<true>()
+
+expect<IsKeyOf<any, 123>>().type.toBe<true>()
+expect<IsKeyOf<any, 'asdf'>>().type.toBe<true>()
+
+expect<IsKeyOf<Example1, 'unknown'>>().type.toBe<false>()
+expect<IsKeyOf<object, 'toString'>>().type.toBe<false>()
+
+expect<IsKeyOf<Example1, never>>().type.toBe<false>()
+expect<IsKeyOf<never, 'toString'>>().type.toBe<false>()
+expect<IsKeyOf<never, 'anystring'>>().type.toBe<false>()
+expect<IsKeyOf<never, 123>>().type.toBe<false>()
+expect<IsKeyOf<any, never>>().type.toBe<false>()
+expect<IsKeyOf<undefined, 123>>().type.toBe<false>()
+
+expect<IsKeyOf<Example1, any>>().type.toBe<boolean>()