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src/05-forgeschrittenes-typsystem/KATA2/solution.ts

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+export {};
+
+// 1. NonNullableProps
+type Response = { success: boolean; message?: string };
+type NonNullableProps<T> = { [K in keyof T]: NonNullable<T[K]> };
+type NonNullableResponse = NonNullableProps<Response>;
+// { success: boolean; message: string; }
+
+// 2. ReadonlyExcept
+type Event = { timestamp: number; type: string };
+type ReadonlyExcept<T, K extends keyof T> = {
+  readonly [P in Exclude<keyof T, K>]: T[P];
+} & { [P in K]: T[P] };
+type ReadonlyExceptType = ReadonlyExcept<Event, 'type'>;
+// { readonly timestamp: number; type: string; }
+
+// 3. KeysOfType
+type Settings = { darkMode: boolean; isActive: boolean; fontSize: number };
+type KeysOfType<T, U> = { [K in keyof T]: T[K] extends U ? K : never }[keyof T];
+type KeysWithBoolean = KeysOfType<Settings, boolean>;
+// "darkMode" | "isActive"
+
+// 4. NumberKeys
+type Employee = { id: number; name: string; salary: number };
+type NumberKeys<T> = {
+  [K in keyof T]: T[K] extends number ? K : never;
+}[keyof T];
+type KeysWithNumbers = NumberKeys<Employee>;
+// "id" | "salary"
+
+// 5. ReturnTypeOf
+type FetchFn = () => Promise<string>;
+type ReturnTypeOf<T> = T extends (...args: any[]) => infer R ? R : never;
+type FetchResult = ReturnTypeOf<FetchFn>;
+// Promise<string>
+
+// 6. FilterStrings
+type Values = { a: string; b: number; c: boolean };
+type FilterStrings<T> = {
+  [K in keyof T as T[K] extends string ? K : never]: T[K];
+};
+type OnlyStrings = FilterStrings<Values>;
+// { a: string; }
+
+// 7. FunctionKeys
+type Button = { text: string; onClick: () => void };
+type FunctionKeys<T> = {
+  [K in keyof T]: T[K] extends Function ? K : never;
+}[keyof T];
+type FnKeys = FunctionKeys<Button>;
+
+// "onClick"
+
+// 8. ToRequired
+type OptionalFields = { foo?: string; bar?: number };
+type ToRequired<T> = { [K in keyof T]-?: T[K] };
+type RequiredFields = ToRequired<OptionalFields>;
+// { foo: string; bar: number; }
+
+// 9. NonFunctionKeys
+type Logger = { log: () => void; level: string };
+type NonFunctionKeys<T> = {
+  [K in keyof T]: T[K] extends Function ? never : K;
+}[keyof T];
+type OnlyProps = NonFunctionKeys<Logger>;
+// "level"
+
+// 10. NullableStrings
+type Product = { price: number; name: string };
+type NullableStrings<T> = {
+  [K in keyof T]: T[K] extends string ? T[K] | null : T[K];
+};
+type NullableProduct = NullableStrings<Product>;
+// { price: number; name: string | null; }