prettier/tests/typescript_conditional_types/conditonal-types.ts

export type DeepReadonly<T> = T extends any[] ? DeepReadonlyArray<T[number]> : T extends object ? DeepReadonlyObject<T> : T;

type NonFunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];

interface DeepReadonlyArray<T> extends ReadonlyArray<DeepReadonly<T>> {}

type DeepReadonlyObject<T> = {
    readonly [P in NonFunctionPropertyNames<T>]: DeepReadonly<T[P]>;
};

type TypeName<T> =
  T extends string ? "string" :
  T extends number ? "number" :
  T extends boolean ? "boolean" :
  T extends undefined ? "undefined" :
  T extends Function ? "function" :
  "object";