How to check if an object type's prototype is Object.prototype in Typescript?

Question

Given an arbitrary type, I want to check if the type is an object whose prototype is Object.prototype . In other words, I want to exclude Functions, Sets, Maps, custom classes, etc. I can't just list every possible object type, since there's an infinite number of custom classes. Is this possible?

I want to do this to recursively apply a mapping to an object. Here's what I have:

type MapDeep<T> = F<
  T extends Primitive ? T
  : T extends Array<infer U> ? Array<MapDeep<U>>
  : T extends ReadonlyArray<infer U> ? ReadonlyArray<MapDeep<U>>
  : T extends Set<infer U> ? Set<MapDeep<U>>
  : T extends Map<infer K, infer V> ? Map<K, MapDeep<V>>
  : T extends Partial<Record<string, any>> ? { [K in keyof T]: MapDeep<T[K]> }
  : T
>;

However, for objects that have prototypes other than Object.prototype , this doesn't work. Eg MapDeep<() => void> returns F<{}> instead of F<() => void> , since it matches Partial<Record<string, any>> . How can I tell if a type's prototype is Object.prototype ?

Answer 1

This is not possible. A typescript type does not contain information about the prototype chain of an object, it only specifies the type interface of properties and their signatures.