neat trick for typing an object transform function

mappedGenericObject.ts

/**
  this lets you type a function that transforms a map 
  from a map with KV such that typeof V is some type A<T> 
  to a a map with KV such that typeof V is some type B<T>
  In this case, we go from DataRequest<T> to ISubscription<T>
*/

class Definition<T> {}
interface DataRequest<T> {
	def: Definition<T>;
}

interface ISubscription<T> {
	def: Definition<T>;
}

function transform<
	TRequests 
>(requests: TRequests): ({ [P in keyof TRequests]: ISubscription<ExtractType<TRequests[P]>> })
{
	return requests as any;
}

interface Project {};
interface Milestone {};
const ProjectDefinition = new Definition<Project>();
const MilestoneDefinition = new Definition<Milestone>();

const projectRequest: DataRequest<Project> = { def: ProjectDefinition };
const milestoneRequest: DataRequest<Milestone> = { def: MilestoneDefinition };
type projectRequestType = ExtractType<typeof projectRequest>;

const subs = transform({ projectRequest, milestoneRequest });

type ExtractType<T> = T extends DataRequest<infer TEntity> ? TEntity : never
type Mapped<T> = ({ [P in keyof T]: ISubscription<ExtractType<T[P]>> });