Great article.
I have used this approach to create a custom RxJS operator to use in Angular with HttpClient

When I receive an array of User, from an HTTP request, my custom RxJS operator can pass, as argument, one of the key of User, that must be a number and, of course, exist on User.

This is how I use the operator:

this.http.get<User[]>(‘https://jsonplaceholder.typicode.com/users'
  // 'qty' must be a number and exists on User
  .pipe(sumProps(‘qty’))

and that’s a simplified version of the operator where I use the conditional type:

type SubType<T, Condition> = Pick<T, {
  [Key in keyof T]: T[Key] extends Condition ? Key : never
}[keyof T]>;
const sumProps = 
<T, K extends keyof SubType<T, number>>(propertyName: K)
:OperatorFunction<T[], number> => {
return input$ => input$.pipe(
  switchMap(res => res),
  reduce((acc: number, user: T) => {
    return acc + +user[propertyName];
  }, 0)
)}

Thank you very much
Fabio