Value mocking example using type inference in conditional types

main.ts

type RT<F> = F extends (...args: any[]) => infer R ? R : never;

interface Proxy<T, K extends keyof T> {
  returnValue(value: RT<T[K]>): this;
}
declare function spyOn<T, K extends keyof T>(obj: T, methodName: K): {
  and: Proxy<T, K>;
};

const serviceToTest = {
  methodToBeMocked() {
    return "SomeStringHardToFetch";
  }
};

spyOn(serviceToTest, "methodToBeMocked").and.returnValue("fake value");
// spyOn(serviceToTest, "methodToBeMocked").and.returnValue(10000); // Compilation Error