Jest/TS: Mock a dependency of the entity under test

eggs.test.ts

import { eggs, eggsAsync } from "../src/eggs";
import { spam, spamAsync } from "../src/spam";

jest.mock("../src/spam");

describe("Synchronous Implementation", () => {
  describe("non-mocked", () => {
    beforeEach(() => { (spam as jest.Mock).mockImplementationOnce((a: number) => 10 * a) });
    
    it("performs some calculations", () => {
      expect(eggs(3)).toEqual(72);
    });
  });

  describe("mocked", () => {
    beforeEach(() => { (spam as jest.Mock).mockImplementationOnce(() => 100) });

    it("performs some calculations", () => {
      expect(eggs(3)).toEqual(142);
    });
  });
});

describe("Asynchronous Implementation", () => {
  describe("non-mocked", () => {
    beforeEach(() => { (spamAsync as jest.Mock).mockResolvedValue(30) });

    it("performs some asynchronous calculations", async() => {
      expect(eggsAsync(3)).resolves.toEqual(72);
    });
  });

  describe("mocked", () => {
    beforeEach(() => { (spamAsync as jest.Mock).mockResolvedValue(100) });

    it("performs some asynchronous calculations", async() => {
      expect(eggsAsync(3)).resolves.toEqual(142);
    });
  });
});

eggs.ts

import { spam, spamAsync } from "./spam";

export const eggs = (a: number) => 42 + spam(a);

export const eggsAsync = async (a: number) =>
  spamAsync(a).then(res => res + 42);

spam.ts

export const spam = (a: number): number => 10 * a;

export const spamAsync = async (a: number): Promise<number> => spam(a);

jest.mock("../src/spam") has to be at the very top (or at least before everything) and once used, all the tests will use the mock, not the real implementation.