FFS

eggs.test.ts

import { eggs } from "../src/eggs";
import { spam } from "../src/spam";

describe("normal", () => {
  it("does some calculations", () => {
    expect(eggs(3)).toEqual(72);
  });
});

describe("mocked", () => {
  // Override `spam` to return 100

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

eggs.ts

import { spam } from "./spam";

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

spam.ts

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