Sample Jest testing with modules

gateway.js

class Axios {
  post() {
    return true;
  }
}

class Gateway {
  constructor() {
    this.api = new Axios();
  }
}

exports.default = new Gateway();

sample.js

const Gateway = require("./gateway").default;

const main = () => {
  const result = Gateway.api.post();
  if (!result) {
    throw new Error("failed");
  }

  return;
};

module.exports = main;

sample.test.js

describe("Sample test", () => {
  it("test failure", () => {
    jest.mock("./gateway");
    const mock = jest.fn(() => false);
    require("./gateway").default.api.post = mock;

    expect(() => require("./sample")()).toThrow("failed");
  });

  it("test success", () => {
    jest.mock("./gateway");
    const mock = jest.fn(() => true);
    require("./gateway").default.api.post = mock;

    expect(() => require("./sample")()).not.toThrow("failed");
  });
});