javascript - 用 Jest 模拟导出的 const 箭头函数

Question

鉴于以下带有基本测试的示例，我希望 bar返回时 sendMessage.message被 Jest mock 。相反，当被调用时，它返回“Hello World”的原始值。

服务总线.js

export const message = () => {
  return "Hello world";
};

export const foo = () => {
  return message();
};

mockDemo.test.js

import * as sendMessage from "../src/services/serviceBus";

describe("sendMessage", () => {
  sendMessage.message = jest.fn(() => {
    return "bar";
  });

  it(`message should return bar`, async () => {
    expect(sendMessage.message()).toBe("bar");
    expect(sendMessage.foo()).toBe("bar");
  });
});

结果如下:

$ ./node_modules/.bin/jest ./__tests__/mockDemo.test.js
 FAIL  __tests__/mockDemo.test.js
  MessageReceiver Router
    ✕ message should return bar (9ms)

  ● MessageReceiver Router › message should return bar

    expect(received).toBe(expected) // Object.is equality

    Expected: "bar"
    Received: "Hello world"

       9 |   it(`message should return bar`, async () => {
      10 |     expect(sendMessage.message()).toBe("bar");
    > 11 |     expect(sendMessage.foo()).toBe("bar");
         |                               ^
      12 |   });
      13 | });
      14 |

      at Object.<anonymous> (__tests__/mockDemo.test.js:11:31)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total

看起来好像sendMessage.foo正在调用 message 导入后的绑定(bind)函数，默认返回 Hello World .

如何模拟 message在这种情况下？

更新

使用 jest.fn().mockImplementation(() => "bar");导致相同的行为:

describe("MessageReceiver Router", () => {
  //   sendMessage.message.mockImplementation = jest.fn(() => {
  //     return "bar";
  //   });

  sendMessage.message = jest.fn().mockImplementation(() => "bar");

  it(`message should return bar`, async () => {
    expect(sendMessage.message()).toBe("bar");
    expect(sendMessage.foo()).toBe("bar");
  });
});

结果:

$ ./node_modules/.bin/jest ./__tests__/mockDemo.test.js
 FAIL  __tests__/mockDemo.test.js
  MessageReceiver Router
    ✕ message should return bar (10ms)

  ● MessageReceiver Router › message should return bar

    expect(received).toBe(expected) // Object.is equality

    Expected: "bar"
    Received: "Hello world"

      11 |   it(`message should return bar`, async () => {
      12 |     expect(sendMessage.message()).toBe("bar");
    > 13 |     expect(sendMessage.foo()).toBe("bar");
         |                               ^
      14 |   });
      15 | });
      16 |

      at Object.<anonymous> (__tests__/mockDemo.test.js:13:31)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        2.175s

Answer 1

您应该使用 spyOn模拟 sendMessage.message 的方法方法。

import * as sendMessage from "../src/services/serviceBus";

describe("sendMessage", () => {
   jest.spyOn(sendMessage, 'message').mockReturnValue("bar")

  it(`message should return bar`, async () => {
    expect(sendMessage.message()).toBe("bar");
    expect(sendMessage.foo()).toBe("bar");
  });
});