使用 Jest 模拟导出的 const 箭头函数

Question

考虑以下具有基本测试的示例，当使用Jest模拟sendMessage.message时，我期望返回bar。但是实际上，当调用时，它返回原始值"Hello World"。

serviceBus.js（服务总线）

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");
  });
});

以下是结果：

$ ./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

看起来好像在导入 message 后，sendMessage.foo 调用了一次绑定的函数，该函数默认返回 Hello World。

在这种情况下，如何模拟 message？

更新

使用 jest.fn().mockImplementation(() => "bar"); 结果是相同的行为：

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

- steadweb

---

sendMessage.message.mockImplementation? - evolutionxbox

很遗憾，它不起作用 :/ - steadweb

啊，它已经被封闭了。所以你可能无法这样做。 - evolutionxbox

Answer 1

内部函数称为无法被模拟或监视。要解决此问题，您需要创建一个服务对象，并导出它。

serviceBus.js

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

const foo = () => {
  return services.message();
};

const services = {
  message,
  foo
};

export default services;

mockDemo.test.js

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

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

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

- steadweb

Answer 2

你应该使用 spyOn 方法来模拟 sendMessage.message 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");
  });
});

- prasana kannan