如何测试开玩笑的 console.log

Question

我正在使用 create-react-app 并尝试编写一个 jest 测试来检查 console.log 的输出。

我要测试的功能是：

export const log = logMsg => console.log(logMsg);

我的测试是：

it('console.log the text "hello"', () => {
  console.log = jest.fn('hello');
  expect(logMsg).toBe('hello');
});

这是我的错误

 FAIL  src/utils/general.test.js
  ● console.log the text hello

    expect(received).toBe(expected)    Expected value to be (using ===):      "hello"
    Received:
      undefined
    Difference:
      Comparing two different types of values. Expected string but received undefined.

Answer 1

如果您想检查console.log 是否收到了正确的参数（您传入的参数），您应该检查jest.fn() 的mock。
您还必须调用 log 函数，否则永远不会调用 console.log：

it('console.log the text "hello"', () => {
  console.log = jest.fn();
  log('hello');
  // The first argument of the first call to the function was 'hello'
  expect(console.log.mock.calls[0][0]).toBe('hello');
});

或

it('console.log the text "hello"', () => {
  console.log = jest.fn();
  log('hello');
  // The first argument of the first call to the function was 'hello'
  expect(console.log).toHaveBeenCalledWith('hello');
});

阅读更多here。

Answer 2

或者你可以这样做：

it('calls console.log with "hello"', () => {
  const consoleSpy = jest.spyOn(console, 'log');

  console.log('hello');

  expect(consoleSpy).toHaveBeenCalledWith('hello');
});

Answer 3

另一种选择是保存对原始日志的引用，为每个测试替换一个开玩笑的模拟，并在所有测试完成后恢复。这对不污染测试输出并仍然能够使用原始日志方法进行调试有一点好处。

describe("Some logging behavior", () => {
  const log = console.log; // save original console.log function
  beforeEach(() => {
    console.log = jest.fn(); // create a new mock function for each test
  });
  afterAll(() => {
    console.log = log; // restore original console.log after all tests
  });
  test("no log", () => {
    // TODO: test something that should not log
    expect(console.log).not.toHaveBeenCalled();
  });
  test("some log", () => {
    // TODO: test something that should log
    expect(console.log).toHaveBeenCalled();
    const message = console.log.mock.calls[0][0]; // get log message
    expect(message).toEqual(expect.stringContaining('something')); // assert on the message content
    log(message); // actually log out what the mock was called with
  });
});

Answer 4

我会考虑 toHaveBeenCalledWith 或 jest 提供的任何其他用于检查模拟调用的方法 (the ones that start with toHaveBeenCalled)。

it('console.log the text "hello"', () => {
  console.log = jest.fn();
  log('hello');
  expect(console.log).toHaveBeenCalledWith('hello');
});