我如何测试一个笑话 console.log

Question

我使用的是创建反应的-app并试图写一个笑话测试检查一个输出console.log 。

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收到了正确的参数（您传入的参数），您应该检查jest.fn() mock 。
你还必须调用你的log函数，否则console.log永远不会被调用：

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');
});

或者

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');
});

在这里阅读更多。

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笑话模拟，并在所有测试完成后恢复。 这对于不污染测试输出并且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或任何其他 jest 提供的用于检查模拟调用的方法（以 toHaveBeenCalled 开头的方法）。

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