如何测试jest console.log

Question

我正在使用create-react-app并尝试编写一个检查console.log输出的jest测试

我的测试功能是:

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

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 2

如果你想检查console.log收到正确的参数(你传入的参数),你应该检查mock你的jest.fn().
您还必须调用您的log函数,否则console.log永远不会调用:

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');
});

在这里阅读更多.

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
  });
});