如何模拟像 new Date() 这样的构造函数 [英] How to mock a constructor like new Date()

问题描述

我有一个方法，它依赖于 new Date 来创建一个日期对象然后操作它.我正在测试操作是否按预期工作，因此我需要将返回日期与预期日期进行比较.为了做到这一点，我需要确保 new Date 在测试和被测试的方法中返回相同的值.我该怎么做?

I have a method which depends on new Date to create a date object and then manipulates it. I'm testing that the manipulation works as expected, so I need to compare the returned date with expected date. In order to do that I need to make sure that new Date returns the same value in the test and in the method being tested. How can I do that?

有没有办法真正模拟构造函数的返回值?

Is there a way to actually mock the return value of a constructor function?

我可以创建一个模块，该模块可能需要一个提供日期对象并且可以被模拟的函数.但这在我的代码中似乎是不必要的抽象.

I could create a module that can be required with a function that provides a date object and can be mocked. But that seems like an unnecessary abstraction in my code.

要测试的示例函数...

an example function to be tested...

module.exports = {
  sameTimeTomorrow: function(){
    var dt = new Date();
        dt.setDate(dt + 1);
    return dt;
  }
};

如何模拟 new Date() 的返回值?

how do I mock the return value of new Date()?

推荐答案

更新:这个答案是 jest <版本 26 请参阅最近玩笑版本的此答案.

Update: this answer is the approach for jest < version 26 see this answer for recent jest versions.

你可以使用 jest.spyOn 模拟像 new Date() 这样的构造函数，如下所示:

You can mock a constructor like new Date() using jest.spyOn as below:

test('mocks a constructor like new Date()', () => {
  console.log('Normal:   ', new Date().getTime())

  const mockDate = new Date(1466424490000)
  const spy = jest
    .spyOn(global, 'Date')
    .mockImplementation(() => mockDate)

  console.log('Mocked:   ', new Date().getTime())
  spy.mockRestore()

  console.log('Restored: ', new Date().getTime())
})

输出如下:

Normal:    1566424897579
Mocked:    1466424490000
Restored:  1566424897608

请参阅 GitHub 上的参考项目.

注意:如果您使用的是 TypeScript 并且会遇到编译错误，Argument of type '() =>日期'不能分配给'() => 类型的参数.细绳'.类型 'Date' 不能分配给类型 'string'.在这种情况下，一种解决方法是使用 mockdate 库，该库可用于在"现在"是.有关详细信息，请参阅这个问题.

Note: If you are using TypeScript and you would encounter a compilation error, Argument of type '() => Date' is not assignable to parameter of type '() => string'. Type 'Date' is not assignable to type 'string'. In this case, a workaround is to use the mockdate library, which can be used to change when "now" is. See this question for more details.

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