Mocking global object methods in Jest

Angelos Chalaris · JavaScript, Testing · Mar 27, 2022

Testing is a big part of the development process. It's also where a lot of mistakes can be overlooked, which can pile up and lead to hard-to-debug issues. A common problem is poorly-written mocks, especially regarding global objects and their methods. Let's take a look at how to mock global object methods in Jest.

When mocking global object methods in Jest, the optimal way to do so is using the jest.spyOn() method. It takes the object and name of the method you want to mock, and returns a mock function. The resulting mock function can then be chained to a mocked implementation or a mocked return value. For example:

jest.spyOn(Math, 'random').mockReturnValue(0.123456789);

jest.spyOn(Date, 'now').mockReturnValue('123456789');

In this example, we mock two global object methods and return a fixed value. You could as easily mock their implementation using mockFn.mockImplementation(). Using either of these options allows you to get predictable values from the mocked methods. This comes in especially handy when working, for example, with Math.random() or Date.now().

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