What are Jest mocking helpers and how do I use them?

What are Jest mocking helpers and how do I use them?

Jest mocking helpers are functions that allow you to create mock implementations of modules or functions in your tests. They help isolate the code being tested by replacing dependencies with controlled versions.

Can you give me an example of how to use a mock function?

Sure! You can use jest.fn() to create a mock function. For example:

const myMock = jest.fn();
myMock('first call');
myMock('second call');
console.log(myMock.mock.calls);

This will log the calls made to myMock.

What about mocking modules?

To mock a module, you can use jest.mock(). For instance:

jest.mock('./myModule');

This will replace all exports from myModule with mock functions.

Are there any built-in matchers for testing mocks?

Yes! Jest provides matchers like toHaveBeenCalled() and toHaveBeenCalledWith(), which you can use to assert how your mocks were called.