When mocking a JavaScript function during a unit test, it can be helpful to force a mock function to return a value of your choosing. This allows you to verify that functions you're testing will behave correctly for every possible use case. Much like [`sinon.spy()`](https://sinonjs.org/releases/v7.0.0/spies/), [`sinon.stub()`](https://sinonjs.org/releases/v7.0.0/stubs/) creates a mock function that prevents a real function from running. But, unlike `spy`, it also allows you to declare what value you'd like the mock function to return.