What is the difference between mocking and spying in testing?

What is the difference between mocking and spying in testing?

Mocking and spying are both techniques used in unit testing. Mocking creates a fake object that simulates the behavior of a real object, allowing you to test interactions without relying on the actual implementation. Spying, on the other hand, allows you to wrap an existing object to track how it was called, without altering its behavior.

Can you give me an example of when to use mocking?

Sure! You would use mocking when you want to isolate the unit of code being tested from its dependencies. For instance, if you're testing a function that makes an API call, you can mock the API response to ensure your test runs quickly and doesn't depend on the actual API.

And what about spying? When should I use that?

Spying is useful when you want to verify that a function was called with specific arguments or a certain number of times, without changing its behavior. For example, if you want to ensure that a callback function is invoked after an event, you can use a spy to check that.

Are there any libraries that support mocking and spying?

Yes, popular testing libraries like Jest and Mocha provide built-in support for both mocking and spying, making it easier to implement these techniques in your tests.