What is the difference between spies, mocks, and stubs in testing?

Can you explain the difference between spies, mocks, and stubs in testing?

Sure! In testing, a stub is a simple implementation that returns predefined responses, while a mock is a more complex object that verifies interactions. A spy, on the other hand, records information about how it was called.

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

Absolutely! Use a stub when you need to simulate a method that returns a fixed value. Use a mock when you want to ensure a method was called with specific parameters. Use a spy when you want to track how a method was called without altering its behavior.

What are some common libraries for implementing these in JavaScript?

Common libraries include Sinon.js for spies, mocks, and stubs, and Jest, which has built-in support for mocking functions.

How do these concepts improve testing?

They improve testing by allowing you to isolate components, ensuring that tests are reliable and focused on specific behaviors.