When we write unit tests, it is important to isolate the code we are testing to ensure that the test does not depend on external parts or unpredictable behaviors.

This is where Sinon.js comes into play, a powerful library for "mocking," "stubbing," and "spying" in JavaScript tests. In this chapter, we will learn how to use Sinon with Mocha to improve our tests.

Installing Sinon

First, we need to install Sinon. You can do this by running the following command at the root of your project:

sh

Stubbing with Sinon

"Stubbing" allows us to replace a function or method with a controlled and predictable version. This is useful when we want to avoid side effects or ensure that the code under test behaves in a specific way.

Function Stubbing Example

Consider the following code in usuario.js:

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We want to test a function that uses fetchUsuario, but we don't want to make real API calls. We can stub fetchUsuario using Sinon.

Writing the Test with Stubbing

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Stubbing Asynchronous Functions

For asynchronous functions, we can return a resolved promise:

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Mocking with Sinon

"Mocking" simulates a complete object and allows setting expectations on how it should be used. Mocks are useful for verifying specific interactions.

Mocking Example in Sinon

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Writing the Test with Mocking

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Using Spies with Sinon

"Spies" allow us to observe the behavior of functions without replacing them. This is useful to verify if a function has been called and with what arguments.

Spying Example

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Conclusion

Sinon.js provides powerful tools for stubbing, mocking, and spying, allowing us to write more robust and isolated unit tests. The combination of Sinon with Mocha and Chai allows us to cover a wide range of test cases and verify all expected interactions in our code.

In the next chapter, we will see how to perform user interaction tests with Mocha, implementing many of the concepts we learned here.