Gain insights into Jest, a versatile testing framework. Learn about fundamental and advanced testing techniques, from unit tests to CI/CD integration, for comprehensive JavaScript application testing.

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Jest is a framework for testing applications developed in JavaScript. It has a well-documented and well-maintained range of tools to run tests in parallel. It is simple and works with projects using Babel, TypeScript, Node, React, Angular, Vue, etc. It works well for both client and server testing. 

This course covers Jest in-depth and is agnostic of any specific JavaScript framework. The course begins with the fundamentals of Jest testing, deconstructing the components of a test and covering how to compose basic unit tests and snapshot tests with Jest correctly. Next, it shows the testing of asynchronous code and the code that relies on external dependencies. Lastly, this course illustrates how Jest testing can effectively integrate into the real-world software development cycle through CI/CD pipeline.

It gives you hands-on experience in setting up Jest, types of testing, testing coverage, mocking, and native tooling depth.

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Mastering Jest: A Complete Guide to Testing

# Why do we mock packages?

Open-source software is a vital part of any modern codebase. It allows us to gain robust features and support complex logic with as little as just a few lines of code. These same external libraries often don’t play nice with testing environments out of the box, though.

This won’t be true for all external packages, and each package should be considered individually. Entire frameworks like `Koa` and `Angular` are packaged as third-party code, and we don’t want to mock these. Additionally, some libraries might provide functionality that just doesn’t need to be mocked, such as the `format` or `joi` packages.

Mocking our external packages can feel overwhelming at first. Often when we start running our tests, if we haven’t mocked a package, the errors we find ourselves facing can feel out of place, unhelpful, and difficult to debug. This is because the code, and therefore the error handling, is intended to be running in a real-world environment, not a test environment. Therefore, it can be confusing to discern what’s happening.


# How do we mock packages?

Mocking all packages comes down to a singular guiding philosophy. Mocking dependencies is about creating an object that _matches the interface_ of the package being mocked. What do we mean when we say “matches the interface”?


Introduction

Setting Up Jest

Jest Matchers

Writing Unit Tests

Writing Snapshot Tests

Asynchronous Code

Mocking

Mocking External Libraries

Working with Jest Mocks

Setup & Teardown

Test Coverage & CI/CD Pipelines

Test Coverage in Jest

Conclusion

Bonus: Best Practices

Bonus: Resources for Continued Learning

Appendix

Introduction to Mocking Packages

Why do we mock packages?