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.

…. To come 

… to come 

Mastering Jest: A Complete Guide to Testing

# Why do we mock timers?

Our discussion on mocking timers largely covers two use cases. The first is mocking timers in our code for efficiency reasons. The second is mocking for the same reasons that we often mock functions: we want to be able to watch them and assert on how timers were called and run.

## Mocking for efficiency

Timers, by nature, _take time_—a resource that we are constantly looking to optimize and reduce in our test runs. We don’t necessarily need our timers to run through their entirety in our test runs. At times we may want to, but we often want to hit the timer and keep running. Mocking the timer at hand allows us to do this.

## Mocking for deeper insights

At other times, we don’t need to speed up the execution. Instead, we want to gain the same insights we do by mocking other functions. We might want to see if a timer was called, how many times it was called, with what argument, and so on.


# How to mock a timer

The good news is we can use the same strategy for mocking in both scenarios. The core of timer mocking comes down to two Jest functions, `jest.useFakeTimers()` and `jest.useRealTimers()`. The first tells Jest to replace all implementations of `setTimeout()` and `setInterval()` with mocked versions of them. This implementation looks something like the example below.

In this example, we see a `newYearsEveCountdown` function that counts down from the number of seconds passed to it, defaulting to `10` if nothing is passed. The function logs the count at each second. Notice how the test that uses fake timers logs all seconds rapidly, while the test that uses real timers actually performs the timed countdown:


Learn how to effectively mock and leverage timers in Jest.

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

Timer Mocks

Why do we mock timers?

Mocking for efficiency

Mocking for deeper insights