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

## Using promises

Promises are an alternative to using callbacks in JavaScript. They are at the core of modern JavaScript asynchronous code. A promise is an object that represents the completion of asynchronous code. Once created, the promise is pending and ultimately either moves into a state of being resolved or rejected, depending on whether or not the asynchronous code was successful.

We can chain the `.then()` and `.catch()` methods onto the function creating the promise. `.then()` would receive the resolved promise, along with any data received from the call, and `.catch()` receives it if rejected. In addition, `Promise.resolve()`, `Promise.rejected()`, and `Promise.all()` can be used to manage the completion of a `Promise` object.

## Jest and the `Promise` object

Thankfully, the `Promise` object is an object (almost) like any other in JavaScript. That means we can work with it in the same way that we would with any other return value, including inside our testing environment. Jest allows us to make assertions directly on the returned promise, testing the completed state and associated value.

In the example below, we are again using our `getCatFacts` call, but this time using promises instead of a callback function. In `promises.js`, we can see we make and return the asynchronous call from our function.

# Using promises

Promises are an alternative to using callbacks in JavaScript. They are at the core of modern JavaScript asynchronous code. A promise is an object that represents the completion of asynchronous code. Once created, the promise is pending and ultimately either moves into a state of being resolved or rejected, depending on whether or not the asynchronous code was successful.

We can chain the `.then()` and `.catch()` methods onto the function creating the promise. `.then()` would receive the resolved promise, along with any data received from the call, and `.catch()` receives it if rejected. In addition, `Promise.resolve()`, `Promise.rejected()`, and `Promise.all()` can be used to manage the completion of a `Promise` object.

# Jest and the `Promise` object

Thankfully, the `Promise` object is an object (almost) like any other in JavaScript. That means we can work with it in the same way that we would with any other return value, including inside our testing environment. Jest allows us to make assertions directly on the returned promise, testing the completed state and associated value.

In the example below, we are again using our `getCatFacts` call, but this time using promises instead of a callback function. In `promises.js`, we can see we make and return the asynchronous call from our function.

Learn how to use Jest to test asynchronous code with promises.


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

Testing with Promises

Using promises