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

# What are matchers?

A **matcher** is what facilitates the assertion portion of a test. It states what we are asserting to be true, ultimately determining the passing or failing of the test.

Jest’s `expect` function provides a number of these as an out-of-the-box feature. All these are called by simply chaining them onto `expect`, as shown below:

```
expect(2 + 2).toEqual(4);
```

## Types of matchers

The matcher library is robust, offering lots of flexibility. Most matchers fall into one of the following categories:

* Truthiness
* Equality
* Numerical comparison
* Exceptions
* Function calls

# Common matchers

**`.toBe()`**: This checks the equality of two primitive values but can’t reliably be used with floating point numbers. Keep in mind that these primitive values can be booleans. Therefore, we can pass our `expect` function something that equates to a `boolean` and then use `.toBe(true)` or `.toBe(false)`.

To compare two objects, we can either use the referential identity to compare a key-value pair from each object, or we can use `Object.is` for comparison:

```
expect(Object.is(obj1, obj2)).toBe(true)
```

This matcher is very flexible, but many of these needs are abstracted into other more explicit matches.

**`.toMatch()`**: It allows us to leverage regex patterns for testing strings. Sometimes, we have dynamic text that we particularly want to ensure is rendering properly. This is an easy and clear way to do this without getting bogged down in the context.

**`.toBeTruthy()`** + **`.toBeFalsy()`**: These are pretty self-explanatory and are great examples of the explicit abstractions of the `.toBe()` testing. It is straightforward and easily readable.

**`.toHaveBeenCalled()`**: It tells us if a mock function has been called. It doesn’t tell us anything about what the function does, just whether or not it was called. It’s incredibly useful when, for instance, we have a low-level button component that can do anything we tell it to—we just need to make sure that clicking it will in fact perform the desired task.

Remember that we can only check if a mock function has been called.

**`.toBeNull()`**: This asserts that the expected value is equal to `null`.

**`.toContain()`**: It checks whether or not an item is present in an array. The array should be passed to `expect`, and the item passed to `toContain()`.

**`.not`**: This lets us test the opposite of any matcher available. It would be equivalent to making an assertion using the `!` operator, but it’s much more readable.

Learn about matchers, the tool at the center of Jest's testing ease.

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

Using Common Matchers

What are matchers?