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

## Expanding our testing toolkit

In addition to the variety of matchers provided to us directly from Jest, there are a number of libraries that extend its functionality with additional matchers. One of the most common is the `jest-extended` [`jest-extended` library](https://www.npmjs.com/package/jest-extended), created and maintained by the Jest community. It includes TypeScript support and extends functionality for asserting upon the following:


* Arrays
* Booleans
* Dates
* Functions
* Mocks
* Numbers
* Objects
* Promises
* Strings
* Symbols


We don’t necessarily get new functionality here, so it won’t make Jest do anything that it couldn’t have done before. What it does, though, is create incredible readability and clarity in our tests while removing the burden of crafting the correct assertions for our needs.

## Extending `expect`

Jest's `expect` function has to be extended in order to make these new matchers available for use. We can do this in one of the two ways discussed below.

Inside our test setup file, we can either extend some or all of these new matchers. To extend all matchers from the library, we can add the following lines of code:

```
import * as matchers from 'jest-extended';
expect.extend(matchers);

```

To extend only the selected matchers, we can extend them by adding the following lines of code:

```
import { toBeAfter, toSatisfy, toReject } from 'jest-extended';
expect.extend({ toBeAfter, toSatisfy, toReject });
```

The preferred method, however, is to automatically extend all matchers via the `"jest"` field in our `package.json`.

```
"jest": {
  "setupFilesAfterEnv": ["jest-extended/all"]
}
```

## The API

It’s a good idea to read through the complete list of available matchers in the  [documentation](https://www.npmjs.com/package/jest-extended#api).  With options like `.toSatisfy(),` which works with custom predicates, `.toBeBetween()` for dates, and `.toContainAllKeys()` for objects, this library really allows us to be specific and explicit about what exactly we are testing.

You can try some of them out below:


# Expanding our testing toolkit

In addition to the variety of matchers provided to us directly from Jest, there are a number of libraries that extend its functionality with additional matchers. One of the most common is the `jest-extended` [`jest-extended` library](https://www.npmjs.com/package/jest-extended), created and maintained by the Jest community. It includes TypeScript support and extends functionality for asserting upon the following:


* Arrays
* Booleans
* Dates
* Functions
* Mocks
* Numbers
* Objects
* Promises
* Strings
* Symbols


We don’t necessarily get new functionality here, so it won’t make Jest do anything that it couldn’t have done before. What it does, though, is create incredible readability and clarity in our tests while removing the burden of crafting the correct assertions for our needs.

# Extending `expect`

Jest's `expect` function has to be extended in order to make these new matchers available for use. We can do this in one of the two ways discussed below.

Inside our test setup file, we can either extend some or all of these new matchers. To extend all matchers from the library, we can add the following lines of code:

```
import * as matchers from 'jest-extended';
expect.extend(matchers);

```

To extend only the selected matchers, we can extend them by adding the following lines of code:

```
import { toBeAfter, toSatisfy, toReject } from 'jest-extended';
expect.extend({ toBeAfter, toSatisfy, toReject });
```

The preferred method, however, is to automatically extend all matchers via the `"jest"` field in our `package.json`.

```
"jest": {
  "setupFilesAfterEnv": ["jest-extended/all"]
}
```

# The API

It’s a good idea to read through the complete list of available matchers in the  [documentation](https://www.npmjs.com/package/jest-extended#api).  With options like `.toSatisfy(),` which works with custom predicates, `.toBeBetween()` for dates, and `.toContainAllKeys()` for objects, this library really allows us to be specific and explicit about what exactly we are testing.

You can try some of them out below:


Learn about jest-extended, an open-source library that easily extends our testing toolkit.

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

The Jest Extended Library

Expanding our testing toolkit