You're a JavaScript developer who wants to learn unit testing?
You come from a non-JS background and want to apply your existing unit testing knowledge in JavaScript?
You're proficient at testing in JavaScript and would like to learn Jest best-practices and featureset?
The Jest Handbook is for you.
Learning outcomes
Gain a deep understanding of the options available to the Jest CLI and how to use different views to write and
debug tests more effectively.
Leverage spying, stubbing and module import interception functionality in tests and create mock JavaScript
object instances, stub ES6 classes and mock out global objects.
Use and contrast 2 approaches to testing backend applications with Jest as well as illustrate synchronous and
asynchronous testing patterns.
Employ advanced Jest partial matchers to write tests with high specificity and reduce toil when updating
application code.
Leverage Jest's built-in coverage tool to set minimum coverage thresholds, find parts of the code that aren't
tested and how to disable it in specific cases.
Table of Contents
Jest/Jest CLI expert - Debugging tests
run jest tests sequentially why/how
run single file/folder
run single test
interactive watch mode (filtering by file name, failing tests, test filename regex)
running in CI mode
Spying, stubbing - become a stub/mock function assertion pro
mock assertion reference
set, clear, reset mocks and spies
example: mocking the current date (how to mock a global object/class) -> 4 examples
stub ES6 class "extends"
master Object.defineProperty to create mock JavaScript application object instances
Mocking of CommonJS and ES6 imports
Testing synchronous vs asynchronous code
test('a', async () => { await codeUnderTest() })
Testing for asynchronous throws (return expect().rejects)
mocking output of synchronous and asynchronous functions
failing a test programmatically fail(), throw 'foo'
Partial matches
objectContaining
arrayContaining
nested arrayContaining, objectContaining
assert over only some of the parameters expect.anything()
expect.any(Function) etc.
Philosophies to write unit tests for Express.js
mock request/response objects in-memory
leverage supertest
Coverage in Jest
coverage thresholds
using Jest coverage reports to find code that's not being exercised
exclude a line, file or statement from coverage reports
Interested in upgrading your Jest and testing game?
Jest is the most widespread solution to test JavaScript applications. Jest is a modern, batteries-included
testing
framework that comes with all features you need to set up for your JavaScript tests including coverage,
interactive
watch mode, advanced matchers.
Jest is the leading test framework in terms of downloads as of 2020 as per npmtrends Jest vs Mocha vs AVA
vs Jasmine vs QUnit vs Chai. Jest came out of efforts from Facebook. It's is used at a plethora of small and
large companies including Facebook, AirBnB, The New York Times and Spotify.
I've looked around the web for resources dedicated to teaching you the ins and outs of Jest. The vast majority of
solutions are GitHub issues or StackOverflow posts, and there are a few React/Vue.js (framework specific) testing
books out there. None really focus on Jest itself and how it can be leveraged to write better tests, faster.
The Jest Handbook is an example and solutions-driven book that will teach you in the ins and outs of testing
JavaScript applications with Jest.
About the Author
Hugo Di Francesco is a co-author of "Professional JavaScript" with Packt. He runs the Code with Hugo
website helping over 100,000 developers every month and holds an MEng in Mathematical Computation from University
College London (UCL). He has used JavaScript extensively to create scalable and performant platforms at companies
such as Canon and Elsevier.
Hugo has also written some of the top articles on the internet about Jest at codewithhugo.com/tags/jest/ including:
