Today we are pleased to ship version 19 of the Jest testing platform. It's the biggest Jest release we have shipped so far and we are quite excited to show you what we've built over the last two months:
Immersive Watch Mode #
We completely rewrote the watch mode to make it instant and more extensible. As a result, the experience of using it really is immersive: tests re-run instantly after a file change and we made it easy to select the right tests.
Snapshot Updates #
We made a couple of changes to the snapshot format. We don't make changes like this often and only consider them if they actually improve how snapshots work. As well as introducing a snapshot version number we accumulated a number of changes we wanted to make to the format for a while:
- We dropped the “test” prefix in snapshot names from top level
- We improved the printing of React elements to cause less changes when the last prop in an element changes.
- We improved the character escaping mechanism to be more bulletproof.
After (no “test” prefix, better JSX rendering, version header):
We decided it's a good time to introduce versioned snapshots to ensure all developers are using a compatible version of Jest. Here's how we warn you about the need to update your snapshot:
Please make sure you revert any local changes before updating to make the transition smooth and to ensure you aren't including any unwanted changes from failing tests in your new snapshots.
Improved printing of skipped tests #
Skipped tests are now printed as a single line instead of showing every individual one when testing in verbose mode or a single suite. Hopefully it will let you focus on currently important tests. It also occupies far less space!
New CLI arguments #
Jest 19 ships with two new coverage-related arguments which you can run from CLI:
We now also error on invalid CLI arguments instead of ignoring them. But we've
got your back with helpful error message like the one below, e.g. when you try
expect Improvements #
We're close to almost full feature parity with the
expect npm package.
Michael Jackson, the author of the package,
agreed to donate it to the Jest
project, which means that
jest-matchers will be renamed to
expect. Since our
expect is not intended to be fully compatible,
Christopher Chedeau is working on a codemod to make
the transition painless. Christopher also worked on a number of improvements to
jest-matchers which enables it to be used outside of Jest and even
works inside browsers.
eslint-plugin-jest – our very own ESLint plugin #
Thanks to Jonathan Kim Jest finally has its own official ESLint plugin. It exposes three rules:
- no-disabled-tests - this rule prevents you from accidentally committing disabled tests.
- no-focused-tests - this rule prevents you from committing focused tests which would disable all other tests in the same suite.
- no-identical-title - disallows identical titles in test names.
You can install it using
npm install eslint-plugin-jest or
yarn add --dev
eslint eslint-plugin-jest and it can be enabled by adding
["jest"]} to your eslint configuration.
New public package: jest-validate #
While we refactored the validation and normalization code for Jest's
configuration, we were so happy with the new error messaging that we extracted
it to its own module to share it with everyone. With Jest 19 we welcome
jest-validate to our self-sustained packages family.
jest-validate is a generic configuration validation tool that helps you with
capable of showing users friendly examples of correct configuration and it comes
with a simple but powerful API. We hope it'll make a good addition to your
We're happy to announce that
jest-validate is validating config options of
Feel free to add it to your project, try it, send us feedback and improve it by
making pull requests on GitHub.
Improved asymmetric matchers #
We moved the asymmetric matchers implementation from Jasmine into Jest, which
enabled us to further improve the user experience around them. As a result,
asymmetric matchers are now pretty-printed nicely, we added the new
matcher and we also paired them with
so you can use the best of both:
Better manual mocks #
With the latest release, manual mocks now finally work with nested folders. For
__mocks__/react-native/Libraries/Text/Text.js will now work as
expected and mock the correct module. We also fixed issues with virtual mocks
and transitive dependencies and improved
moduleNameMapper to not overwrite
mocks when many patterns map to the same file.
Breaking Changes #
As a part of our cleanups and fixes we removed the
option which was never officially supported. We also renamed the
configuration option to
roots which better explains what the option can be
used for. The default configuration for
["<rootDir>"] and can be
customized to include any number of directories. The rootDir configuration
option has always been used mostly as a token for other configuration options
and this rename should make configuring Jest clearer.
Revamped documentation #
- Under Docs you can find an introduction to Jest, including Getting Started or Testing Asynchronous Code and handy guides like Snapshot Testing, Testing React Native App, Using with webpack or Migrating to Jest and many more!
- The API section on the other
hand lists all available methods exposed by Jest: the
jestobjects, mock functions, globals, along with configuration options from package.json and from the CLI.
The homepage was completely redesigned to be more descriptive of what Jest is about: “Zero configuration testing platform”. We also made sure it reads better on mobile devices. And for those using RSS – we finally provide a feed for our blog.
Community Updates #
- We really loved this talk: “Introduction to Jest“ by Vas Boroviak.
- Follow @fbjest on Twitter.
- The Jest Core team syncs once a week to discuss current and future issues. If you'd like to work on Jest, let us know, submit a few pull requests and join our weekly team meetings.
- The awesome engineers at Artsy wrote about Jest as part of their 2017 frontend stack.
- Stephen Scott wrote a detailed article about testing React components in which he weighs the pros and cons of different approaches.
- Using Jest with vue.js got a lot easier after reading Cristian Carlesso's blog post.
- Michele Bertoli wrote a book about React Design Patterns and Best Practices which features an entire section about Jest.
--notifycommand that shows an operating system notification which can now also re-run tests from the notification. This is actually a Jest feature and we are just checking if you are still reading this blog post.
- Jest is now part of react-boilerplate.
- Read about the hidden powers of Jest's matchers.
Finally, we are happy to announce that the ava test runner has adopted parts of the Jest platform and is now shipping with basic snapshot support and is using pretty-format. Consolidating test infrastructure makes it easier to learn how to test applications and enables us to share best practices. We are looking forward to see what we can learn from existing test libraries in the future.