In the Jest 26 blog post about a year ago, we announced that after two major releases with few breaking changes, Jest 27 will flip some switches to set better defaults for projects that are new or can migrate smoothly. This gives us the opportunity to remove some packages from the default distribution of Jest 28 and publish them as separately installable and pluggable modules instead. Everyone on the new defaults can benefit from a smaller install size, while people needing these packages can still install them separately.

With the first major change of defaults since the New Defaults for Jest that came with the seminal version 15, Jest 27 is now here, to keep Jest fast, lean, and relevant in the future. We will explain those changes of defaults and other notable breaking changes in this post, but first, let's get into some exciting new features!

We are excited to present the new Jest website!

The Jest site was upgraded from Docusaurus 1 to Docusaurus 2, and is an early adopter of the newly released Docusaurus 2 i18n support.

So, what's new exactly?

When we started rebuilding Jest five years ago our goal was to provide a batteries-included zero-configuration test runner that is approachable for beginners, extensible for almost all testing use cases and scalable to large projects. One of the instrumental releases was Jest 15 which tied everything together and provided good defaults that allowed people to run Jest often without any setup. However, this approach has a big downside as Jest installs a lot of dependencies into your projects that you may not need.

We are now beginning to address this shortcoming and are working on reducing Jest’s install size while keeping it approachable and extensible. We have made the following breaking changes in Jest 26:

Jest 25 is laying the groundwork for many major changes in the future. As such, we kept breaking changes to a minimum, but internal architecture changes may require attention during the upgrade. The main changes are an upgrade of JSDOM from v11 to v15, 10-15% faster test runs, a new diff view for outdated snapshots and dropped Node 6 support.

There has been more than 200 commits since Jest 24.9 by more than 80 different contributors, so as always, take a look at the changelog for a full list of changes.

Today we are happy to announce the next major release of Jest - version 24! It's been 4 months since the last minor release, and 8 months since Jest 23, so this upgrade is a big one, with something for everyone! Highlights include built-in support for TypeScript by upgrading the Jest internals to Babel 7, fixing some long-standing issues with missing console output and performance issues when computing large diffs, and a brand new sparkling website. ✨

For a full list of all changes see the changelog.

Jest is maintained by a community of open source contributors and Facebook employees.

Our open source contributors work on behalf of the community to create new features, fix bugs, and maintain the issue backlog. Facebook employees do the same, focused on the problems specific to Facebook's scale, and run all proposed releases on Facebook's enormous test suite before publishing.

All of us do our part to make testing delightful.

At the Jest Summit we announced the Jest Open Collective. This is our way to help support the efforts of the open source Jest community. To be absolutely clear: Facebook employees cannot benefit from the Jest Open Collective. The collective's sole purpose is to support the open source side of the Jest community.

In this post we'll outline what the Jest Open Collective is, the structure, and the goals we have.

Today we are excited to announce Jest 23, our largest major release to date! Together with over 100 contributors, we've shipped a ton of features and bug fixes. Thank you to everyone in the community for helping make JavaScript Testing Delightful.

We would also like to welcome both Babel and Webpack to the Jest community! After converting from Mocha to Jest 23 Beta, Webpack saw their total test suite time reduced 6x from over 13 minutes to 2 minutes 20 seconds. #blazingmeansgood

Here's are some of the Jest 23 highlights and breaking changes.

Today we are announcing a new major version of Jest which refines almost all parts of Jest to provide a more solid testing foundation. Together with the Jest community we made a number of changes across the board that will help you get more out of Jest. We are also graduating the custom runners feature out of the experimental stage and added a new package, jest-worker, for parallelizing work across multiple processes. We have compiled a list of highlights below but make sure to check out the (as always) massive changelog.

A few months ago we announced Jest 19 which came with major new features and was the biggest Jest release until today. Jest 20 has twice the amount of changes compared to the previous version, features a complete rewrite of the test runner, adds new testing APIs. The new release enables a new level of customization and configuration for projects all while making it effortless to upgrade. Beyond Painless JavaScript Testing, we believe Jest is now delivering a Delightful JavaScript Testing experience. Let's take a look at the best new features and changes in depth:

Multi-Project-Runner & Configuration Overhaul#

Until now, Jest could only operate in one project at a time. This is often cumbersome if you are working on many smaller projects that each have their own setup and configuration. With Jest 20, we rewrote the test runner completely to run many projects at the same time within a single instance of Jest, for example if you are working on a React frontend and a node.js backend. Here is a video of Jest running tests for React, Relay, Yarn and Jest all at the same time:

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.