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!
Firstly, the interactive mode you may know from reviewing and updating failed snapshots can now also be used to step through failed tests one at a time. Credit goes to first-time contributor @NullDivision for implementing this feature!
Speaking of snapshots, one of the more exciting features we've shipped in recent years are Inline Snapshots, which landed in a minor release of Jest 23 almost three years ago. However, they came with the restriction that projects wanting to utilize them must be using Prettier to format their code, because that's what Jest would use to make sure the file it writes the snapshots into remains properly formatted.
And so for most of these years, we've had a pull request in the pipeline to eliminate this restriction and allow using Inline Snapshots without Prettier. It has amassed well above a hundred comments, not even taking into account PRs split out from it and landed first, and even changed owner once after the initial submission by another first-time contributor, @mmkal under the hilarious working title 'Uglier Inline Snapshots'. With the stellar rise of Prettier in recent times, this improvement is now maybe less needed than back in 2018, but still, we know that feeling of getting into a project that does not use Prettier, and suddenly not being able to use inline snapshots anymore. Nevermore!
The main reason why it took us so long to land this was, somewhat surprisingly, an out of memory error on our build pipeline. It turns out that the dependencies we load for each test file to perform the parsing, snapshot insertion, and printing do incur a significant time and memory overhead.
So with some tricks, we've speed up the initialization per test file by roughly 70% compared to Jest 26. Note that you will almost certainly not see this big of a performance improvement on your project—you would need a lot of test files that each run very quickly to best notice this, and the overhead when using a JSDOM environment dwarfs any such improvement.
In other news, the native ESM support is progressing, but some major complexities, for instance around mocking, are still ahead of us, and we continue to observe the migration to ESM as a huge ecosystem effort, where Node and a lot of crucial tools and packages all have to rely on each other to deliver an overall compelling experience.
ESM support for plugging modules into Jest is more advanced—custom runners, reporters, watch plugins, and many other modules can already be loaded as ES modules.