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2020-05-06-jest-26.md

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---
title: Jest 26: Tick Tock
author: Christoph Nakazawa
authorURL: http://twitter.com/cpojer
authorFBID: 100000023028168
---

Jest 26: Tick Tock

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 most important releases was Jest 15 which tied all the bits and pieces together and provided great defaults that allowed people to install and 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:

  • [expect, jest-mock, pretty-format] Remove ES5 build files with a new minimum of support of ES2015 (Node 8) which were only used for browser builds (#9945)
  • [jest-config, jest-resolve] Remove support for browser field (#9943)
  • TypeScript definitions requires a minimum of TypeScript v3.8 (#9823)

With the above changes Jest 26 is now 4 MiB smaller than Jest 25.5.4 (53 → 49 MiB). Please keep in mind that many dependencies like Babel are likely already part of your project. Jest's own size was reduced by 1.2 MiB (4.3 -> 3.1 MiB).

While this is a good start, it isn’t enough to make Jest meaningfully leaner. We are planning on gradually reducing Jest's and its dependency tree size by about 70%. Most of the changes to reduce Jest’s default setup will be small breaking changes and we want to bring the community with us as much as possible. Therefore we will make the following changes in Jest 27 and Jest 28 using a “Tick-Tock" release process:

  • Jest 27 will ship with a new test runner (jest-circus) and the node.js environment by default. jest-jasmine2 and jsdom will still be bundled so users can keep using them by changing one-line each in the configuration.
  • Jest 28 will remove jest-jasmine2 and jest-environment-jsdom from the default distribution of Jest. The packages will still be actively maintained as part of the Jest project and be published separately. Users will need to install these packages to use them.

Upon upgrading to these major releases, Jest will guide you through the necessary changes. If you’d like to get ahead and migrate to the new architecture now you can upgrade to Jest 26 and add the following configuration options:

{
  "jest": {
    "testEnvironment": "node",
    "testRunner": "jest-circus/runner"
  }
}

Jest will continue to ship with babel-jest enabled by default. It currently powers a few Jest features beyond compiling modern JS syntax to something understood by current versions of Node.js and browsers. It also powers Jest's code coverage and mocking of ES modules. Jest currently ships with experimental support for V8 coverage and native support for ES Modules (more on that below!). It is not possible to mock static ES Modules per the specification without transforming the code first, so we will be encouraging patterns that work without the transformation we use Babel for today. Once V8 coverage and native ESM support stabilizes in Jest, we will also be able remove babel-jest as a default but we will keep maintaining it.

Other Breaking Changes in Jest 26

  • Dropped support for Node 8 (#9423)
  • [jest-environment-jsdom] Upgrade jsdom to v16 (#9606)
  • [jest-runtime] Remove long-deprecated require.requireActual and require.requireMock methods (#9854)
  • [jest-haste-map] Removed providesModuleNodeModules (#8535)
  • [jest-circus] Fail tests if a test takes a done callback and have return values (#9129)
  • [jest-circus] Throw a proper error if a test / hooks is defined asynchronously (#8096)
  • TODO

Stay Safe

We are all currently experiencing an unprecedented time of uncertainty. If you are struggling financially, we would like to use Jest’s OpenCollective fund to help new and existing contributors. We have an existing list of issues with a bounty but are open to offering a bounty on any of our current open issues - you can mention that this issue should have a bounty in the issue or contact @cpojer via private message on Twitter.

Please stay safe.