Restoring the Jest cache in Circle CI

Jest is a great testing framework. Jest utilizes a cache to speed up subsequent test runs. I was trying to find information about saving / restoring the Jest cache on CircleCI, but I couldn’t find anything. So, I implemented a solution from scratch.

To help other folks out with saving and restoring the Jest Cache on CircleCI, I created a reusable CircleCI orb, @blimmer/jest to easily restore the Jest cache between runs. To jump right in, check out the Orb Registry Listing and/or the GitHub repository.

Example

To utilize the cache, you can use the jest/test job exposed by the orb. In your .circleci/config.yml file, declare the job like this:

# .circleci/config.yml
version: 2.1

orbs:
  # Replace x.y.z. with a real version number. All versions are listed here:
  # https://circleci.com/developer/orbs/orb/blimmer/jest
  jest: blimmer/jest@x.y.z

  workflows:
    test:
      jobs:
        - jest/test

You’ll also need to configure the Jest cache to be stored in a predictible location. By default, the orb expects it to be located at .jest-cache. Make sure you have defined the cacheDirectory in your Jest configuration file.

// jest.config.js
module.exports = {
  // other configuration properties
  cacheDirectory: ".jest-cache",
};

Benefits of Storing the Jest Cache

Jest stores useful information in the cache, which improves the performance of your test suite.

• Reduced startup time Jest stores information about the file structure and mocks you create in your tests. By restoring the previous jest-haste-map data, it reduces the amount of work jest has to do at startup.

• Even distribution of test suites across workers If you’re running your test suites in parallel (enabled by default), jest will cache information about how long each of your test suites takes to run. It then uses this information to evenly distribute your test suites across the jest workers, so they all complete their work around the same time. This prevents one slow test suite from holding up the entire jest run.

• Fast fail with bail config. Jest also stores whether each individual test succeeded or failed on the previous run, and runs failed tests as soon as possible on the next run. This is very handy when used in conjunction with the bail configuration, so you get super-fast feedback on previously failed tests.

• Faster Typescript transpiling. If you’re using TypeScript with jest, your files are likely transpiled to Javascript before each run. By restoring the cache, jest will only transpile changed files.

If you want to learn more about the jest cache, check out Jest Architecture video on YouTube.

Other CI Providers

This same idea could be applied to CI providers other than CircleCI, like GitHub actions, TravisCI, GitLab CI, CodeShip, etc. Check out the orb source to see the method I’m using.

Is there another CI provider I should support with an open-source package? Let me know! My contact details are in the footer.

Final Thoughts

Did you try out the CircleCI orb? If it helped you out, help me out by starring the GitHub repo. Thanks!