Common troubleshooting tips

Configuration

rules_jest configures Jest to work optimally with Bazel and rules_js. Custom configuration may conflict with the default rules_jest configurations and care should be taken when configuring Jest.

rules_js virtual store

Similar to pnpm, rules_js uses a virtual store for npm packages. This can cause issues with Jest and may require package hoisting similar to pnpm. See the pnpm public hoist pattern documentation for details and rules_js public_hoist_packages for configuring rules_js package hoisting.

Performance

Jest is designed to run as a standalone long running application, often having a slow startup time then running incrementally using caching for performance. This conflicts with Bazel which designed for short-lived processes that can be run in parallel and cached.

Common performance issues include:

transformIgnorePatterns

By default Jest ignores and does not transform node_modules/*. This is often overridden to transform a subset of modules. When using rules_js or pnpm the ignore pattern must match the package within the virtual store. See the transformIgnorePatterns notes for pnpm. If this is misconfigured to not properly ignore node_modules/* it may cause significant performance issues.

Example configuration of transformIgnorePatterns with rules_jest:

const config = {
  transformIgnorePatterns: [
    '<rootDir>/node_modules/.aspect_rules_js/(?!(package-a|@scope\\+pkg-b)@)',
    /* or using relative pattern to match the second 'node_modules/' in 'node_modules/.aspect_rules_js/@scope+pkg-b@x.x.x/node_modules/@scope/pkg-b/' */
    'node_modules/(?!.aspect_rules_js|package-a|@scope/pkg-b)',
  ],
};

Note the .aspect_rules_js directory, and use of \\+ for scoped packages.

Concurrency - Bazel sharding and Jest concurrency

By default Jest runs tests concurrently using workers. This conflicts with Bazel which also runs tests and actions in parallel. Jest concurrency will be configured by rules_jest to work optimally with Bazel actions and Bazel test sharding. If necessary this can be overridden using jest_test(run_in_band = False).

ts-jest

ts-jest is often used to write and run Jest tests using TypeScript. With short-lived Bazel actions compiling TypeScript on each invocation can cause performance issues. Using rules_ts is recommended to run and cache the TypeScript compilation, removing the need for ts-jest and runtime compilation.

Jest haste

By default Jest uses jest-haste-map to optimize and cache fs operations which must be configured to work with Bazel. rules_jest will automatically configure haste for compatibility withrules_jest and rules_js. Care must be taken with custom Jest configurations when configuring haste.