# Jest Knowledge Patch > Jest - Skill: `nevaberry/jest-knowledge-patch` (Agent Skill, multi-file: 7 files) - Install (CLI): `npx skillmds@latest add nevaberry/jest-knowledge-patch` - Raw SKILL.md: https://api.skillmd.com/api/skills/nevaberry/jest-knowledge-patch/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: MIT - Author: nevaberry (https://skillmd.com/u/nevaberry) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/nevaberry/jest-knowledge-patch --- # Jest Knowledge Patch Use this skill when upgrading, configuring, or debugging Jest 30 projects. Load the reference that matches the task before changing configuration, tests, custom environments, runners, sequencers, transforms, or integrations. ## Reference index | Reference | Topics | | --- | --- | | [references/migration-and-runtime.md](references/migration-and-runtime.md) | Runtime requirements, removed APIs, renamed CLI options, package boundaries, path behavior | | [references/configuration-and-cli.md](references/configuration-and-cli.md) | Configuration helpers, project settings, discovery commands, reporting, coverage, runners | | [references/modules-transforms-and-environments.md](references/modules-transforms-and-environments.md) | ESM/CJS interop, TypeScript, Babel, import attributes, JSDOM environments | | [references/assertions-mocks-and-snapshots.md](references/assertions-mocks-and-snapshots.md) | Matcher behavior, mock hooks, spies, data-driven tests, snapshots, formatting | | [references/execution-and-timers.md](references/execution-and-timers.md) | Rejections, retries, fake timers, cleanup, leak detection, worker shutdown, setup hooks | ## Working method 1. Inspect the installed Jest, Node.js, TypeScript, transformer, and test environment versions. 2. Read the relevant reference file before editing code. 3. Search for removed names and invalid configuration shapes. 4. Update behavior-sensitive tests separately from mechanical renames. 5. Run focused tests, then the full suite, coverage, and snapshot review. 6. Treat failures in custom runners, environments, sequencers, and reporters as possible public-contract changes before blaming test code. ## Breaking changes first ### Check the runtime Jest 30 does not support Node.js 14, 16, 19, 21, or 23. Its type definitions require TypeScript 5.4 or newer. Confirm CI images, local toolchains, editor TypeScript, and package-manager constraints before investigating test failures. The bundled JSDOM environment moved from JSDOM 21 to 26. Recheck DOM behavior, especially tests that mock `window.location`. A custom environment can use a different JSDOM version through `@jest/environment-jsdom-abstract`. ### Replace removed matcher aliases Use the canonical matcher names: | Removed | Replacement | | --- | --- | | `toBeCalled` | `toHaveBeenCalled` | | `toBeCalledTimes` | `toHaveBeenCalledTimes` | | `toBeCalledWith` | `toHaveBeenCalledWith` | | `lastCalledWith` | `toHaveBeenLastCalledWith` | | `nthCalledWith` | `toHaveBeenNthCalledWith` | | `toReturn` | `toHaveReturned` | | `toReturnTimes` | `toHaveReturnedTimes` | | `toReturnWith` | `toHaveReturnedWith` | | `lastReturnedWith` | `toHaveLastReturnedWith` | | `nthReturnedWith` | `toHaveNthReturnedWith` | | `toThrowError` | `toThrow` | Called-with matchers now infer the mocked function's parameter types. A previously running assertion can become a TypeScript error when its expected arguments do not match the mock signature. ### Migrate CLI path filters Rename `--testPathPattern` to plural `--testPathPatterns`; it accepts multiple patterns: ```bash jest --testPathPatterns "unit/.*" "integration/.*" ``` Programmatic watch integrations must construct `TestPathPatterns`. CLI paths are matched against relative test-file paths, so remove assumptions that they match absolute paths. `jest --init` is gone; initialize with: ```bash npm init jest@latest ``` Pass explicit values to options such as `--maxWorkers` and `--selectProjects`. A custom `--filter` must return `{filtered: Array}`, not an array. ### Replace removed mock APIs and types Replace `jest.genMockFromModule()` with `jest.createMockFromModule()`. Replace uses of `jest.SpyInstance` with `jest.Spied`; the public `MockFunctionMetadata`, `MockFunctionMetadataType`, and `SpyInstance` types were removed. Match `jest.mock()` paths to the filename's exact casing even on case-insensitive file systems. ### Stop deep-importing Jest packages Import public package entry points. Jest packages are bundled and expose ESM wrappers through package exports, so a path such as `jest-runner/build/testWorker` is not a stable import. Recheck custom glob patterns after the move to `glob` v10, particularly brace expansion and extglob behavior. ## High-value behavior changes ### Equality and subset matching `toEqual` and `objectContaining` ignore non-enumerable properties by default. Equality includes symbol-keyed properties. `toStrictEqual` accepts compatible cross-realm constructors produced by `structuredClone`, and subset matching no longer throws on exotic iterables. Review failures and newly passing tests as semantic changes; do not update expectations mechanically. ### Snapshots and formatting Snapshot text can change because: - serialized errors include `cause`; - React empty-string children are omitted; - `ArrayBuffer` and `DataView` have readable formatting; - React 19 values have formatter support; - the deprecated `goo.gl` documentation URL is replaced by the full URL. Review every snapshot update. Use static `SERIALIZABLE_PROPERTIES` on a custom object when only selected properties should appear in snapshots and matcher errors. ### Module interop Native ESM supports `import.meta.*`, `file://`, and validated import attributes. On supported Node.js versions, `require()` can load ES modules. When ESM imports CommonJS, the complete `module.exports` value is the default export. Do not expect Babel-style `__esModule` default unwrapping. Named imports can see own properties on a function assigned to `module.exports`, and all importers share the same CommonJS singleton. ### Test discovery Default module extensions and test matching include `.mjs`, `.cjs`, `.mts`, and `.cts`. If non-test files now match, constrain `testMatch` or `testRegex`. Discover tests without executing them: ```bash jest --collect-tests ``` Write test-file discovery to a file with: ```bash jest --listTests --outputFile test-files.json ``` ### Project configuration Project entries can set `testTimeout`, `coverageReporters`, `reporters`, `verbose`, `silent`, `collectCoverage`, and `coverageProvider`. Project-level `verbose` and `silent` override the corresponding global settings. The `jest` field in `package.json` may be a path to a configuration file: ```json {"jest": "./config/jest.config.js"} ``` Use `defineConfig` and `mergeConfig` from `jest-config` for typed configuration declaration and composition. Import `GlobalConfig` and `ProjectConfig` types from `jest`. ## Useful APIs ### Auto-restoring spies With explicit resource management, `using` restores a spy at block exit: ```ts using warnSpy = jest.spyOn(console, 'warn'); ``` ### Generated mock customization Each `jest.onGenerateMock()` callback must return the mock: ```js jest.onGenerateMock((modulePath, moduleMock) => { if (modulePath.includes('Database')) moduleMock.connect = jest.fn(); return moduleMock; }); ``` ### Retry controls Configure delay or immediate retry: ```js jest.retryTimes(3, {waitBeforeRetry: 1000}); jest.retryTimes(3, {retryImmediately: true}); ``` ### Data-driven tests Use `%$` for the test-case sequence number. Readonly tables are accepted and receive improved callback argument inference: ```ts const cases = [[1, 2, 3]] as const; test.each(cases)('case %$: %i + %i = %i', (a, b, total) => { expect(a + b).toBe(total); }); ``` ### Fake timers Run pending animation-frame callbacks with `advanceTimersToFrame()`. Timer advancement and clock setup also accept supported Temporal values. Consult the execution reference before mixing fake timers, asynchronous work, and retries. ## Verification checklist - Confirm supported Node.js and TypeScript versions in every environment. - Search for removed matcher, mock, type, and CLI names. - Check mock path casing and package deep imports. - Revalidate `.mjs`, `.cjs`, `.mts`, and `.cts` discovery. - Review relative CLI path filters and custom filter return values. - Exercise custom runners, sequencers, environments, transformers, and reporters. - Review changed equality semantics instead of blindly accepting results. - Inspect snapshots for error, React, binary-data, and URL formatting changes. - Test ESM/CJS boundaries and JSON import attributes. - Run discovery-only commands and confirm selected test files. - Validate per-project reporting and coverage behavior. - Check cleanup warnings, leak detection, and worker shutdown behavior. - Run the full suite after focused migration checks pass.