Jest v30 operator

Write and repair Jest tests for promises, async functions, callbacks, rejected errors, assertion counts, and test timeouts. Use when an async test passes too early, hangs, reports an unhandled rejection, or needs a reliable success and failure path.

Answer quick Jest command, flag, API, configuration, and test-isolation questions from a version-aware reference. Use when the user asks how to run, target, mock, snapshot, time, cover, configure, or troubleshoot a common Jest operation.

Design or verify deterministic Jest commands for CI, including non-interactive execution, coverage and JSON output, worker limits, project selection, retries, and sharding. Use when a Jest run behaves differently in CI or a pipeline needs a reliable test command.

Inspect and make deliberate, minimal changes to Jest configuration for discovery, environments, transforms, module mapping, setup files, projects, coverage, and mock isolation. Use when Jest cannot find tests or modules, uses the wrong environment, or needs an explicit config change.

Collect and interpret Jest coverage for the requested tests or source files, inspect uncovered paths, and make threshold changes only when explicitly requested. Use when the user asks for coverage, a coverage report, a threshold check, or help understanding missing coverage.

Inspect a repository's Jest installation, package manager, test scripts, configuration, test environment, and project layout before another Jest operation. Use when Jest is missing, the active config is unclear, a command behaves differently from expectation, or you need a safe baseline without changing files.

Reproduce and fix nondeterministic Jest tests, covering repeat and randomized runs, order dependence from shared state and leaked mocks, real timers and unawaited promises, open handles that hang a suite, time zone and locale assumptions, concurrency between workers, and why retries hide a defect rather than fixing it. Use when a test passes locally and fails in the pipeline, when a suite fails only in a particular order, or when a run hangs or times out intermittently.

Connect Jest to the tools around it, covering Testing Library and component rendering, request interception with Mock Service Worker, real databases and containers in integration tests, coverage reporting to a pipeline or coverage service, JUnit and JSON reporters for build artifacts, editor and debugger integration, and monorepo project selection. Use when tests must exercise a component, a stubbed network call, or a real dependency, or when the pipeline needs machine-readable results and coverage.

Create and diagnose Jest mock functions, spies, module mocks, manual mocks, implementations, return values, and mock cleanup. Use when a test needs to isolate a dependency or fails because mock state or restoration is wrong.

Diagnose and reduce Jest suite runtime and memory use, covering per-file timing, worker count and its tradeoffs, transform and cache costs, the environment choice, module resolution and barrel imports, heavy setup files, memory growth between files, sharding across machines, and measuring before and after each change. Use when a suite takes too long, when a run exhausts memory, or when the pipeline test step dominates build time.

Run Jest tests at the right scope: the full suite, an exact file, a file pattern, a test name, changed files, related tests, or only previous failures. Use when the user wants to execute, narrow, reproduce, or verify Jest tests without changing test code.

Install and configure Jest in a repository from zero, choosing a TypeScript transform such as ts-jest, Babel, or SWC, deciding between CommonJS and native ECMAScript modules, selecting the node or jsdom test environment, wiring setup files, module name mapping, and package scripts. Use when a project has no test runner yet, when Jest is installed but cannot parse or resolve the project's files, or when the user asks how to add Jest to an application or library.

Review, update, and troubleshoot Jest snapshot tests with a narrow test scope and explicit approval for snapshot file changes. Use when snapshots fail, a snapshot needs intentional regeneration, or the user asks how to inspect snapshot differences.

Write focused Jest tests from an existing behavior contract, covering deriving cases from the contract rather than the implementation, naming and arranging a test, table-driven cases, choosing the right assertion, boundary and error paths, what to stub and what to leave real, and proving a test can fail. Use when new tests must be added for existing behavior, when a bug needs a regression test, or when a suite passes without actually checking anything.

Control Jest fake timers and diagnose timer-driven tests, intervals, polling, debounced work, and timer leaks. Use when time-dependent tests are slow, flaky, stuck, or need deterministic advancement without sleeping.

Diagnose Jest failures systematically across test discovery, transforms, ESM and TypeScript, environments, module mocks, async work, timers, snapshots, coverage, workers, and open handles. Use when a Jest command fails, hangs, flakes, or behaves differently locally and in CI.

Start and control Jest watch mode for fast local feedback, selecting changed tests, all tests, a file, or a test name while preserving an interactive terminal workflow. Use when the user wants Jest to rerun tests as files change or needs help with watch-mode controls.