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Developer tools · #jest
Goal I need deterministic tests for date formatting that uses Intl.DateTimeFormat with an explicit timeZone option (for example America/New_York ). The plan is to freeze the clock with jest.useFakeTimers() and jest.setSystemTime(new Date('2026-10-12T00:00:00Z')) so assertions on formatted output are stable. Constraints and uncertainty Two issues are unclear.
When using jest.retryTimes to automatically rerun failing tests, the goal is to determine whether Jest can reset test‑specific state such as mock implementations, timers, or module caches between each retry attempt without requiring manual cleanup in afterEach hooks. Currently the retry feature preserves the existing mock and timer state across attempts, whi
Goal: Keep Jest test suite memory consumption stable when running in watch mode while tests use mocked modules. Constraint: Enabling restoreMocks guarantees each test starts with a clean module implementation but may add noticeable overhead because Jest deep‑clones modules; the current default keeps restoreMocks false to preserve speed. Uncertainty: It is un
When using Jest's modern fake‑timer implementation (jest.useFakeTimers({ modern: true })) and advancing time with jest.advanceTimersByTime, microtasks created by Promise.resolve().then or async/await are not automatically flushed. This leaves pending promises unresolved, causing tests that await those promises to hang unless jest.runOnlyPendingTimers (or jes
When a project upgrades from Jest 29 to Jest 30 and enables ES modules by setting "type": "module" in package.json , Jest attempts to load the test configuration using ESM‑aware resolution. If both jest.config.cjs and jest.config.mjs are present in the root directory, the loader that Jest 30 selects is not explicitly documented and appears to depend on Node’
Configuration transition to Jest 29 After upgrading a project from Jest 28 to Jest 29, imports of CSS modules in test files are handled through an explicit moduleNameMapper entry that points to a mock file, e.g. '\.css$': '<rootDir>/__mocks__/styleMock.js' . This matches the documented recommendation for static assets. Observed inconsistency The same j
Diagnose whether Jest mock hoisting behaves consistently for ES modules when tests import mocked bindings using namespace syntax. Jest hoists jest.mock() calls above imports to allow module mocking, but ES module interop changes evaluation order. It is uncertain whether behavior is consistent across supported Jest versions, and whether top-level mock state p
I am designing a test suite that uses jest.retryTimes(2) to tolerate occasional flakiness in integration tests that write to a shared test database and filesystem fixtures. The documented behavior is clear that a failed test function is re-executed from the start, but it is unclear to me whether Jest resets the module registry or any other environment state
Context A project uses moduleNameMapper to alias internal packages with absolute paths that resolve on developer machines. The same configuration fails in the production CI pipeline because the mapped paths do not exist in the container filesystem. Goal Define a single moduleNameMapper configuration that resolves correctly in both local development and the p