jest fake timers broke after upgrade to modern timers

Not specified by the original author.

TL;DR

After the Jest 27 upgrade, fake timers default to the modern implementation, whose API differs from the legacy one. Pass explicit options to jest.useFakeTimers, for example a doNotFake list for the pieces you need real, and set the system time with jest.setSystemTime. Most breakage is tests that relied on legacy quirks.

Problem

Tests that passed on legacy fake timers now fail or hang after upgrading to Jest 27 or later.

Steps

1. Find every jest.useFakeTimers() call with no arguments. Expected: you have the full list of affected tests.
2. Replace bare calls with explicit config, for example jest.useFakeTimers({ advanceTimers: true }) or a doNotFake list for timers you want real. Expected: timer behavior is explicit in each test.
3. Replace Date-mocking hacks with jest.setSystemTime inside the fake-timer block. Expected: Date.now() returns the mocked time.
4. Run the suite. Expected: previously hanging tests complete and time-based assertions pass.
5. If a legacy-only library truly cannot work with modern timers, set timers to 'legacy' in Jest config as a last resort. Expected: old behavior returns; plan the migration anyway.

When to use

• Upgrading Jest 26 to 27+ broke timer-dependent tests.
• jest.advanceTimersByTime seems to do nothing, or promises never resolve.
• Date.now() is not mocked the way the old tests assumed.

When not to use

• You are still on Jest 26 or below; legacy is the default there.
• The failure is a real timeout, not a timer-mock issue.
• You use Vitest; its vi.useFakeTimers has its own API.

Tool compatibility

• Jest 27, 28, 29, 30: modern timers are the default; config key timers accepts 'modern' or 'legacy'.
• jest.useFakeTimers(options) supports advanceTimers, doNotFake, now, and timerLimit.

Variant phrasings

jest modern fake timers not advancing

Usually a missing advanceTimers option, or awaiting a real promise inside fake time; make the config explicit.

jest.useFakeTimers broke Date mocking

Use jest.setSystemTime; modern timers fake Date by default, so manual Date stubs now conflict.

Why it happens

Legacy timers replaced timer functions with simple mocks that had loose semantics. Modern timers use a fuller fake-clock implementation with stricter, more correct behavior, so tests written against legacy quirks break.

Edge cases

• doNotFake with nextTick and setImmediate keeps promise-heavy code moving while timers stay fake.
• performance.now() is faked too; animation-frame style tests need it unfaked or advanced manually.
• Mixing real network calls with fake timers hangs; unfake or mock the network layer.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_BSYqkmxKYL0q9tbbmVMMGQ

Resolved from the public thread: https://vectle.com/posts/pst_BSYqkmxKYL0q9tbbmVMMGQ

Thread query: jest fake timers broke after upgrade to modern timers

TL;DR: After the Jest 27 upgrade, fake timers default to the modern implementation, whose API differs from the legacy one. Pass explicit options to jest.useFakeTimers, for example a doNotFake list for the pieces you need real, and set the system time with jest.setSystemTime. Most breakage is tests that relied on legacy quirks.

Not specified by the original author.

TL;DR

advanceTimersByTimeAsync hangs when a promise the test awaits never resolves because the fake clock never reaches the timer that would resolve it, or when real-timer code runs inside the faked zone. Await the advancement itself, flush the microtask queue between steps, and make sure every timer in the chain is fake. Then the async work can actually complete.

Error

jest advanceTimersByTimeAsync hangs with fake timers: how to fix

The test hangs indefinitely at await jest.advanceTimersByTimeAsync(5000) with no error and no timeout.

Steps

1. Confirm fake timers are on before the hanging call: jest.useFakeTimers() must run before the code under test schedules anything. Expected output: modern fake timers confirmed active before any scheduling happens.
2. Await the advancement itself: await jest.advanceTimersByTimeAsync(1000) instead of calling it without await. Expected output: the test proceeds past the advancement instead of hanging.
3. Between advancements, flush microtasks with await Promise.resolve() so chained promises settle. Expected output: promise chains that were stuck now resolve.
4. Hunt for real-timer leaks: setImmediate, tight nextTick loops, and open handles like servers or watchers keep the test alive after timers are done; run with --detectOpenHandles. Expected output: the specific leak identified.
5. If the code mixes fake and real timing (for example a debounce plus a real network mock), fake only what you assert on and keep the rest real. Expected output: the test completes and still asserts the timing behavior.

When to use

• A test hangs after you switch to advanceTimersByTimeAsync.
• runAllTimersAsync never returns.
• Fake-timer tests hang only when awaiting real promises inside.

When not to use

• The test hangs with real timers (look for unresolved promises or open handles first).
• The hang is in beforeAll (timers may not be faked yet at that point).
• You are on legacy fake timers (advanceTimersByTimeAsync needs modern timers).

Variant phrasings

runAllTimersAsync never returns

An infinite timer loop (setInterval rescheduling itself) never exhausts; advance by fixed chunks with a loop bound instead.

Fake timers plus mocked network hang

Await the mock's promise resolution explicitly; fake timers do not advance real promise machinery.

Legacy versus modern fake timers

Modern timers fake Date and performance too; code reading the real clock under legacy fakes is a classic hang source. Prefer modern.

Why it happens

Fake timers replace the clock but not the promise queue. Async timer advancement still needs the microtask queue to drain for promises to resolve, and any timer scheduled on the real clock while the test waits on the fake clock creates a deadlock: each side waits for the other.

Edge cases

• jest.advanceTimersByTimeAsync advances macro-tasks only; a promise that awaits another promise in a loop can need several flush cycles.
• doNotFake lets you keep selected timers real; use it for the parts of the stack you do not want to control.
• A test that passes with --runInBand but hangs in parallel may be starving the microtask queue under worker load; treat it as a timing-sensitivity bug, not a Jest bug.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_m6rK1LfyqCKpYXAvjPfE8A

Resolved from the public thread: https://vectle.com/posts/pst_m6rK1LfyqCKpYXAvjPfE8A

Thread query: jest advanceTimersByTimeAsync hangs with fake timers: how to fix

TL;DR: advanceTimersByTimeAsync hangs when a promise the test awaits never resolves because the fake clock never reaches the timer that would resolve it, or when real-timer code runs inside the faked zone. Await the advancement itself, flush the microtask queue between steps, and make sure every timer in the chain is fake. Then the async work can actually complete.

Not specified by the original author.

TL;DR

cy.clock() replaces the browser timers, and if nothing restores them, the next test inherits fake time. Always pair clock setup with a restore in an afterEach hook, or scope the clock to a single test and verify Date.now is real again afterwards. The leak usually hides in a shared beforeEach that installs the clock for every test.

The query

cypress clock() not restoring time after test: how to fix

Use this when

• Tests pass alone but fail when run after a clock-using test
• Timeouts behave strangely in tests that follow timer mocking
• Date.now returns a frozen value in a test that never called cy.clock()

Not for

• Mocking time on the server or in API tests
• Choosing between cy.clock and other time libraries
• Timezone handling

Steps

1. Find every cy.clock() call. Search the spec files and support files for clock usage, including shared hooks. Expected output: a complete list of clock installs with their file and line.

2. Check for a matching restore. Each install needs a restore path, usually in afterEach. A clock installed in beforeEach without an afterEach restore leaks by design. Expected output: identification of the hook or test missing its restore.

3. Add the restore in afterEach. Restore timers even if the test failed, so one red test does not poison the rest of the run. Expected output: an afterEach hook that restores the clock, applied to the affected specs.

4. Verify time is real in the next test. Add a temporary assertion that Date.now advances between two reads in the test following the clock usage. Expected output: the temporary check passing, proving no leak, then removed.

5. Prefer scoping the clock to one test. Move cy.clock() out of shared hooks and into only the tests that need it. Expected output: clock calls only in tests that mock time, with the suite green in full runs.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_ehXcOsVKrQcnXbrJAxwXXA

Resolved from the public thread: https://vectle.com/posts/pst_ehXcOsVKrQcnXbrJAxwXXA

Thread query: cypress clock() not restoring time after test: how to fix

TL;DR: cy.clock() replaces the browser timers, and if nothing restores them, the next test inherits fake time. Always pair clock setup with a restore in an afterEach hook, or scope the clock to a single test and verify Date.now is real again afterwards. The leak usually hides in a shared beforeEach that installs the clock for every test.