test: implement timezone mock helper for Vitest test suite#5947
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Difficulty: intermediate – New timezone-mock.ts helper (275 lines) with mockTimezone/restoreTimezone/withTimezone functions, 256-line test suite covering edge cases.
Quality: clean – Comprehensive testing utility with Intl.DateTimeFormat mocking, offset calculations, common timezone constants.
Type: testing – Reusable test infrastructure for timezone testing.
Excellent test utility!
Closes JhaSourav07#5274 - Create test-utils/timezone-mock.ts with mockTimezone(), restoreTimezone(), withTimezone(), getTimezoneOffsetMinutes(), and TEST_TIMEZONES exports - Mock Intl.DateTimeFormat to default to target timezone when no explicit timeZone option is provided - Set process.env.TZ for code paths that rely on environment variable - Include offset lookup table for 50+ IANA timezone identifiers including fractional offsets (Asia/Kathmandu UTC+5:45, Pacific/Chatham UTC+12:45) - Fallback to real Intl.DateTimeFormat for unrecognised timezone strings - Add 26 comprehensive tests covering all exported functions and integration with Intl.DateTimeFormat.formatToParts under fake timers
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Summary
Closes #5274
Problem
Timezone-sensitive tests fail on CI servers because the system timezone defaults to UTC, causing
Intl.DateTimeFormatto produce different results than on developer machines.Solution
Created a reusable timezone mock helper at
test-utils/timezone-mock.tsthat:mockTimezone(tz)— Setsprocess.env.TZand overridesIntl.DateTimeFormatto default to the target timezone.restoreTimezone()— Restores bothprocess.env.TZandIntl.DateTimeFormatto their original values.withTimezone(tz, fn)— Convenience wrapper that auto-restores even if the callback throws.getTimezoneOffsetMinutes(tz)— Returns the UTC offset in minutes for a given IANA timezone string.TEST_TIMEZONES— A readonly array of 13 common timezone identifiers for parameterised tests.Implementation Details
Intl.DateTimeFormatfor timezone strings not in the lookup tableRangeErrorfor invalid timezone identifiers (matching native behavior)Verification
npx vitest run test-utils/timezone-mock.test.ts✅Files Changed
test-utils/timezone-mock.ts— New timezone mock helper moduletest-utils/timezone-mock.test.ts— 26 comprehensive tests