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#vitest

2 stories

Programming Languages Bug Report

jest.mock silently ignored after switching a Node.js project to native ESM

Daniel Tran · Sep 18, 2025 · 9 min read

After we set "type": "module" in package.json, jest.mock stopped replacing our mailer and tests started calling the real SMTP client. Static ESM imports are linked before any mock runs. Here is the mechanism and the fix with unstable_mockModule.

Banner: a calendar where the 31st of January jumps past February into March during a failing test

Testing & QA Bug Report

Flaky test fails only on the 31st: month arithmetic with the real clock

Daniel Tran · Jan 31, 2025 · 8 min read

A billing test passed for weeks and failed this morning, 31 January, for every engineer at once. JavaScript's setMonth rolled 31 January into 3 March, and the test read the real clock. We fixed the arithmetic, injected the clock and added property tests over every day.