Tag See all →

#esm

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: Node.js event loop phases with timers, check and microtask queues racing in different orders

Programming Languages Bug Report

Flaky Node.js ordering: setTimeout 0 vs setImmediate vs process.nextTick

John Nguyen · Dec 19, 2024 · 8 min read

A batching test in our Node.js telemetry library failed about one run in twenty on CI and never locally. The code assumed setTimeout 0 fires before setImmediate, which Node does not guarantee in the main module. Moving to ESM later flipped nextTick and promise order too.