Upgrade your Test Suite with fn-monitor
Modern JavaScript testing focuses heavily on testing public outputs and user behavior rather than chasing line-by-line or branch-by-branch coverage.…
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Fake timers and await are fighting in your Vitest test
You mock the clock, advance it, and assert. The assertion runs before the code under test has finished. async function poll () { await new Promise (…
Why your vi.mock in Vitest silently does nothing
You wrote a mock. The test still hits the real module. Nothing in the error message points at why. import { fetchUser } from ' ./api ' import { vi ,…
Testing AI Agents with Vitest 4 — Mocking LLM Calls and Streaming Responses in Practice
At some point when writing AI agent code, testing comes to a halt. Without an ANTHROPIC_API_KEY , the code won't even run. With one, every test run…
Migrating from Jest to Vitest 4: A Complete 2026 Guide
Last month I overhauled the test pipeline for a side project and switched from Jest to Vitest. The reason was straightforward: maintaining Jest in a…
The Unexpected optimization in ESLint vs Vitest: A Head-to-Head
In a 2024 benchmark of 10,000+ line TypeScript codebases, Vitest outperformed ESLint ’s native rule execution by 47% in cold-start lint speed when c…