QABattle

JavaScript & TypeScript / Scenario

Boss Fight: The 800-Test Migration

Jest out, Vitest in, one sprint, no coverage dips allowed. Globals, factories, requireActual, aliases, and the order of operations that decides whether this lands or burns.

Difficulty
Boss
Format
Scenario
Points
300
Estimate
20 min

// MISSION BRIEF

Your Mission

Leadership approved the Jest-to-Vitest migration you have been advocating for a year: 800 tests, 14 packages, one sprint, and a hard rule that main stays green throughout.

The first mechanical attempt (rename jest to vi, swap the config) produced a wall of distinct failures. Each one is a category, not a one-off. Read the artifacts, decode each failure class, and sequence the migration like someone who has done this before.

This is a boss fight: five weighted decisions, partial credit where reasoning is partially right.

// FIRST CONTACT

Battle teaser

First artifact

jest.config.js (being replaced)

Failure [1], 'jest is not defined' even with globals: true enabled. The correct understanding:

  1. AAdd a globalThis.jest = vi shim in setup and keep the old spelling forever, cost-free
  2. Bglobals: true injects describe/it/expect/vi, but the jest NAMESPACE never exists in Vitest; every jest.fn/jest.spyOn/jest.mock call must become vi.* (a mechanical, codemod-friendly rename)
  3. CThe tests must stop using mocks to be Vitest-compatible
  4. Dglobals: true is broken; import jest from 'vitest' instead
Answers, scoring, hints, and the full battle stay sealed.

// SKILL TAGS

javascripttypescriptvitestjestmigration