Updated November 2026

Migrating from Jest to Vitest

Class Duration

7 hours of live training delivered over 1 day

Student Prerequisites

Professional JavaScript or TypeScript development experience
Experience writing and maintaining tests with Jest
Familiarity with npm-based tooling and CI pipelines

Target Audience

Developers, tech leads, and platform engineers who maintain Jest test suites and plan to move them to Vitest. It fits front-end teams on Vite, Node.js and TypeScript teams with ESM transform problems, and monorepo owners migrating package by package. For more on AI-assisted workflows, see AI-Assisted Development with JavaScript and TypeScript.

Description

This one-day course moves an existing Jest suite to Vitest without a long freeze or lost coverage. Students map Jest configuration to Vitest and convert the APIs that the official migration guide calls out. These include vi mocks, module mock factories, vi.importActual, mock reset behavior, hooks, and fake timers. The course also covers snapshots, ESM and TypeScript tests, DOM environments, coverage, and CI. The last part covers migration at scale with codemods, AI coding agents, and Jest and Vitest running side by side.

Learning Outcomes

Decide whether a Jest suite should move to Vitest.
Translate Jest configuration to Vitest configuration.
Convert Jest mocks, spies, hooks, and timers to Vitest APIs.
Migrate snapshot tests without churning snapshot files.
Run ES module and TypeScript tests without a transform pipeline.
Choose between jsdom, happy-dom, and Browser Mode.
Match Jest coverage and CI behavior with V8 coverage and sharding.
Automate conversion with codemods and AI coding agents.
Migrate a monorepo incrementally with Vitest projects.

Training Materials

All students receive comprehensive courseware covering all topics in the course. Courseware is distributed via GitHub in the form of documentation and extensive code samples. Students practice the topics covered through challenging hands-on lab exercises.

Software Requirements

Students will need a free, personal GitHub account to access the courseware. No local student setup is required, this course is delivered with GitHub Codespaces. The free allotment of Codespaces hours from GitHub is sufficient to complete the course.

Versions covered: Vitest 5 (with Vite 6.4 or later) on Node.js 22.12 or later, migrating from Jest 29 or 30.

Training Topics

Why Teams Move to Vitest

Native ESM and TypeScript
Sharing Vite config with the app
Watch mode and smart re-runs
Parallel execution
What carries over unchanged
When to stay on Jest

Planning the Migration

Inventorying a Jest setup
Custom matchers and shared mocks
Big-bang vs. incremental migration
Success criteria and CI time

Configuration Mapping

jest.config to vitest.config.ts
The test block in vite.config
testEnvironment to environment
setupFilesAfterEnv to setupFiles
moduleNameMapper to resolve.alias
Transforms to Vite plugins
testMatch to include and exclude
globals: true vs. explicit imports
vitest/globals types
JEST_WORKER_ID to VITEST_POOL_ID

API Differences

vi.fn, vi.spyOn, and vi.mocked
Module mocking with vi.mock
Hoisting and vi.hoisted
Mock factories that return every export
jest.requireActual to vi.importActual
The importOriginal helper
When __mocks__ folders apply
mockReset semantics
Clearing mocks by default
Converting done callbacks to promises
Hooks that return teardown functions
Hook ordering
jest.setTimeout to vi.setConfig
Test-name filtering with -t

Timers and Dates

vi.useFakeTimers and vi.advanceTimersByTime
Async timer methods
No legacy fake timers
Faking Date and Temporal
vi.setSystemTime

Snapshots

toMatchSnapshot and inline snapshots
File snapshots
Snapshot format differences
Regenerating vs. preserving snapshots
Reviewing snapshot diffs
Custom serializers

ES Modules and TypeScript

Tests without Babel or ts-jest
Path aliases and tsconfig
Type tests with expectTypeOf
The --typecheck flag
Mixed CommonJS code

Test Environments

node, jsdom, and happy-dom
Per-file @vitest-environment comments
Browser Mode with Playwright
Real browser vs. simulated DOM
Testing Library under Vitest

Coverage

V8 and Istanbul coverage
coverage.include for untested files
Coverage thresholds
Comparing with Jest reports
Reporters for CI and code review

CI

JUnit and GitHub Actions reporters
Sharding with --shard
Blob reports and --merge-reports
Affected tests with --changed
Forks and threads pools
Isolation and run-time tuning

Codemods

Community Jest-to-Vitest codemods
What codemods miss
Dry runs and diff review
Finishing the manual cases

Migrating with AI Coding Agents

Migration instructions for agents
File-by-file conversion
The test runner as verifier
Small, reviewable batches
Catching invented APIs
Catching skipped tests
Catching weakened assertions
Codemods, agents, and human review

Incremental Migration in Monorepos

Jest and Vitest side by side
Vitest projects per package
Sharing configuration across packages
Task runners and CI during transition
Retiring Jest