Jest

• Ditch Jest for Vitest: A Ready-Made Migration Prompt

  If you’ve ever sat there watching Jest crawl through your TypeScript test suite, you know pain. I mean, I know your pain.

  When Switching to Vitest, and the speed difference is genuinely dramatic. The answers to why it’s slow are easy to figure out. There’s plenty of explanations on that, so I’ll leave that to you to go look up why.

  I put together a prompt you can hand to Claude (or any AI assistant) that will handle the migration for you. Let me know how it goes!

  The Migration Prompt

  Convert all Jest tests in this project to Vitest. Here's what to do:
  
  ## Setup
  
  1. Remove Jest dependencies (`jest`, `ts-jest`, `@types/jest`, `babel-jest`, any jest presets)
  2. Install Vitest: `pnpm add -D vitest`
  3. Remove `jest.config.*` files
  4. Add a `test` section to `vite.config.ts` (or create `vitest.config.ts` if no Vite config exists):
  
  import { defineConfig } from 'vitest/config'
  
  export default defineConfig({
    test: {
      globals: true,
    },
  })
  
  5. Update the `test` script in `package.json` to `vitest`
  
  ## Test File Migration
  
  For every test file:
  
  1. Replace imports — Remove any `import ... from '@jest/globals'`. If `globals: true` is set, no imports needed. Otherwise add:
     import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
  
  2. Replace `jest` with `vi` everywhere:
     - jest.fn() → vi.fn()
     - jest.mock() → vi.mock()
     - jest.spyOn() → vi.spyOn()
     - jest.useFakeTimers() → vi.useFakeTimers()
     - jest.useRealTimers() → vi.useRealTimers()
     - jest.advanceTimersByTime() → vi.advanceTimersByTime()
     - jest.clearAllMocks() → vi.clearAllMocks()
     - jest.resetAllMocks() → vi.resetAllMocks()
     - jest.restoreAllMocks() → vi.restoreAllMocks()
     - jest.requireActual() → vi.importActual() (note: async in Vitest)
  
  3. Fix mock hoisting — vi.mock() is hoisted automatically, but variables used in the mock factory must be prefixed with `vi` or declared inside the factory.
  
  4. Fix jest.requireActual — This becomes vi.importActual() and returns a Promise:
  
     // Jest
     jest.mock('./utils', () => ({
       ...jest.requireActual('./utils'),
       fetchData: jest.fn(),
     }))
  
     // Vitest
     vi.mock('./utils', async () => ({
       ...(await vi.importActual('./utils')),
       fetchData: vi.fn(),
     }))
  
  5. Snapshot tests work the same way. No changes needed.
  6. Timer mocks — same API after the jest → vi rename.
  7. Module mocks with __mocks__ directories work identically.
  
  ## TypeScript Config
  
  Add Vitest types to tsconfig.json:
  {
    "compilerOptions": {
      "types": ["vitest/globals"]
    }
  }
  
  ## After Migration
  
  1. Delete leftover Jest config files
  2. Update CI config to use `vitest run`
  3. Run tests and fix any remaining failures
  

  ---

  If you’re still on Jest and your TypeScript test suite is dragging, give Vitest a shot. The migration is low-risk, the speed improvements are real, and with the prompt above, the hardest parts are handled. ☕

  Feb 19, 2026 / Development / Testing / Typescript / Vitest / Jest

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