Effect ts test integration with vitest

Skill kjuhwa/skills-hub/skills/testing/effect-ts-test-integration-with-vitest

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Test effect-based services by composing layers with Vitest, using drain() instead of sleep for deterministic async

SKILL.md

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When to Apply

You write tests for Effect.ts-based services with side effects (I/O, async, external services)
You want tests to be deterministic and not flaky
You're using Vitest as your test runner (not Jest)
You need to stub external services (git, providers, filesystem)

Steps

Define your service layer as Layer.effect(TAG) that yields a { method1, method2 } interface
Create test doubles (stubs) that implement the same interface but return fixed/controlled values
In test, build a layer graph that includes test doubles instead of real implementations
Use Effect.gen to compose the test effect
Instead of sleep, use worker.drain() or stream.toArray() to wait for async work
Run effect via Effect.runPromise() or Vitest's Effect integration
Assert state after all async milestones complete

Example

const mockGitCore = Layer.succeed(GitCore, {
  executeGit: (cmd) => Effect.succeed({ stdout: '...' }),
  getStatus: () => Effect.succeed({ isRepo: true })
})

const testEffect = Effect.gen(function*() {
  const orchestration = yield* OrchestrationEngine
  const worker = yield* checkpointReactor

  yield* orchestration.dispatch({ type: 'thread.turn.start', ... })
  yield* worker.drain()  // wait for checkpoint work to finish

  const checkpoints = yield* orchestration.getCheckpoints(...)
  expect(checkpoints).toHaveLength(1)
})

const layers = Layer.compose(mockGitCore, OrchestrateLayer)
const result = yield* testEffect.pipe(Effect.provide(layers))

Counter / Caveats

Layer composition order matters; dependencies must be available before use
Test doubles can diverge from real implementations; keep in sync or use property-based tests
drain() only works for DrainableWorker; other async patterns need Effect.sleep or Future
Large test suites with many layers can slow down test startup; consider shared layer fixtures