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Overview

Effect provides first-class testing support through the @effect/vitest package. This integration makes it easy to test Effect programs with:
  • it.effect: Write tests using Effect.gen syntax
  • TestClock: Control time for testing scheduled operations
  • Layer testing: Share service layers across tests
  • Property-based testing: Generate test cases with Schema

​
Installation

Install the testing package: 
npm install --save-dev @effect/vitest

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Basic Tests with it.effect

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Writing Your First Test

Use it.effect instead of regular it to test Effect code: 
import { assert, describe, it } from "@effect/vitest"
import { Effect } from "effect"

describe("@effect/vitest basics", () => {
  it.effect("runs Effect code with assert helpers", () =>
    Effect.gen(function*() {
      const upper = ["ada", "lin"].map((name) => name.toUpperCase())
      assert.deepStrictEqual(upper, ["ADA", "LIN"])
      assert.strictEqual(upper.length, 2)
      assert.isTrue(upper.includes("ADA"))
    })
  )
})

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Parameterized Tests

Test multiple cases with it.effect.each: 
import { assert, it } from "@effect/vitest"
import { Effect } from "effect"

it.effect.each([
  { input: " Ada ", expected: "ada" },
  { input: " Lin ", expected: "lin" },
  { input: " Nia ", expected: "nia" }
])("parameterized normalization %#", ({ input, expected }) =>
  Effect.gen(function*() {
    assert.strictEqual(input.trim().toLowerCase(), expected)
  })
)

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Controlling Time with TestClock

The TestClock service lets you control virtual time in tests: 
import { assert, it } from "@effect/vitest"
import { Effect, Fiber } from "effect"
import { TestClock } from "effect/testing"

it.effect("controls time with TestClock", () =>
  Effect.gen(function*() {
    const fiber = yield* Effect.forkChild(
      Effect.sleep(60_000).pipe(Effect.as("done" as const))
    )
    
    // Move virtual time forward to complete sleeping fibers immediately
    yield* TestClock.adjust(60_000)
    
    const value = yield* Fiber.join(fiber)
    assert.strictEqual(value, "done")
  })
)
This is essential for testing:
  • Scheduled operations
  • Retry logic with delays
  • Timeout behavior
  • Rate limiting

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Testing with Real Runtime Services

Use it.live when you need real runtime services (like actual time): 
import { assert, it } from "@effect/vitest"
import { Effect } from "effect"

it.live("uses real runtime services", () =>
  Effect.gen(function*() {
    const startedAt = Date.now()
    yield* Effect.sleep(1)
    assert.isTrue(Date.now() >= startedAt)
  })
)

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Property-Based Testing

Generate test cases automatically using Schema arbitraries: 
import { assert, it } from "@effect/vitest"
import { Effect, Schema } from "effect"

it.effect.prop(
  "reversing twice is identity",
  [Schema.String],
  ([value]) =>
    Effect.gen(function*() {
      const reversedTwice = value.split("").reverse().reverse().join("")  
      assert.strictEqual(reversedTwice, value)
    })
)

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Testing Services with Layers

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Creating Test Services

Define test-specific implementations using test refs: 
import { Array, Effect, Layer, Ref, ServiceMap } from "effect"

export interface Todo {
  readonly id: number
  readonly title: string
}

// Test ref service for storing test data
export class TodoRepoTestRef extends ServiceMap.Service<
  TodoRepoTestRef,
  Ref.Ref<Array<Todo>>
>()("app/TodoRepoTestRef") {
  static readonly layer = Layer.effect(TodoRepoTestRef, Ref.make(Array.empty()))
}

class TodoRepo extends ServiceMap.Service<TodoRepo, {
  create(title: string): Effect.Effect<Todo>
  readonly list: Effect.Effect<ReadonlyArray<Todo>>
}>()("app/TodoRepo") {
  static readonly layerTest = Layer.effect(
    TodoRepo,
    Effect.gen(function*() {
      const store = yield* TodoRepoTestRef
      
      const create = Effect.fn("TodoRepo.create")(function*(title: string) {
        const todos = yield* Ref.get(store)
        const todo = { id: todos.length + 1, title }
        yield* Ref.set(store, [...todos, todo])
        return todo
      })
      
      const list = Ref.get(store)
      
      return TodoRepo.of({ create, list })
    })
  ).pipe(
    // Provide test ref as a dependency
    Layer.provideMerge(TodoRepoTestRef.layer)
  )
}

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Testing with Shared Layers

Use layer(...) to create one shared layer for multiple tests: 
import { assert, layer } from "@effect/vitest"
import { Effect } from "effect"

layer(TodoRepo.layerTest)("TodoRepo", (it) => {
  it.effect("tests repository behavior", () =>
    Effect.gen(function*() {
      const repo = yield* TodoRepo
      const before = (yield* repo.list).length
      assert.strictEqual(before, 0)
      
      yield* repo.create("Write docs")
      
      const after = (yield* repo.list).length
      assert.strictEqual(after, 1)
    })
  )
  
  it.effect("layer is shared", () =>
    Effect.gen(function*() {
      const repo = yield* TodoRepo
      const before = (yield* repo.list).length
      assert.strictEqual(before, 1) // Previous test data is still present
      
      yield* repo.create("Write docs again")
      
      const after = (yield* repo.list).length
      assert.strictEqual(after, 2)
    })
  )
})
The layer(...) function:
  • Creates the layer once before all tests
  • Shares it across all tests in the block
  • Tears it down in afterAll

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Testing Higher-Level Services

Test services that depend on other services: 
import { assert, describe, it } from "@effect/vitest"
import { Effect, Layer, ServiceMap } from "effect"

class TodoService extends ServiceMap.Service<TodoService, {
  addAndCount(title: string): Effect.Effect<number>
  readonly titles: Effect.Effect<ReadonlyArray<string>>
}>()("app/TodoService") {
  static readonly layerNoDeps = Layer.effect(
    TodoService,
    Effect.gen(function*() {
      const repo = yield* TodoRepo
      
      const addAndCount = Effect.fn("TodoService.addAndCount")(
        function*(title: string) {
          yield* repo.create(title)
          const todos = yield* repo.list
          return todos.length
        }
      )
      
      const titles = repo.list.pipe(
        Effect.map((todos) => todos.map((todo) => todo.title))
      )
      
      return TodoService.of({ addAndCount, titles })
    })
  )
  
  static readonly layerTest = this.layerNoDeps.pipe(
    // Provide test repo layer and its dependencies
    Layer.provideMerge(TodoRepo.layerTest)
  )
}

describe("TodoService", () => {
  it.effect("tests higher-level service logic", () =>
    Effect.gen(function*() {
      const ref = yield* TodoRepoTestRef
      const service = yield* TodoService
      
      const count = yield* service.addAndCount("Review docs")
      const titles = yield* service.titles
      
      assert.isTrue(count >= 1)
      assert.isTrue(titles.some((title) => title.includes("Review docs")))
      
      // Access test ref directly for assertions
      const todos = yield* Ref.get(ref)
      assert.isTrue(todos.length >= 1)
    }).pipe(
      Effect.provide(TodoService.layerTest)
    )
  )
})

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Test Patterns

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Testing Error Handling

import { assert, it } from "@effect/vitest"
import { Effect, Exit } from "effect"

class UserNotFound extends Schema.TaggedErrorClass<UserNotFound>()(
  "UserNotFound",
  { id: Schema.Number }
) {}

it.effect("handles user not found error", () =>
  Effect.gen(function*() {
    const result = yield* getUserById(999).pipe(
      Effect.exit
    )
    
    assert.isTrue(Exit.isFailure(result))
    
    if (Exit.isFailure(result)) {
      const error = result.cause.failures[0]
      assert.instanceOf(error, UserNotFound)
      assert.strictEqual(error.id, 999)
    }
  })
)

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Testing Retry Logic

import { assert, it } from "@effect/vitest"
import { Effect, Ref, Schedule } from "effect"
import { TestClock } from "effect/testing"

it.effect("retries with exponential backoff", () =>
  Effect.gen(function*() {
    const attempts = yield* Ref.make(0)
    
    const task = Effect.gen(function*() {
      const count = yield* Ref.updateAndGet(attempts, (n) => n + 1)
      if (count < 3) {
        return yield* Effect.fail("temporary error")
      }
      return "success"
    })
    
    const fiber = yield* task.pipe(
      Effect.retry(Schedule.exponential("100 millis")),
      Effect.forkChild
    )
    
    // Fast-forward through retry delays
    yield* TestClock.adjust("100 millis")
    yield* TestClock.adjust("200 millis")
    
    const result = yield* Fiber.join(fiber)
    assert.strictEqual(result, "success")
    
    const finalAttempts = yield* Ref.get(attempts)
    assert.strictEqual(finalAttempts, 3)
  })
)

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Testing Concurrent Operations

import { assert, it } from "@effect/vitest"
import { Effect, Ref } from "effect"

it.effect("handles concurrent updates", () =>
  Effect.gen(function*() {
    const counter = yield* Ref.make(0)
    
    yield* Effect.forEach(
      Array.from({ length: 10 }, (_, i) => i),
      () => Ref.update(counter, (n) => n + 1),
      { concurrency: "unbounded" }
    )
    
    const final = yield* Ref.get(counter)
    assert.strictEqual(final, 10)
  })
)

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Best Practices

  1. Use it.effect for Effect code: Always use it.effect when testing Effect programs
  2. Use TestClock for time-based tests: Don’t use real delays in tests
  3. Share layers with layer(…): Avoid recreating services for each test
  4. Create test-specific layers: Use layerTest for test implementations
  5. Use property-based testing: Catch edge cases with generated inputs
  6. Test error cases: Use Effect.exit to test failure scenarios
  7. Mock external dependencies: Replace real services with test implementations
  8. Test at the right level: Test business logic through service interfaces

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Running Tests

# Run all tests
npm test

# Run tests in watch mode
npm test -- --watch

# Run tests with coverage
npm test -- --coverage

# Run specific test file
npm test -- path/to/test.test.ts

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Assertion Helpers

The assert object from @effect/vitest provides:
  • assert.strictEqual(actual, expected)
  • assert.deepStrictEqual(actual, expected)
  • assert.isTrue(value)
  • assert.isFalse(value)
  • assert.instanceOf(value, Class)
  • assert.throws(() => code)
  • assert.rejects(async () => code)
See the Vitest assertion API for the complete list.

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