Effect Testing

Effect-TS Testing

Basic Test Structure

import { describe, it, expect } from "vitest"
import { Effect, Layer } from "effect"
import { Foo, FooLive } from "../services/Foo.js"

describe("Foo", () => {
  it("does the thing", async () => {
    const result = await Effect.runPromise(
      Effect.gen(function* () {
        const foo = yield* Foo
        return yield* foo.doThing()
      }).pipe(Effect.provide(testLayer))
    )
    expect(result).toBe(expected)
  })
})

Mock Services with Layer.succeed

const MockDep = Layer.succeed(Dep, {
  getValue: () => Effect.succeed("mock-value"),
  save: () => Effect.succeed(undefined),
})

Composing Test Layers

const testLayer = FooLive.pipe(
  Layer.provide(MockDep),
  Layer.provide(MockConfig),
)

Testing Errors

it("fails with NotFound", async () => {
  const exit = await Effect.runPromiseExit(
    Effect.gen(function* () {
      const foo = yield* Foo
      return yield* foo.getById("nonexistent")
    }).pipe(Effect.provide(testLayer))
  )
  expect(exit._tag).toBe("Failure")
})

Config in Tests

const MockConfig = Layer.succeed(ConfigService, {
  database: { path: ":memory:" },
  server: { port: 0 },
  // ... minimal config for the test
})

Rules

• Every test layer must provide ALL dependencies the service needs
• Use Layer.succeed for mocks — not Layer.effect
• Do NOT use Effect.runSync for async operations in tests
• Test timeout is 10 seconds — keep tests fast
• Name test files *.test.ts in src/__tests__/

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