mocking-dependencies-in-tests

description: Provide mock service implementations via a test-specific Layer to isolate the unit under test. globs: "**/*.ts" alwaysApply: true

Mocking Dependencies in Tests

Rule: Provide mock service implementations via a test-specific Layer to isolate the unit under test.

Example

We want to test a Notifier service that uses an EmailClient to send emails. In our test, we provide a mock EmailClient that doesn't actually send emails but just returns a success value.

import { Effect, Layer } from "effect";

// --- The Services ---
interface EmailClientService {
  send: (address: string, body: string) => Effect.Effect<void>;
}

class EmailClient extends Effect.Service<EmailClientService>()("EmailClient", {
  sync: () => ({
    send: (address: string, body: string) =>
      Effect.sync(() => Effect.log(`Sending email to ${address}: ${body}`)),
  }),
}) {}

interface NotifierService {
  notifyUser: (userId: number, message: string) => Effect.Effect<void>;
}

class Notifier extends Effect.Service<NotifierService>()("Notifier", {
  effect: Effect.gen(function* () {
    const emailClient = yield* EmailClient;
    return {
      notifyUser: (userId: number, message: string) =>
        emailClient.send(`user-${userId}@example.com`, message),
    };
  }),
  dependencies: [EmailClient.Default],
}) {}

// Create a program that uses the Notifier service
const program = Effect.gen(function* () {
  yield* Effect.log("Using default EmailClient implementation...");
  const notifier = yield* Notifier;
  yield* notifier.notifyUser(123, "Your invoice is ready.");

  // Create mock EmailClient that logs differently
  yield* Effect.log("\nUsing mock EmailClient implementation...");
  const mockEmailClient = Layer.succeed(EmailClient, {
    send: (address: string, body: string) =>
      // Directly return the Effect.log without nesting it in Effect.sync
      Effect.log(`MOCK: Would send to ${address} with body: ${body}`),
  } as EmailClientService);

  // Run the same notification with mock client
  yield* Effect.gen(function* () {
    const notifier = yield* Notifier;
    yield* notifier.notifyUser(123, "Your invoice is ready.");
  }).pipe(Effect.provide(mockEmailClient));
});

// Run the program
Effect.runPromise(Effect.provide(program, Notifier.Default));