DI Overrides

The container's central role makes testing easy: replace any provider with a test double, and every consumer resolves the double without code changes. This page is the reference for how to swap a provider in a Titan test — at the container level, at the Application.create level, and with the @omnitron-dev/testing/titan helpers.

How overriding works

A Nexus container keys registrations by token, and the last registration for a token wins. Overriding is therefore just re-registering a token with a different provider definition after the original is in place.

A provider definition is the standard Nexus shape (one of useClass, useValue, useFactory, useToken, or a bare constructor — see Provider in Provider in packages/titan/src/nexus/types.ts):

import { Container } from '@omnitron-dev/titan/nexus';

const container = new Container();

container.register(DATABASE, { useClass: FakeDatabase });
container.register(USERS_SERVICE, { useClass: UsersService });
// UsersService resolves with FakeDatabase as its `database` dep.

To force a re-registration over an existing one, pass { override: true }:

container.register(DATABASE, { useClass: RealDatabase });
container.register(DATABASE, { useClass: FakeDatabase }, { override: true });
// resolve(DATABASE) → FakeDatabase

Application exposes the same surface as app.register(token, provider, { override: true }).

Overriding through Application.create

When you boot a full app for an integration test, pass the providers option — an array of [token, providerDefinition] tuples:

import { Application } from '@omnitron-dev/titan';

const app = await Application.create({
  modules:   [AppModule],
  providers: [
    [Database, { useClass: FakeDatabase }],
    [REDIS,    { useValue: fakeRedis     }],
  ],
  disableGracefulShutdown: true,
});

The shape of providers is defined by CreateOptions in IModuleOptions.imports in packages/titan/src/application/application.ts:

providers?: Array<[InjectionToken<unknown>, Provider<unknown>]>;

create() registers these into the container after the modules are wired (Application.create–288 — modules at lines 269–274, providers immediately after), so each tuple's definition wins over whatever the module declared for the same token. There is no overrides option — providers is the override mechanism.

Each tuple's provider definition uses the standard Nexus shape:

providers: [
  [Database, { useClass: FakeDatabase }],          // construct a fake
  [Clock,    { useValue: { now: () => 0 } }],       // pin a value
  [Cache,    { useFactory: () => new MapCache() }], // build per resolution
]

After all modules and providers are registered, create() calls container.eagerlyInitialize() (Application.create), so singleton providers — including your fakes — are constructed before create() resolves.

Fake vs mock vs stub

Three styles, all valid:

• Fake — a working implementation backed by a simpler store (in-memory database, in-memory cache). Best for integration tests of wiring.

• Mock — a programmable double that records calls and returns configured responses (vi.fn()). Best for unit tests of business logic.

• Stub — a hard-coded response, no programming.

A canonical fake

import type { Database, User, NewUser } from './types.js';

export class FakeDatabase implements Database {
  private readonly users = new Map<string, User>();

  async findUser(id: string): Promise<User | null> {
    return this.users.get(id) ?? null;
  }

  async createUser(input: NewUser): Promise<User> {
    const user = { id: crypto.randomUUID(), ...input };
    this.users.set(user.id, user);
    return user;
  }

  async deleteUser(id: string): Promise<void> {
    this.users.delete(id);
  }

  /** Test affordance: wipe between tests. */
  reset(): void {
    this.users.clear();
  }
}

Wire it through providers, then reset it in beforeEach:

const app = await Application.create({
  modules:   [AppModule],
  providers: [[Database, { useClass: FakeDatabase }]],
  disableGracefulShutdown: true,
});
await app.start();

beforeEach(() => {
  (app.resolve(Database) as FakeDatabase).reset();
});

The same fake works at every test level — unit, integration, e2e.

The @omnitron-dev/testing/titan helpers

The testing package ships container-level test helpers under the @omnitron-dev/testing/titan subpath (the "./titan" entry in the package's exports map). They are not exported from the package root — always import from the subpath:

import {
  createTestModule,
  MockProvider,
  createMockProvider,
} from '@omnitron-dev/testing/titan';

These wrap a TestContainer (a Container subclass), not a booted Application. Use them when you want fine-grained mocking, spying, and interaction recording without standing up the full lifecycle.

createTestModule / TestModule

createTestModule(options) returns a TestModule (createTestModule in packages/testing/src/titan/test-module.ts). Its options (TestModuleOptions, same file lines 11–21):

interface TestModuleOptions {
  modules?:   IModule[];
  providers?: Array<[InjectionToken<any>, ProviderDefinition<any>]>;
  mocks?:     Array<{ token: InjectionToken<any>; mock: any; spy?: boolean }>;
  config?:    Partial<IApplicationConfig>;
  autoMock?:  boolean;
}

mocks is the override channel here — each entry registers { useValue: mock } for its token (with optional spy wrapping). TestModule also exposes a fluent API:

import { createTestModule } from '@omnitron-dev/testing/titan';

const harness = createTestModule({
  providers: [[USERS_SERVICE, { useClass: UsersService }]],
  mocks:     [{ token: DATABASE, mock: new FakeDatabase() }],
});

// Fluent overrides (each returns `this`):
harness
  .override(CLOCK).useValue({ now: () => 0 })
  .mock(MAILER, { send: vi.fn() }, /* spy */ true)
  .stub(FEATURE_FLAGS, { isEnabled: true });

const users = harness.get(USERS_SERVICE);  // resolves from the test container

Method reference (TestModuleBuilder.mock–159):

TestModule.createApplication() constructs an Application and attaches the test container, but does not call start() — it is for container-resolution tests, not lifecycle tests. To exercise the real lifecycle (onStart, module ordering, hooks), boot with Application.create({ ..., providers: [...] }) and call app.start() yourself (see Integration).

MockProvider and createMockProvider

Two distinct helpers (packages/testing/src/titan/nexus/mock-provider.ts):

MockProvider (lines 112–126) — wrap a plain object so every function becomes a vi.fn(). Hand the instance straight to a useValue provider:

import { MockProvider } from '@omnitron-dev/testing/titan';

const mailer = new MockProvider({
  send:  async () => ({ id: 'msg_1' }),
  flush: () => {},
});

const app = await Application.create({
  modules:   [AppModule],
  providers: [[MAILER, { useValue: mailer }]],
  disableGracefulShutdown: true,
});

// `mailer.send` is a vi.fn — assert on it directly:
expect(mailer.send).toHaveBeenCalledWith(expect.objectContaining({ to: 'ada@x.com' }));

createMockProvider(config) (lines 95–97) — builds a MockProviderDI from a config object (MockProviderConfig, lines 9–15: { token, value?, factory?, spy?, autoMock? }). Call .getProvider() to get the Nexus provider definition:

import { createMockProvider } from '@omnitron-dev/testing/titan';

const dbMock = createMockProvider({
  token: DATABASE,
  value: new FakeDatabase(),
});

const app = await Application.create({
  modules:   [AppModule],
  providers: [[DATABASE, dbMock.getProvider()]],  // → { useValue: ... }
  disableGracefulShutdown: true,
});

With autoMock: true (and no value/factory), the provider is a proxy that lazily returns a vi.fn() for every accessed property — handy for a dependency you must satisfy but never call.

Module-level overrides

Sometimes you want to override a whole module, not a single provider. Define a test variant and import it instead of the real one:

@Module({
  providers: [{ provide: Database, useClass: FakeDatabase }],
  exports:   [Database],
})
class FakeDatabaseModule {}

@Module({
  imports: [
    FakeDatabaseModule,   // instead of DatabaseModule
    UsersModule,
    OrdersModule,
  ],
})
class TestAppModule {}

const app = await Application.create(TestAppModule, {
  disableGracefulShutdown: true,
});

Useful for swapping infrastructure (database, redis, file storage) to test-friendly implementations across many tests. Note this uses the module-decorator { provide, useClass } shape, distinct from the create({ providers: [[token, def]] }) tuple shape.

Spying without replacing

Sometimes you want to observe calls without changing behaviour:

import { vi } from 'vitest';

const real = app.resolve(LoggerService);
const spy  = vi.spyOn(real, 'info');

// run code that should log
expect(spy).toHaveBeenCalledWith('ready', expect.objectContaining({ port: 3000 }));

The spy wraps the implementation; it doesn't replace it. The TestModule.spy(token, method) helper does the same against the test container.

The "two real, one fake" pattern

A common integration setup — real logger, real config, fake database, real services:

const app = await Application.create({
  modules:   [LoggerModule, ConfigModule.forRoot({ /* ... */ }), UsersModule],
  providers: [[Database, { useClass: FakeDatabase }]],
  disableGracefulShutdown: true,
});

Catches wiring bugs without paying for a real database.

Anti-patterns

• Overriding too much. A test where every provider is mocked is testing the test, not the code. Override only what you need to control.

• Mutable shared fakes across tests. Two tests that share one FakeDatabase see each other's data. Use a fresh fake per test, or reset() in beforeEach.

• Testing the framework. "Does the container resolve providers?" is the framework's job. Test your code; trust the framework.