[RFC] Testing Utilities

[Evanion]

Overview

A set of utilities to make testing with NexusDI easier, including test containers, provider overrides/mocks, and helpers for setup/teardown. The goal is to streamline unit and integration testing for DI-based applications, regardless of the test runner.

Motivation

• Simplify DI container setup in tests: Reduce boilerplate for creating and configuring containers in test environments.
• Easy mocking/overriding of providers: Allow tests to replace real services with mocks, stubs, or spies.
• Helpers for lifecycle management: Make it easy to manage container setup/teardown and resource cleanup.
• Test runner agnostic: Work with Jest, Vitest, Mocha, etc.

Proposed Features

• createTestContainer() utility for fast test container setup
• Provider overrides/mocks for easy dependency replacement
• Helpers for setup/teardown (e.g., beforeEach, afterEach)
• Utilities for asserting DI graph, provider resolution, and instance lifetimes
• Support for async setup/teardown and resource disposal
• Optionally, built-in mocks for common modules (e.g., cache, logger)

Example Workflow

1. Create a test container with overrides and get the SUT:

   import { createTestContainer } from '@nexusdi/testing';
   import { UserService } from '../src/services/UserService';
   import { OrderService } from '../src/services/OrderService';
   
   // Mock dependency
   const mockUserService = {
     getUser: jest.fn().mockResolvedValue({ id: 1, name: 'Test User' }),
   };
   
   let container: ReturnType<typeof createTestContainer>;
   let orderService: OrderService;
   
   beforeEach(() => {
     container = createTestContainer({
       overrides: [
         { provide: UserService, useValue: mockUserService },
       ],
     });
     orderService = container.get(OrderService); // SUT
   });
   
   afterEach(() => container.dispose());

2. Use the SUT in your tests:

   test('should fetch user info when processing order', async () => {
     await orderService.processOrder({ orderId: 123, userId: 1 });
     expect(mockUserService.getUser).toHaveBeenCalledWith(1);
     // ...other assertions on orderService behavior...
   });

3. Assert provider resolution and DI graph:

   import { assertProviderResolved } from '@nexusdi/testing';
   
   test('should resolve OrderService and its dependencies', () => {
     assertProviderResolved(container, OrderService);
     assertProviderResolved(container, UserService);
   });

Open Questions

• What test runners should be supported out of the box (Jest, Vitest, Mocha, etc.)?
• Should we provide built-in mocks for common modules (cache, logger, etc.)?
• How to best support async setup/teardown and resource disposal?
• Should we provide utilities for snapshotting or visualizing the DI graph?
• Any other helpers/utilities that would make testing easier?

API Sketch

import { createTestContainer } from '@nexusdi/testing';
import { UserService } from '../src/services/UserService';
import { OrderService } from '../src/services/OrderService';

const mockUserService = {
  getUser: jest.fn().mockResolvedValue({ id: 1, name: 'Test User' }),
};

let container: ReturnType<typeof createTestContainer>;
let orderService: OrderService;

beforeEach(() => {
  container = createTestContainer({
    overrides: [
      { provide: UserService, useValue: mockUserService },
    ],
  });
  orderService = container.get(OrderService); // SUT
});

afterEach(() => container.dispose());

test('should fetch user info when processing order', async () => {
  await orderService.processOrder({ orderId: 123, userId: 1 });
  expect(mockUserService.getUser).toHaveBeenCalledWith(1);
});

Call for Feedback

• What testing patterns do you use with DI?
• What helpers/utilities would make your tests easier?
• Any pain points with testing DI-based code?
• Should we provide built-in mocks for common modules?
• Any other features or utilities you’d want?