TestingInjector

Kind: Class

Source: packages/testing/testing-injector.ts

Part of: Testing

TestingInjector extends Nest’s dependency injector for testing contexts. It coordinates the test container and mock factory, resolving real providers when available and supplying mocks for unresolved dependencies during testing module compilation.

Extends: Injector

Methods

Properties

Where it refuses work

• TestingInjector stops the work with Error when !internalCoreModule — “Expected to have internal core module reference at this point.”.
• TestingInjector stops the work with an early return when !this.mocker.
• TestingInjector stops the work with an early return when !mockedInstance.

When something fails

• TestingInjector handles failure in 2 places: it turns it into a return value in all 2.

Diagram

mermaid
graph LR
  Builder[TestingModuleBuilder] -->|setMocker()| Injector[TestingInjector]
  Builder -->|setContainer()| Injector
  Injector -->|resolveComponentWrapper()| CoreInjector[Nest Injector]
  CoreInjector -->|Provider found| RealProvider[Resolved provider]
  CoreInjector -->|Provider missing| Injector
  Injector -->|mocker(token)| MockFactory[Mock factory]
  MockFactory --> MockProvider[Mock provider instance]
  MockProvider --> Container[NestContainer]

Usage

ts
import { Test } from '@nestjs/testing';
import { HttpService } from '@nestjs/axios';
import { UsersService } from './users.service';

const moduleRef = await Test.createTestingModule({
  providers: [UsersService],
})
  // TestingInjector receives and uses this mock factory internally.
  .useMocker((token) => {
    if (token === HttpService) {
      return {
        get: jest.fn().mockResolvedValue({
          data: { id: 'user-1', name: 'Ada' },
        }),
      };
    }

    // Return undefined for tokens that should not be mocked.
    return undefined;
  })
  .compile();

const usersService = moduleRef.get(UsersService);

AI Coding Instructions

• Configure mocks through TestingModuleBuilder.useMocker(); TestingInjector is an internal integration point used during test module compilation.
• Ensure mock factories return an object or instance for dependencies that cannot be resolved from the testing module.
• Return undefined only when a token should be resolved normally or intentionally left unresolved.
• Call setContainer() before resolving providers directly so generated mock wrappers can be registered in Nest’s internal core module.
• Preserve normal injector behavior by delegating to the base resolution flow before applying mock fallback logic.

Relationships

• IMPORTS → NestContainer
• IMPORTS → STATIC_CONTEXT
• IMPORTS → Injector
• IMPORTS → InjectorDependencyContext
• IMPORTS → ContextId
• IMPORTS → InstanceWrapper
• IMPORTS → Module