# OverrideBy

**Kind:** Interface

**Source:** [`packages/testing/interfaces/override-by.interface.ts`](https://github.com/nestjs/nest/blob/master/packages/testing/interfaces/override-by.interface.ts#L7)

**Part of:** [Testing](subsystem-packages-testing)

`OverrideBy` defines the strategies available when overriding a provider, guard, interceptor, pipe, or filter in a testing module. Each strategy returns a `TestingModuleBuilder`, allowing overrides to be chained before compiling the module.

## Properties

| Property | Type |
|---|---|
| `useValue` | `(value: any) => TestingModuleBuilder` |
| `useFactory` | `(options: OverrideByFactoryOptions) => TestingModuleBuilder` |
| `useClass` | `(metatype: any) => TestingModuleBuilder` |

## Diagram

```mermaid
graph LR
  A[TestingModuleBuilder<br/>overrideProvider / overrideGuard] --> B[OverrideBy]
  B --> C[useValue]
  B --> D[useFactory]
  B --> E[useClass]
  C --> F[TestingModuleBuilder]
  D --> F
  E --> F
  F --> G[compile()]
```

## Usage

```ts
import { Test } from '@nestjs/testing';
import { UsersService } from './users.service';
import { UsersController } from './users.controller';

const usersServiceMock = {
  findAll: jest.fn().mockResolvedValue([{ id: 1, name: 'Ada' }]),
};

const moduleRef = await Test.createTestingModule({
  controllers: [UsersController],
  providers: [UsersService],
})
  .overrideProvider(UsersService)
  .useValue(usersServiceMock)
  .compile();

// Alternative override strategies:
// .overrideProvider(UsersService).useClass(MockUsersService)
// .overrideProvider(UsersService).useFactory({
//   factory: () => usersServiceMock,
//   inject: [],
// })
```

## AI Coding Instructions

- Use `useValue` for simple mocks, stubs, and fixed test doubles that do not require dependency injection.
- Use `useClass` when the replacement should be instantiated as a provider and may have its own dependencies.
- Use `useFactory` for dynamically created overrides; provide required dependencies through `OverrideByFactoryOptions`.
- Always call an override method such as `overrideProvider()` before calling `useValue`, `useFactory`, or `useClass`.
- Preserve the returned `TestingModuleBuilder` to support chaining additional overrides before calling `compile()`.