# ModuleOverride

**Kind:** Interface

**Source:** [`packages/core/interfaces/module-override.interface.ts`](https://github.com/nestjs/nest/blob/master/packages/core/interfaces/module-override.interface.ts#L3)

**Part of:** [Core](subsystem-packages-core)

`ModuleOverride` defines a replacement mapping between an existing `ModuleDefinition` and a new implementation. It is used by the module resolution or dependency injection system to substitute one module definition with another during configuration or application setup.

## Properties

| Property | Type |
|---|---|
| `moduleToReplace` | `ModuleDefinition` |
| `newModule` | `ModuleDefinition` |

## Diagram

```mermaid
graph LR
  A[Original ModuleDefinition] --> B[ModuleOverride]
  C[Replacement ModuleDefinition] --> B
  B --> D[Module Resolution / DI System]
  D --> E[Uses newModule instead of moduleToReplace]
```

## Usage

```ts
import type { ModuleOverride } from '@your-scope/core';

const moduleOverride: ModuleOverride = {
  moduleToReplace: {
    name: 'NotificationModule',
    providers: [EmailNotificationService],
  },
  newModule: {
    name: 'NotificationModule',
    providers: [MockNotificationService],
  },
};

// Pass the override into the application's module configuration.
createApplication({
  moduleOverrides: [moduleOverride],
});
```

## AI Coding Instructions

- Set `moduleToReplace` to the exact `ModuleDefinition` targeted by the application configuration.
- Ensure `newModule` is a valid `ModuleDefinition` with compatible exports, providers, and dependencies.
- Use overrides for environment-specific implementations, such as mocks in tests or alternate infrastructure adapters.
- Avoid mutating either module definition after creating the override; treat override configuration as immutable setup data.
- Verify that the module resolution integration consumes overrides before modules are instantiated.

## Used by

1 reference from 1 file. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

### Imported by (1)

- `TestingModuleOptions` — `packages/testing/testing-module.builder.ts`:29