ExistingProvider

Kind: Interface

Source: packages/common/interfaces/modules/provider.interface.ts

Part of: Common

Interface defining an Existing (aliased) type provider.

For example:

typescript
const loggerAliasProvider = {
  provide: 'AliasedLoggerService',
  useExisting: LoggerService
};

ExistingProvider defines an alias for an already-registered dependency injection provider. Instead of creating a new instance, it maps a new provide token to the same instance represented by useExisting.

Properties

Diagram

mermaid
graph LR
  A[Injection Token: AliasedLoggerService] --> B[ExistingProvider]
  B -->|useExisting| C[Existing Provider: LoggerService]
  C --> D[Shared LoggerService Instance]

Usage

typescript
import { ExistingProvider } from '@nestjs/common';

class LoggerService {
  log(message: string) {
    console.log(message);
  }
}

const loggerAliasProvider: ExistingProvider = {
  provide: 'AliasedLoggerService',
  useExisting: LoggerService,
};

@Module({
  providers: [LoggerService, loggerAliasProvider],
})
export class AppModule {}

AI Coding Instructions

• Use useExisting when multiple injection tokens must resolve to the exact same provider instance.
• Ensure the target referenced by useExisting is registered in the same module or available through imported/exported modules.
• Use a distinct provide token for the alias, such as a string, symbol, class, or custom injection token.
• Do not use useExisting when a separate instance is required; use useClass, useFactory, or useValue instead.
• Prefer aliases for backwards-compatible token migrations or interface-based dependency injection.

Used by

3 references from 3 files. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

Imported by (3)

• InternalCoreModule — packages/core/injector/internal-core-module/internal-core-module.ts:16
• Module — packages/core/injector/module.ts:44
• DependenciesScanner — packages/core/scanner.ts:75