The issue here is that NestJS does not automatically inject dependencies into a parent class constructor. When you extend a class, the super() constructor is called before the configService is available, leading to undefined.

Further explanation: NestJS automatically resolves dependencies without needing @Inject() because of TypeScript's metadata reflection (enabled by emitDecoratorMetadata and reflect-metadata).

• When you extend the class PassportStrategy, TypeScript loses the type metadata for ConfigService in the subclass constructor. NestJS can't infer ConfigService from readonly configService: ConfigService because the super() call is required first. Since ConfigService is injected before the call to super(), the metadata isn't available at that point.

• @Inject(ConfigService) explicitly tells NestJS: Ignore the missing metadata, manually provide the correct dependency and force NestJS to resolve ConfigService properly.