Providers

Providers in Nest are used to create services, factories, helpers, and more that can be injected into controllers and other providers using Nest’s built-in dependency injection. The @Injectable() decorator is used to create a provider class.

The AuthenticationService in our blog application, for example, is a provider that injects and uses the UsersService component.

@Injectable()
export class AuthenticationService {
    constructor(private readonly userService: UserService) {}

    async validateUser(payload: {
        email: string;
        password: string;
    }): Promise<boolean> {
        const user = await this.userService.findOne({
            where: { email: payload.email }
        });
        return !!user;
    }
}

We’ll talk more about dependency injection in the the Dependency Injection chapter.