nestjs-dependency-injection

name	nestjs-dependency-injection
user-invocable	false
description	Use when nestJS dependency injection with providers, modules, and decorators. Use when building modular NestJS applications.
allowed-tools	["Bash","Read","Write","Edit","Glob","Grep"]

NestJS Dependency Injection

Master NestJS dependency injection for building modular, testable Node.js applications with proper service architecture, provider patterns, and module organization.

Table of Contents

• Provider Patterns
• Module System
• Injection Scopes
• Advanced Patterns
• Best Practices
• Common Pitfalls
• Resources

Provider Patterns

Class Providers (Standard Pattern)

import { Injectable } from '@nestjs/common';

@Injectable()
export class UserService {
  private users: User[] = [];

  findAll(): User[] {
    return this.users;
  }

  findById(id: string): User | undefined {
    return this.users.find(user => user.id === id);
  }

  create(user: User): User {
    this.users.push(user);
    return user;
  }
}

// Module registration
@Module({
  providers: [UserService],
  exports: [UserService],
})
export class UserModule {}

Value Providers

import { Module } from '@nestjs/common';

// Simple value provider
const DATABASE_CONNECTION = {
  provide: 'DATABASE_CONNECTION',
  useValue: {
    host: 'localhost',
    port: 5432,
    database: 'mydb',
  },
};

// Configuration value provider
const APP_CONFIG = {
  provide: 'APP_CONFIG',
  useValue: {
    apiUrl: process.env.API_URL,
    timeout: 5000,
    retries: 3,
  },
};

@Module({
  providers: [DATABASE_CONNECTION, APP_CONFIG],
  exports: [DATABASE_CONNECTION, APP_CONFIG],
})
export class ConfigModule {}

// Usage in service
@Injectable()
export class ApiService {
  constructor(
    @Inject('APP_CONFIG') private config: AppConfig,
  ) {}

  async (): <> {
     response =  (.., {
      : ..,
    });
     response.();
  }
}

Factory Providers

import { Injectable, Module } from '@nestjs/common';

// Simple factory provider
const CONNECTION_FACTORY = {
  provide: 'DATABASE_CONNECTION',
  useFactory: () => {
    return createConnection({
      type: 'postgres',
      host: process.env.DB_HOST,
      port: parseInt(process.env.DB_PORT),
      database: process.env.DB_NAME,
    });
  },
};

// Factory with dependencies
const CACHE_MANAGER = {
  provide: 'CACHE_MANAGER',
  useFactory: (config: ConfigService) => {
    return createCacheManager({
      store: config.get('CACHE_STORE'),
      ttl: config.get('CACHE_TTL'),
      max: config.get('CACHE_MAX_ITEMS'),
    });
  },
  inject: [ConfigService],
};

@Module({
  providers: [
    ,
    ,
    ,
  ],
  : [, ],
})
   {}

Async Providers with useFactory

import { Module } from '@nestjs/common';
import { ConfigService } from '@nestjs/config';

const DATABASE_PROVIDER = {
  provide: 'DATABASE_CONNECTION',
  useFactory: async (config: ConfigService) => {
    const connection = await createConnection({
      type: 'postgres',
      host: config.get('DB_HOST'),
      port: config.get('DB_PORT'),
      username: config.get('DB_USER'),
      password: config.get('DB_PASSWORD'),
      database: config.get('DB_NAME'),
    });

    await connection.runMigrations();
    return connection;
  },
  inject: [ConfigService],
};

const REDIS_PROVIDER = {
  provide: 'REDIS_CLIENT',
  useFactory: async (config: ConfigService) => {
    const client = ({
      : config.(),
    });

     client.();

    client.(,  {
      .(, err);
    });

     client;
  },
  : [],
};

({
  : [, ],
  : [, ],
})
   {}

Token-Based Injection with String Tokens

import { Inject, Injectable, Module } from '@nestjs/common';

// Define string tokens as constants
export const LOGGER_TOKEN = 'LOGGER';
export const METRICS_TOKEN = 'METRICS';
export const API_CLIENT_TOKEN = 'API_CLIENT';

// Provider definitions
const LOGGER_PROVIDER = {
  provide: LOGGER_TOKEN,
  useFactory: () => {
    return createLogger({
      level: process.env.LOG_LEVEL || 'info',
      format: 'json',
    });
  },
};

const METRICS_PROVIDER = {
  provide: METRICS_TOKEN,
  useValue: createMetricsClient(),
};

@Module({
  providers: [LOGGER_PROVIDER, METRICS_PROVIDER],
  exports: [LOGGER_TOKEN, METRICS_TOKEN],
})
export class ObservabilityModule {}


()
   {
  () {}

   (: ): <> {
    ..(, { : data. });
    ..();

     user =  ..(data);
     user;
  }
}

Token-Based Injection with Symbol Tokens

import { Inject, Injectable, Module } from '@nestjs/common';

// Define symbol tokens for better type safety
export const DATABASE_CONNECTION = Symbol('DATABASE_CONNECTION');
export const CACHE_MANAGER = Symbol('CACHE_MANAGER');
export const EVENT_BUS = Symbol('EVENT_BUS');

// Provider with symbol token
const DB_PROVIDER = {
  provide: DATABASE_CONNECTION,
  useFactory: async () => {
    return await createDatabaseConnection();
  },
};

const CACHE_PROVIDER = {
  provide: CACHE_MANAGER,
  useClass: RedisCacheManager,
};

@Module({
  providers: [DB_PROVIDER, CACHE_PROVIDER],
  exports: [DATABASE_CONNECTION, CACHE_MANAGER],
})
export class InfrastructureModule {}

// Usage with symbol tokens
()
   {
  () {}

   (: ): <> {
     cached =  ..();
     (cached)  cached;

     product =  .
      .()
      .({ : { id } });

     ..(, product, );
     product;
  }
}

Optional Dependencies with @Optional()

import { Injectable, Optional, Inject } from '@nestjs/common';

@Injectable()
export class NotificationService {
  constructor(
    @Optional()
    @Inject('EMAIL_SERVICE')
    private emailService?: EmailService,
    @Optional()
    @Inject('SMS_SERVICE')
    private smsService?: SmsService,
  ) {}

  async notify(user: User, message: string): Promise<void> {
    // Gracefully handle missing optional dependencies
    if (this.emailService) {
      await this.emailService.send(user.email, message);
    }

    if (this.smsService && user.phone) {
      await this.smsService.send(user.phone, message);
    }

    // Always have a fallback notification method
     .(user, message);
  }

    (
    : ,
    : ,
  ): <> {
    .();
  }
}


({
  : [
    ,
    
    
  ],
  : [],
})
   {}

Property-Based Injection

import { Injectable, Inject } from '@nestjs/common';

// Property-based injection (less preferred)
@Injectable()
export class PaymentService {
  @Inject('PAYMENT_GATEWAY')
  private paymentGateway: PaymentGateway;

  @Inject('FRAUD_DETECTOR')
  private fraudDetector: FraudDetector;

  async processPayment(
    amount: number,
    card: CardDetails,
  ): Promise<PaymentResult> {
    const isFraudulent = await this.fraudDetector.check(card);
    if (isFraudulent) {
      throw new FraudDetectedException();
    }

    return await this.paymentGateway.charge(amount, card);
  }
}

// Constructor-based injection (preferred)
@Injectable()
export class OrderService {
  constructor(
    @Inject()
      : ,
    ()
      : ,
  ) {}

   (: ): <> {
     ..(data.);
     ..(
      data.,
      data.,
    );

      ..(data);
  }
}

Class Provider with useClass

import { Injectable, Module } from '@nestjs/common';

// Abstract interface
export abstract class LoggerService {
  abstract log(message: string): void;
  abstract error(message: string, trace: string): void;
}

// Concrete implementations
@Injectable()
export class ConsoleLoggerService extends LoggerService {
  log(message: string): void {
    console.log(message);
  }

  error(message: string, trace: string): void {
    console.error(message, trace);
  }
}

@Injectable()
export class FileLoggerService extends LoggerService {
  log(message: string):  {
    fs.(, );
  }

  (: , : ):  {
    fs.(, );
  }
}


({
  : [
    {
      : ,
      :
        process.. === 
          ? 
          : ,
    },
  ],
  : [],
})
   {}


()
   {
  () {}

  ():  {
    ..();
  }
}

Alias Providers (useExisting)

import { Injectable, Module } from '@nestjs/common';

@Injectable()
export class UserService {
  findAll(): User[] {
    return [];
  }
}

// Create an alias for the service
@Module({
  providers: [
    UserService,
    {
      provide: 'IUserService',
      useExisting: UserService,
    },
    {
      provide: 'UserRepository',
      useExisting: UserService,
    },
  ],
  exports: [
    UserService,
    'IUserService',
    'UserRepository',
  ],
})
export class UserModule {}

// Can inject using any of the tokens
@Injectable()
export class ReportService {
  constructor(
    @Inject('IUserService') private userService: UserService,
  ) {}

  async generateReport(): Promise<Report> {
    const users = this.userService.();
     .(users);
  }
}

Module System

Module Organization and Encapsulation

import { Module } from '@nestjs/common';

// Feature module with proper encapsulation
@Module({
  imports: [DatabaseModule, CacheModule],
  providers: [
    UserService,
    UserRepository,
    UserValidator,
  ],
  controllers: [UserController],
  exports: [UserService], // Only export what's needed
})
export class UserModule {}

// Domain module grouping related features
@Module({
  imports: [
    UserModule,
    AuthModule,
    ProfileModule,
  ],
})
export class IdentityModule {}

// Application module
@Module({
  imports: [
    ConfigModule.forRoot(),
    IdentityModule,
    ProductModule,
    OrderModule,
  ],
})
export class AppModule {}

Global Modules with @Global()

import { Module, Global } from '@nestjs/common';

// Global module available everywhere
@Global()
@Module({
  providers: [
    {
      provide: 'LOGGER',
      useFactory: () => createLogger(),
    },
    {
      provide: 'CONFIG',
      useValue: loadConfiguration(),
    },
  ],
  exports: ['LOGGER', 'CONFIG'],
})
export class CoreModule {}

// Usage in any module without importing
@Injectable()
export class AnyService {
  constructor(
    @Inject('LOGGER') private logger: Logger,
    @Inject('CONFIG') private config: Config,
  ) {}
}

// Register global module once in AppModule
@Module({
  imports: [
    CoreModule, // Only import once
    FeatureModule1,
    FeatureModule2,
  ],
})
export class AppModule {}

Dynamic Modules with forRoot

import { Module, DynamicModule, Provider } from '@nestjs/common';

export interface DatabaseModuleOptions {
  host: string;
  port: number;
  username: string;
  password: string;
  database: string;
}

@Module({})
export class DatabaseModule {
  static forRoot(
    options: DatabaseModuleOptions,
  ): DynamicModule {
    const connectionProvider: Provider = {
      provide: 'DATABASE_CONNECTION',
      useFactory: async () => {
        return await createConnection(options);
      },
    };

    return {
      module: DatabaseModule,
      providers: [
        connectionProvider,
        DatabaseService,
      ],
      exports: [
        'DATABASE_CONNECTION',
        DatabaseService,
      ],
      global: true,
    };
  }
}

// Usage in AppModule
({
  : [
    .({
      : ,
      : ,
      : ,
      : ,
      : ,
    }),
  ],
})
   {}

Dynamic Modules with forRootAsync

import {
  Module,
  DynamicModule,
  Provider,
} from '@nestjs/common';
import { ConfigService } from '@nestjs/config';

export interface CacheModuleAsyncOptions {
  useFactory: (...args: any[]) => Promise<CacheOptions>;
  inject?: any[];
}

@Module({})
export class CacheModule {
  static forRootAsync(
    options: CacheModuleAsyncOptions,
  ): DynamicModule {
    const cacheOptionsProvider: Provider = {
      provide: 'CACHE_OPTIONS',
      useFactory: options.useFactory,
      inject: options.inject || [],
    };

    const cacheProvider: Provider = {
      provide: 'CACHE_MANAGER',
      useFactory: async (cacheOptions: CacheOptions) => {
        return await (cacheOptions);
      },
      : [],
    };

     {
      : ,
      : [
        cacheOptionsProvider,
        cacheProvider,
        ,
      ],
      : [, ],
      : ,
    };
  }
}


({
  : [
    .(),
    .({
      :  (: ) => ({
        : config.(),
        : config.(),
        : config.(),
      }),
      : [],
    }),
  ],
})
   {}

Module Re-exporting

import { Module } from '@nestjs/common';

// Low-level infrastructure modules
@Module({
  providers: [DatabaseService],
  exports: [DatabaseService],
})
export class DatabaseModule {}

@Module({
  providers: [CacheService],
  exports: [CacheService],
})
export class CacheModule {}

@Module({
  providers: [QueueService],
  exports: [QueueService],
})
export class QueueModule {}

// Shared module that re-exports common services
@Module({
  imports: [
    DatabaseModule,
    CacheModule,
    QueueModule,
  ],
  exports: [
    DatabaseModule,
    CacheModule,
    QueueModule,
  ],
})
export class SharedModule {}

// Feature modules import SharedModule instead of individual modules
@Module({
  imports: [SharedModule],
  providers: [UserService],
  controllers: [],
})
   {}

({
  : [],
  : [],
  : [],
})
   {}

Circular Dependencies Handling

import { Module, forwardRef } from '@nestjs/common';

// UserModule depends on AuthModule
@Module({
  imports: [forwardRef(() => AuthModule)],
  providers: [UserService],
  exports: [UserService],
})
export class UserModule {}

// AuthModule depends on UserModule
@Module({
  imports: [forwardRef(() => UserModule)],
  providers: [AuthService],
  exports: [AuthService],
})
export class AuthModule {}

// Service-level circular dependency
@Injectable()
export class UserService {
  constructor(
    @Inject(forwardRef(() => AuthService))
    private authService: AuthService,
  ) {}
}

@Injectable()
export class AuthService {
  constructor(
    @Inject(forwardRef(() => UserService))
    private userService: UserService,
  ) {}
}

Feature Modules with Lazy Loading

import { Module } from '@nestjs/common';

// Admin feature module
@Module({
  imports: [SharedModule],
  providers: [
    AdminService,
    AdminGuard,
  ],
  controllers: [AdminController],
})
export class AdminModule {}

// Lazy load admin module only when needed
@Module({
  imports: [
    // Other modules loaded eagerly
    CoreModule,
    UserModule,
  ],
})
export class AppModule {}

// In a controller or service, load AdminModule dynamically
@Injectable()
export class AppService {
  constructor(private readonly lazyModuleLoader: LazyModuleLoader) {}

  async performAdminTask(): Promise<void> {
    const moduleRef = await this.lazyModuleLoader.load(
      () => AdminModule,
    );
    const adminService = moduleRef.get();
     adminService.();
  }
}

Module Configuration Pattern

import { Module, DynamicModule } from '@nestjs/common';

export interface EmailModuleOptions {
  from: string;
  host: string;
  port: number;
  secure: boolean;
}

@Module({})
export class EmailModule {
  static forRoot(
    options: EmailModuleOptions,
  ): DynamicModule {
    return {
      module: EmailModule,
      providers: [
        {
          provide: 'EMAIL_OPTIONS',
          useValue: options,
        },
        EmailService,
      ],
      exports: [EmailService],
    };
  }

  static forFeature(): DynamicModule {
    return {
      module: EmailModule,
      providers: [EmailTemplateService],
      exports: [EmailTemplateService],
    };
  }
}

// Root module configuration
@Module({
  imports: [
    EmailModule.forRoot({
      : ,
      : ,
      : ,
      : ,
    }),
  ],
})
   {}


({
  : [.()],
  : [],
})
   {}

Shared Module Pattern

import { Module, Global } from '@nestjs/common';

// Shared utilities module
@Global()
@Module({
  providers: [
    DateService,
    StringService,
    ValidationService,
  ],
  exports: [
    DateService,
    StringService,
    ValidationService,
  ],
})
export class UtilsModule {}

// Shared data access module
@Module({
  providers: [
    DataSource,
    TransactionManager,
    UnitOfWork,
  ],
  exports: [
    DataSource,
    TransactionManager,
    UnitOfWork,
  ],
})
export class DataAccessModule {}

// Combine into SharedModule
@Module({
  imports: [
    UtilsModule,
    DataAccessModule,
  ],
  exports: [
    UtilsModule,
    DataAccessModule,
  ],
})
export class SharedModule {}

Injection Scopes

DEFAULT Scope (Singleton)

import { Injectable, Scope } from '@nestjs/common';

// Default scope - single instance shared across the app
@Injectable()
export class ConfigService {
  private config: Record<string, any>;

  constructor() {
    this.config = this.loadConfiguration();
  }

  get(key: string): any {
    return this.config[key];
  }

  private loadConfiguration(): Record<string, any> {
    // Loaded once when application starts
    return {
      apiUrl: process.env.API_URL,
      dbHost: process.env.DB_HOST,
    };
  }
}

// Singleton service with state
@Injectable()
export class CacheService {
  private cache = new Map<string, any>();

  (: , : ):  {
    ..(key, value);
  }

  (: ):  {
     ..(key);
  }

  
  ():  {
    ..();
  }
}

REQUEST Scope with Performance Implications

import { Injectable, Scope, Inject } from '@nestjs/common';
import { REQUEST } from '@nestjs/core';
import { Request } from 'express';

// Request-scoped provider - new instance per request
@Injectable({ scope: Scope.REQUEST })
export class RequestContextService {
  constructor(@Inject(REQUEST) private request: Request) {}

  getUserId(): string {
    return this.request.user?.id;
  }

  getTenantId(): string {
    return this.request.headers['x-tenant-id'] as string;
  }

  getTraceId(): string {
    return this.request.headers['x-trace-id'] as string;
  }
}


({ : . })
   {
  () {}

   (: ): <> {
     ..({
      : ..(),
      : ..(),
      action,
      :  (),
    });
  }
}


({ : . })
   {
  
  
  () {
    .();
  }

   (: ): <> {
     user =  ..(data);
     ..();
     user;
  }
}

TRANSIENT Scope

import { Injectable, Scope } from '@nestjs/common';

// Transient scope - new instance every time it's injected
@Injectable({ scope: Scope.TRANSIENT })
export class UniqueIdGenerator {
  private readonly id: string;

  constructor() {
    this.id = Math.random().toString(36).substring(7);
    console.log(`New generator created with id: ${this.id}`);
  }

  generate(): string {
    return `${this.id}-${Date.now()}`;
  }
}

// Each injection point gets its own instance
@Injectable()
export class OrderService {
  constructor(
    private readonly idGen1: UniqueIdGenerator, // Instance 1
  ) {}
}

()
   {
  () {}
}


({ : . })
   {
   : [] = [];
   : [] = [];

  (: , ...: []):  {
    ..(condition);
    ..(...params);
     ;
  }

  (): { : ; : [] } {
     {
      : ,
      : .,
    };
  }
}

Durable Providers

import { Injectable, Scope } from '@nestjs/common';

// Durable provider - survives across requests
@Injectable({ scope: Scope.DEFAULT, durable: true })
export class ConnectionPoolService {
  private pool: Pool;

  constructor() {
    this.pool = createPool({
      host: 'localhost',
      port: 5432,
      max: 20,
    });
  }

  getConnection(): Promise<Connection> {
    return this.pool.connect();
  }

  async onModuleDestroy(): Promise<void> {
    await this.pool.end();
  }
}

// Durable request-scoped provider
@Injectable({
  scope: Scope.REQUEST,
  durable: true,
})
export   {
   : [] = [];

  (: ):  {
    ..(message);
  }

  (): [] {
     .;
  }
}

Scope Inheritance

import { Injectable, Scope } from '@nestjs/common';

// Parent service with DEFAULT scope
@Injectable()
export class DatabaseService {
  query(sql: string): Promise<any> {
    return this.pool.query(sql);
  }
}

// Child service inherits scope from parent
@Injectable()
export class UserRepository {
  constructor(private readonly db: DatabaseService) {}

  findAll(): Promise<User[]> {
    return this.db.query('SELECT * FROM users');
  }
}

// Request-scoped parent
@Injectable({ scope: Scope.REQUEST })
export class RequestContext {
  constructor(@Inject(REQUEST) private request: Request) {}

  ():  {
     ..[]  ;
  }
}


({ : . })
   {
  () {}

   (): <[]> {
     tenantId = ..();
     ..(
      ,
    );
  }
}

Scope Configuration in Modules

import { Module } from '@nestjs/common';

@Module({
  providers: [
    // Default scope (singleton)
    ConfigService,

    // Request scope
    {
      provide: 'REQUEST_LOGGER',
      scope: Scope.REQUEST,
      useClass: RequestLoggerService,
    },

    // Transient scope
    {
      provide: 'ID_GENERATOR',
      scope: Scope.TRANSIENT,
      useClass: IdGeneratorService,
    },
  ],
})
export class AppModule {}

Advanced Patterns

Custom Decorators for Injection

import { Inject } from '@nestjs/common';

// Custom decorator for logger injection
export const InjectLogger = () => Inject('LOGGER');

// Custom decorator for repository injection
export function InjectRepository(
  entity: Function,
): ParameterDecorator {
  return Inject(`${entity.name}Repository`);
}

// Custom decorator with options
export function InjectCache(
  namespace?: string,
): ParameterDecorator {
  const token = namespace ? `CACHE:${namespace}` : 'CACHE';
  return Inject(token);
}

// Usage in services
@Injectable()
export class UserService {
  constructor(
    @InjectLogger() private logger: Logger,
    @InjectRepository(User) private repo: Repository<>,
    ()  : ,
  ) {}

   (): <[]> {
    ..();

     cached =  ..();
     (cached)  cached;

     users =  ..();
     ..(, users, );
     users;
  }
}


({
  : [
    {
      : ,
      :  (),
    },
    {
      : ,
      : ,
    },
    {
      : ,
      :  (),
    },
  ],
})
   {}

Provider Arrays and Multi-Providers

import { Module, Inject } from '@nestjs/common';

// Define token for array of providers
export const EVENT_HANDLERS = 'EVENT_HANDLERS';

// Multiple implementations of event handler
@Injectable()
export class UserEventHandler implements EventHandler {
  handle(event: Event): void {
    console.log('User event:', event);
  }
}

@Injectable()
export class AuditEventHandler implements EventHandler {
  handle(event: Event): void {
    console.log('Audit event:', event);
  }
}

@Injectable()
export class NotificationEventHandler implements EventHandler {
  handle(event: Event): void {
    console.log('Notification event:', event);
  }
}


({
  : [
    ,
    ,
    ,
    {
      : ,
      :  [userHandler, auditHandler, notificationHandler],
      : [
        ,
        ,
        ,
      ],
    },
  ],
  : [],
})
   {}


()
   {
  () {}

  (: ):  {
    ..( {
      handler.(event);
    });
  }
}

Lazy Module Loading

import {
  Injectable,
  LazyModuleLoader,
} from '@nestjs/common';

@Injectable()
export class ReportService {
  constructor(
    private readonly lazyModuleLoader: LazyModuleLoader,
  ) {}

  async generateComplexReport(): Promise<Report> {
    // Load heavy module only when needed
    const moduleRef = await this.lazyModuleLoader.load(
      () => import('./analytics/analytics.module')
        .then((m) => m.AnalyticsModule),
    );

    const analyticsService = moduleRef.get(AnalyticsService);
    const data = await analyticsService.analyze();

    return this.buildReport(data);
  }

  async exportToPdf(report: Report): Promise<Buffer> {
    // Load PDF module lazily
    const moduleRef =  ..(
       ()
        .( m.),
    );

     pdfService = moduleRef.();
      pdfService.(report);
  }
}

Testing with Dependency Injection

import { Test, TestingModule } from '@nestjs/testing';

describe('UserService', () => {
  let service: UserService;
  let repository: Repository<User>;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [
        UserService,
        {
          provide: 'UserRepository',
          useValue: {
            find: jest.fn(),
            findOne: jest.fn(),
            create: jest.fn(),
            save: jest.fn(),
          },
        },
        {
          provide: 'LOGGER',
          useValue: {
            log: jest.fn(),
            error: jest.fn(),
          },
        },
      ],
    }).compile();

    service = module.get<UserService>(UserService);
    repository = .();
  });

  (,  () => {
     users = [{ : , :  }];
    jest.(repository, ).(users);

     result =  service.();

    (result).(users);
    (repository.).();
  });

  (,  () => {
     createDto = { : , :  };
     user = { : , ...createDto };

    jest.(repository, ).(user);
    jest.(repository, ).(user);

     result =  service.(createDto);

    (result).(user);
    (repository.).(createDto);
    (repository.).(user);
  });
});


(,  {
   : ;
   : ;

  ( () => {
     moduleRef =  .({
      : [
        .({
          : ,
          : ,
          : ,
          : ,
        }),
        ,
      ],
    }).();

    app = moduleRef.();
     app.();

    service = moduleRef.<>();
  });

  ( () => {
     app.();
  });

  (,  () => {
     createDto = { : , :  };
     user =  service.(createDto);

    (user.).();
    (user.).(createDto.);

     found =  service.(user.);
    (found).(user);
  });
});

ModuleRef for Dynamic Provider Resolution

import { Injectable, ModuleRef } from '@nestjs/core';

@Injectable()
export class DynamicService {
  constructor(private readonly moduleRef: ModuleRef) {}

  async processWithStrategy(
    strategyName: string,
    data: any,
  ): Promise<any> {
    // Dynamically resolve provider at runtime
    const strategy = this.moduleRef.get(
      `${strategyName}Strategy`,
      { strict: false },
    );

    return await strategy.process(data);
  }

  async getServiceByTenant(tenantId: string): Promise<any> {
    // Get service instance dynamically
    const token = `TenantService:${tenantId}`;
    try {
      return this.moduleRef.get(token, { strict:  });
    }  {
      
       ..();
    }
  }
}


({ : . })
   {
  () {}

   (): <> {
    
     context =  ..(
      ,
    );
     userId = context.();

     userService = ..();
      userService.(userId);
  }
}

Plugin Pattern with Dependency Injection

import { Module, DynamicModule, Type } from '@nestjs/common';

export interface Plugin {
  name: string;
  initialize(): Promise<void>;
  execute(data: any): Promise<any>;
}

export interface PluginModuleOptions {
  plugins: Type<Plugin>[];
}

@Module({})
export class PluginModule {
  static forRoot(
    options: PluginModuleOptions,
  ): DynamicModule {
    const pluginProviders = options.plugins.map((plugin) => ({
      provide: plugin,
      useClass: plugin,
    }));

    const pluginRegistryProvider = {
      provide: 'PLUGIN_REGISTRY',
      useFactory: (...plugins: Plugin[]) => plugins,
      inject: options.plugins,
    };

     {
      : ,
      : [
        ...pluginProviders,
        pluginRegistryProvider,
        ,
      ],
      : [],
    };
  }
}


()
     {
  name = ;

   (): <> {
    .();
  }

   (: ): <> {
    
     data;
  }
}

()
     {
  name = ;

   (): <> {
    .();
  }

   (: ): <> {
    
     data;
  }
}


()
   {
  () {}

   (: ): <> {
     result = data;
     ( plugin  .) {
      result =  plugin.(result);
    }
     result;
  }
}


({
  : [
    .({
      : [, ],
    }),
  ],
})
   {}

Conditional Provider Registration

import { Module, DynamicModule } from '@nestjs/common';

@Module({})
export class StorageModule {
  static forRoot(): DynamicModule {
    const providers = [];

    // Conditional provider based on environment
    if (process.env.STORAGE_TYPE === 's3') {
      providers.push({
        provide: 'STORAGE_SERVICE',
        useClass: S3StorageService,
      });
    } else if (process.env.STORAGE_TYPE === 'gcs') {
      providers.push({
        provide: 'STORAGE_SERVICE',
        useClass: GcsStorageService,
      });
    } else {
      providers.push({
        provide: 'STORAGE_SERVICE',
        useClass: LocalStorageService,
      });
    }

    // Conditional feature providers
    if (process.env.ENABLE_COMPRESSION === 'true') {
      providers.push(CompressionService);
    }

    if (process.env.ENABLE_ENCRYPTION === ) {
      providers.();
    }

     {
      : ,
      providers,
      : [],
    };
  }
}

Best Practices

1. Use constructor injection over property injection: Constructor injection makes dependencies explicit, ensures they're available when the class is instantiated, and works better with TypeScript's type system.

2. Prefer class-based providers for services: Class providers are more idiomatic in NestJS, provide better type safety, and integrate seamlessly with decorators like @Injectable().

3. Use factory providers for complex initialization: When providers need async initialization, depend on other services, or require conditional logic, factory providers offer the flexibility needed.

4. Avoid circular dependencies with forwardRef: While forwardRef() solves circular dependencies, it's better to restructure your modules to eliminate the circular reference entirely.

5. Keep modules focused and cohesive: Each module should represent a single feature or domain. This improves maintainability, makes testing easier, and enables better code organization.

6. Use dynamic modules for configurable features: When building reusable modules that need configuration, implement forRoot() and forRootAsync() methods to provide flexible initialization.

7. Leverage REQUEST scope only when needed: Request-scoped providers have performance overhead. Use them only when you truly need per-request state, like request context or tenant isolation.

8. Use symbol tokens for better type safety: Symbol tokens prevent naming conflicts and provide better IntelliSense support compared to string tokens.

9. Export only what's needed from modules: Keep module interfaces minimal by exporting only the providers that other modules need to use. This maintains encapsulation and reduces coupling.

10. Test providers in isolation: Write unit tests that mock dependencies to test providers in isolation. Use integration tests to verify the full dependency graph works correctly.

Common Pitfalls

1. Circular dependency errors: Occurs when Module A imports Module B, and Module B imports Module A. Restructure your modules or use forwardRef() as a last resort.

2. REQUEST scope performance overhead: Request-scoped providers are created for every request, which adds memory and CPU overhead. All dependent providers also become request-scoped.

3. Not handling async provider initialization: Forgetting to use async/await in factory providers can lead to providers being injected before they're fully initialized.

4. Overusing global modules: Global modules are convenient but can lead to tight coupling. Use them sparingly for truly global services like logging and configuration.

5. Missing provider exports in modules: If a provider is not listed in the module's exports array, it won't be available to other modules that import it.

6. Token name conflicts: Using generic string tokens like 'config' or 'service' across multiple modules can cause conflicts. Use descriptive, namespaced tokens.

7. Memory leaks with REQUEST scope: Request-scoped providers that hold references to large objects or don't clean up resources can cause memory leaks over time.

8. Not cleaning up resources in onModuleDestroy: Providers that create connections, timers, or other resources should implement onModuleDestroy to clean up properly.

9. Tight coupling between modules: Importing too many modules or depending on internal implementation details creates tight coupling that makes refactoring difficult.

10. Missing @Injectable() decorator: Forgetting to add @Injectable() to a class that should be a provider results in runtime errors when NestJS tries to inject it.

Resources

• NestJS Dependency Injection
• NestJS Modules
• NestJS Injection Scopes
• NestJS Dynamic Modules
• NestJS Testing
• NestJS Circular Dependency
• NestJS Module Reference
• NestJS Lifecycle Events