lcbp3/.agents/skills/nestjs-best-practices/rules/perf-lazy-loading.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Use Lazy Loading for Large Modules	MEDIUM	Improves startup time for large applications	performance, lazy-loading, modules, optimization

Use Lazy Loading for Large Modules

NestJS supports lazy-loading modules, which defers initialization until first use. This is valuable for large applications where some features are rarely used, serverless deployments where cold start time matters, or when certain modules have heavy initialization costs.

Incorrect (loading everything eagerly):

// Load everything eagerly in a large app
@Module({
  imports: [
    UsersModule,
    OrdersModule,
    PaymentsModule,
    ReportsModule, // Heavy, rarely used
    AnalyticsModule, // Heavy, rarely used
    AdminModule, // Only admins use this
    LegacyModule, // Migration module, rarely used
    BulkImportModule, // Used once a month
  ],
})
export class AppModule {}

// All modules initialize at startup, even if never used
// Slow cold starts in serverless
// Memory wasted on unused modules

Correct (lazy load rarely-used modules):

// Use LazyModuleLoader for optional modules
import { LazyModuleLoader } from '@nestjs/core';

@Injectable()
export class ReportsService {
  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  async generateReport(type: string): Promise<Report> {
    // Load module only when needed
    const { ReportsModule } = await import('./reports/reports.module');
    const moduleRef = await this.lazyModuleLoader.load(() => ReportsModule);

    const reportsService = moduleRef.get(ReportsGeneratorService);
    return reportsService.generate(type);
  }
}

// Lazy load admin features with caching
@Injectable()
export class AdminService {
  private adminModule: ModuleRef | null = null;

  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  private async getAdminModule(): Promise<ModuleRef> {
    if (!this.adminModule) {
      const { AdminModule } = await import('./admin/admin.module');
      this.adminModule = await this.lazyModuleLoader.load(() => AdminModule);
    }
    return this.adminModule;
  }

  async runAdminTask(task: string): Promise<void> {
    const moduleRef = await this.getAdminModule();
    const taskRunner = moduleRef.get(AdminTaskRunner);
    await taskRunner.run(task);
  }
}

// Reusable lazy loader service
@Injectable()
export class ModuleLoaderService {
  private loadedModules = new Map<string, ModuleRef>();

  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  async load<T>(
    key: string,
    importFn: () => Promise<{ default: Type<T> } | Type<T>>,
  ): Promise<ModuleRef> {
    if (!this.loadedModules.has(key)) {
      const module = await importFn();
      const moduleType = 'default' in module ? module.default : module;
      const moduleRef = await this.lazyModuleLoader.load(() => moduleType);
      this.loadedModules.set(key, moduleRef);
    }
    return this.loadedModules.get(key)!;
  }
}

// Preload modules in background after startup
@Injectable()
export class ModulePreloader implements OnApplicationBootstrap {
  constructor(private lazyModuleLoader: LazyModuleLoader) {}

  async onApplicationBootstrap(): Promise<void> {
    setTimeout(async () => {
      await this.preloadModule(() => import('./reports/reports.module'));
    }, 5000); // 5 seconds after startup
  }

  private async preloadModule(importFn: () => Promise<any>): Promise<void> {
    try {
      const module = await importFn();
      const moduleType = module.default || Object.values(module)[0];
      await this.lazyModuleLoader.load(() => moduleType);
    } catch (error) {
      console.warn('Failed to preload module', error);
    }
  }
}

Reference: NestJS Lazy Loading Modules