1---
2title: Use Lazy Loading for Large Modules
3impact: MEDIUM
4impactDescription: Improves startup time for large applications
5tags: performance, lazy-loading, modules, optimization
6---
7 
8## Use Lazy Loading for Large Modules
9 
10NestJS 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.
11 
12**Incorrect (loading everything eagerly):**
13 
14```typescript
15// Load everything eagerly in a large app
16@Module({
17  imports: [
18    UsersModule,
19    OrdersModule,
20    PaymentsModule,
21    ReportsModule, // Heavy, rarely used
22    AnalyticsModule, // Heavy, rarely used
23    AdminModule, // Only admins use this
24    LegacyModule, // Migration module, rarely used
25    BulkImportModule, // Used once a month
26  ],
27})
28export class AppModule {}
29 
30// All modules initialize at startup, even if never used
31// Slow cold starts in serverless
32// Memory wasted on unused modules
33```
34 
35**Correct (lazy load rarely-used modules):**
36 
37```typescript
38// Use LazyModuleLoader for optional modules
39import { LazyModuleLoader } from '@nestjs/core';
40 
41@Injectable()
42export class ReportsService {
43  constructor(private lazyModuleLoader: LazyModuleLoader) {}
44 
45  async generateReport(type: string): Promise<Report> {
46    // Load module only when needed
47    const { ReportsModule } = await import('./reports/reports.module');
48    const moduleRef = await this.lazyModuleLoader.load(() => ReportsModule);
49 
50    const reportsService = moduleRef.get(ReportsGeneratorService);
51    return reportsService.generate(type);
52  }
53}
54 
55// Lazy load admin features with caching
56@Injectable()
57export class AdminService {
58  private adminModule: ModuleRef | null = null;
59 
60  constructor(private lazyModuleLoader: LazyModuleLoader) {}
61 
62  private async getAdminModule(): Promise<ModuleRef> {
63    if (!this.adminModule) {
64      const { AdminModule } = await import('./admin/admin.module');
65      this.adminModule = await this.lazyModuleLoader.load(() => AdminModule);
66    }
67    return this.adminModule;
68  }
69 
70  async runAdminTask(task: string): Promise<void> {
71    const moduleRef = await this.getAdminModule();
72    const taskRunner = moduleRef.get(AdminTaskRunner);
73    await taskRunner.run(task);
74  }
75}
76 
77// Reusable lazy loader service
78@Injectable()
79export class ModuleLoaderService {
80  private loadedModules = new Map<string, ModuleRef>();
81 
82  constructor(private lazyModuleLoader: LazyModuleLoader) {}
83 
84  async load<T>(
85    key: string,
86    importFn: () => Promise<{ default: Type<T> } | Type<T>>,
87  ): Promise<ModuleRef> {
88    if (!this.loadedModules.has(key)) {
89      const module = await importFn();
90      const moduleType = 'default' in module ? module.default : module;
91      const moduleRef = await this.lazyModuleLoader.load(() => moduleType);
92      this.loadedModules.set(key, moduleRef);
93    }
94    return this.loadedModules.get(key)!;
95  }
96}
97 
98// Preload modules in background after startup
99@Injectable()
100export class ModulePreloader implements OnApplicationBootstrap {
101  constructor(private lazyModuleLoader: LazyModuleLoader) {}
102 
103  async onApplicationBootstrap(): Promise<void> {
104    setTimeout(async () => {
105      await this.preloadModule(() => import('./reports/reports.module'));
106    }, 5000); // 5 seconds after startup
107  }
108 
109  private async preloadModule(importFn: () => Promise<any>): Promise<void> {
110    try {
111      const module = await importFn();
112      const moduleType = module.default || Object.values(module)[0];
113      await this.lazyModuleLoader.load(() => moduleType);
114    } catch (error) {
115      console.warn('Failed to preload module', error);
116    }
117  }
118}
119```
120 
121Reference: [NestJS Lazy Loading Modules](https://docs.nestjs.com/fundamentals/lazy-loading-modules)
122