1---
2title: Understand Provider Scopes
3impact: CRITICAL
4impactDescription: Prevents data leaks and performance issues
5tags: dependency-injection, scopes, request-context
6---
7 
8## Understand Provider Scopes
9 
10NestJS has three provider scopes: DEFAULT (singleton), REQUEST (per-request instance), and TRANSIENT (new instance for each injection). Most providers should be singletons. Request-scoped providers have performance implications as they bubble up through the dependency tree. Understanding scopes prevents memory leaks and incorrect data sharing.
11 
12**Incorrect (wrong scope usage):**
13 
14```typescript
15// Request-scoped when not needed (performance hit)
16@Injectable({ scope: Scope.REQUEST })
17export class UsersService {
18  // This creates a new instance for EVERY request
19  // All dependencies also become request-scoped
20  async findAll() {
21    return this.userRepo.find();
22  }
23}
24 
25// Singleton with mutable request state
26@Injectable() // Default: singleton
27export class RequestContextService {
28  private userId: string; // DANGER: Shared across all requests!
29 
30  setUser(userId: string) {
31    this.userId = userId; // Overwrites for all concurrent requests
32  }
33 
34  getUser() {
35    return this.userId; // Returns wrong user!
36  }
37}
38```
39 
40**Correct (appropriate scope for each use case):**
41 
42```typescript
43// Singleton for stateless services (default, most common)
44@Injectable()
45export class UsersService {
46  constructor(private readonly userRepo: UserRepository) {}
47 
48  async findById(id: string): Promise<User> {
49    return this.userRepo.findOne({ where: { id } });
50  }
51}
52 
53// Request-scoped ONLY when you need request context
54@Injectable({ scope: Scope.REQUEST })
55export class RequestContextService {
56  private userId: string;
57 
58  setUser(userId: string) {
59    this.userId = userId;
60  }
61 
62  getUser(): string {
63    return this.userId;
64  }
65}
66 
67// Better: Use NestJS built-in request context
68import { REQUEST } from '@nestjs/core';
69import { Request } from 'express';
70 
71@Injectable({ scope: Scope.REQUEST })
72export class AuditService {
73  constructor(@Inject(REQUEST) private request: Request) {}
74 
75  log(action: string) {
76    console.log(`User ${this.request.user?.id} performed ${action}`);
77  }
78}
79 
80// Best: Use ClsModule for async context (no scope bubble-up)
81import { ClsService } from 'nestjs-cls';
82 
83@Injectable() // Stays singleton!
84export class AuditService {
85  constructor(private cls: ClsService) {}
86 
87  log(action: string) {
88    const userId = this.cls.get('userId');
89    console.log(`User ${userId} performed ${action}`);
90  }
91}
92```
93 
94Reference: [NestJS Injection Scopes](https://docs.nestjs.com/fundamentals/injection-scopes)
95