Provider scope (default, request, transient) in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the default provider scope in NestJS?

The default provider scope is Singleton. This means one instance of the provider is created and shared across the entire application.

intermediate

Explain the Request provider scope in NestJS.

The Request scope creates a new instance of the provider for each incoming HTTP request. This helps isolate data per request.

intermediate

What does the Transient provider scope do in NestJS?

The Transient scope creates a new instance of the provider every time it is injected, regardless of the request or module.

advanced

How does the Request scope differ from the Transient scope?

The Request scope creates one instance per HTTP request, shared within that request. The Transient scope creates a new instance every time it is injected, even multiple times in the same request.

intermediate

Why would you use Transient scope for a provider?

Use Transient when you want a fresh instance every time to avoid shared state or side effects, like for stateless helpers or utilities.

What scope does NestJS use by default for providers?

A Session

B Singleton

C Transient

D Request

By default, NestJS providers are singletons, meaning one instance is shared across the app.

Which provider scope creates a new instance for each HTTP request?

A Singleton

B Transient

C Global

D Request

The Request scope creates a new provider instance per HTTP request.

If you want a new provider instance every time it is injected, which scope should you use?

A Singleton

B Request

C Transient

D Scoped

Transient scope creates a new instance every time the provider is injected.

Which scope shares the same instance within a single HTTP request but creates new instances for different requests?

A Request

B Singleton

C Transient

D Global

Request scope shares one instance per HTTP request.

Why might you avoid using Singleton scope for stateful providers?

A Because Singleton shares the same instance across all requests, causing shared state issues

B Because Singleton creates multiple instances

C Because Singleton creates new instances per injection

D Because Singleton is slower

Singleton scope shares one instance app-wide, which can cause unwanted shared state for stateful providers.

Describe the differences between default, request, and transient provider scopes in NestJS.

Think about how many instances are created and when.

When would you choose to use the transient scope over the request or default scopes in NestJS?

Consider cases where sharing instances could cause problems.

Practice

1. In NestJS, what does the default provider scope mean for a service instance?

easy

A. A new instance is created for each HTTP request.

B. A single instance is shared across the entire application.

C. A new instance is created every time the service is injected.

D. The service instance is destroyed after each method call.

Solution

1. Step 1: Understand default provider scope behavior

   By default, NestJS creates one instance of a provider and shares it across the whole app.

2. Step 2: Compare with other scopes

   Request scope creates per HTTP request instances, transient creates per injection instances, so default is the shared singleton.

3. Final Answer:

   A single instance is shared across the entire application. -> Option B

4. Quick Check:

   Default scope = singleton instance [OK]

Hint: Default scope means one shared instance app-wide [OK]

Common Mistakes:

• Confusing default with request scope
• Thinking default creates new instances per injection
• Assuming default destroys instance after use

2. Which of the following is the correct way to set a provider's scope to transient in NestJS?

easy

A. @Injectable({ scope: Scope.REQUEST })

B. @Injectable({ scope: 'transient' })

C. @Injectable({ scope: Scope.TRANSIENT })

D. @Injectable({ transient: true })

Solution

1. Step 1: Recall correct syntax for setting scope

   In NestJS, the scope is set using the Scope enum inside @Injectable decorator like { scope: Scope.TRANSIENT }.

2. Step 2: Eliminate incorrect options

   @Injectable({ scope: Scope.REQUEST }) uses REQUEST scope, @Injectable({ scope: 'transient' }) uses string instead of enum, @Injectable({ transient: true }) uses invalid property.

3. Final Answer:

   @Injectable({ scope: Scope.TRANSIENT }) -> Option C

4. Quick Check:

   Use Scope.TRANSIENT enum for transient scope [OK]

Hint: Use Scope.TRANSIENT enum inside @Injectable decorator [OK]

Common Mistakes:

• Using string 'transient' instead of Scope enum
• Confusing transient with request scope
• Using invalid decorator properties

3. Consider this NestJS service with transient scope:

@Injectable({ scope: Scope.TRANSIENT })
export class MyService {
  id = Math.random();
}

If this service is injected twice in the same controller, what will be true about the id values?

medium

A. Both injections share the same id value.

B. The service will throw an error on multiple injections.

C. The id value is always zero.

D. Each injection has a different id value.

Solution

1. Step 1: Understand transient scope behavior

   Transient scope creates a new instance every time the service is injected, so each injection gets a fresh object.

2. Step 2: Apply to the id property

   Since id is set to a random number on instance creation, each injection will have a different id.

3. Final Answer:

   Each injection has a different id value. -> Option D

4. Quick Check:

   Transient scope = new instance per injection [OK]

Hint: Transient scope means new instance each injection [OK]

Common Mistakes:

• Assuming transient shares instance like default
• Thinking id is constant or zero
• Expecting errors on multiple injections

4. You set a service to request scope but notice it behaves like a singleton. What is the most likely cause?

medium

A. The controller injecting the service is not handling HTTP requests.

B. The service was not decorated with @Injectable.

C. The service scope was set incorrectly using a string instead of Scope.REQUEST.

D. The service is injected only once in the entire app.

Solution

1. Step 1: Understand request scope requirements

   Request scope creates a new instance per HTTP request, so it only works if the provider is injected in a context handling requests.

2. Step 2: Analyze the controller context

   If the controller is not handling HTTP requests (e.g., injected in a service or singleton), the request scope behaves like singleton.

3. Final Answer:

   The controller injecting the service is not handling HTTP requests. -> Option A

4. Quick Check:

   Request scope needs HTTP request context [OK]

Hint: Request scope needs HTTP request context to work [OK]

Common Mistakes:

• Forgetting request scope needs HTTP context
• Confusing decorator absence with scope behavior
• Assuming string scope works like enum

5. You have a service with request scope injected into a transient-scoped service, which is then injected into a controller. How many instances of the request-scoped service are created per HTTP request?

hard

A. One instance per HTTP request.

B. One instance per injection of the transient service.

C. One instance per controller instance.

D. Multiple instances per HTTP request depending on injections.

Solution

1. Step 1: Understand request scope lifetime

   Request-scoped services create one instance per HTTP request regardless of how many times they are injected.

2. Step 2: Analyze injection chain

   Even if injected into a transient service (new instance per injection), the request-scoped service instance remains the same within the same HTTP request.

3. Final Answer:

   One instance per HTTP request. -> Option A

4. Quick Check:

   Request scope = one instance per HTTP request [OK]

Hint: Request scope always one instance per HTTP request [OK]

Common Mistakes:

• Thinking transient causes multiple request-scoped instances
• Confusing transient and request scope lifetimes
• Assuming controller scope affects request-scoped service count