Provider scope (default, request, transient) in NestJS - Mini Project: Build & Apply

Understanding Provider Scope in NestJS

📖 Scenario: You are building a simple NestJS service that demonstrates how different provider scopes work. This will help you understand how NestJS manages instances of services in different scopes.

🎯 Goal: Create three services with different provider scopes: default (singleton), request-scoped, and transient. Then, inject them into a controller to observe their behavior.

📋 What You'll Learn

Create a service called DefaultService with default scope

Create a service called RequestService with request scope

Create a service called TransientService with transient scope

Inject all three services into a controller called AppController

Add a method in AppController called getScopes() that returns the instance IDs of each service

💡 Why This Matters

🌍 Real World

Understanding provider scopes is essential for building scalable and efficient NestJS applications, especially when managing state or resources per request or per injection.

💼 Career

Many backend developer roles using NestJS require knowledge of provider scopes to optimize performance and resource management in server-side applications.

1

Create DefaultService with default scope

Create a service class called DefaultService with a property id set to a random number using Math.random(). Use the default provider scope (no special decorator needed).

NestJS

// Create DefaultService class with an id property set to Math.random()
// Your code here

Hint

Use @Injectable() without any scope option to create a default singleton service.

2

Create RequestService with request scope

Create a service class called RequestService with a property id set to Math.random(). Use @Injectable({ scope: Scope.REQUEST }) to make it request-scoped. Import Scope from @nestjs/common.

NestJS

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

@Injectable()
export class DefaultService {
  id = Math.random();
}

// Create RequestService class with request scope and id property
// Your code here

Hint

Use @Injectable({ scope: Scope.REQUEST }) to create a request-scoped service.

3

Create TransientService with transient scope

Create a service class called TransientService with a property id set to Math.random(). Use @Injectable({ scope: Scope.TRANSIENT }) to make it transient-scoped. Import Scope from @nestjs/common if not already imported.

NestJS

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

@Injectable()
export class DefaultService {
  id = Math.random();
}

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

// Create TransientService class with transient scope and id property
// Your code here

Hint

Use @Injectable({ scope: Scope.TRANSIENT }) to create a transient-scoped service.

4

Inject services into AppController and add getScopes() method

Create a controller class called AppController. Inject DefaultService, RequestService, and TransientService via the constructor. Add a method called getScopes() that returns an object with keys defaultId, requestId, and transientId holding the id values from each service instance.

NestJS

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

import { DefaultService } from './default.service';
import { RequestService } from './request.service';
import { TransientService } from './transient.service';

// Create AppController class
// Inject DefaultService, RequestService, TransientService
// Add getScopes() method returning their id values
// Your code here

Hint

Inject the services in the constructor and return their id properties in getScopes().

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