Optional providers in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mark the provider as optional in the constructor.

NestJS

constructor(@[1]() private readonly service?: MyService) {}

Drag options to blanks, or click blank then click option'

AInject()

BController()

CInjectable()

DOptional()

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Injectable() instead of @Optional()

Forgetting to add the decorator

Using @Inject() alone without @Optional()

The @Optional() decorator marks the provider as optional, so NestJS won't throw an error if it is missing.

2fill in blank

medium

Complete the code to import the Optional decorator from the correct package.

NestJS

import { [1] } from '@nestjs/common';

Drag options to blanks, or click blank then click option'

AInjectable

BOptional

CController

DModule

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Injectable instead of Optional

Importing from wrong package

Misspelling the decorator name

The Optional decorator is imported from @nestjs/common to mark providers as optional.

3fill in blank

hard

Fix the error in the constructor to correctly inject an optional provider.

NestJS

constructor(@Optional() private readonly service?: [1]) {}

Drag options to blanks, or click blank then click option'

AOptional<MyService>

BMyService?

CMyService

DMyService | null

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the type to a union or optional type instead of using the decorator

Not using @Optional() decorator

Using incorrect syntax for optional types

To inject an optional provider, you must use the @Optional() decorator on the parameter, not change the type. The correct way is to decorate the parameter, not change its type.

4fill in blank

hard

Fill both blanks to correctly inject an optional provider with a custom token.

NestJS

constructor(@[1]('MY_TOKEN') @[2]() private readonly service?: MyService) {}

Drag options to blanks, or click blank then click option'

AInject

BOptional

CController

DInjectable

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of decorators

Using wrong decorators like @Controller or @Injectable

Omitting @Optional() decorator

When injecting a provider by a custom token, use @Inject('TOKEN'). To make it optional, add @Optional() decorator.

5fill in blank

hard

Fill all three blanks to create an optional provider with a default value fallback.

NestJS

const defaultService = [3];

constructor(@[1]() private readonly service?: MyService) {
  this.service = this.service ?? [2];
}

Drag options to blanks, or click blank then click option'

AOptional

BdefaultService

Cnew MyService()

DInject

Attempts:

3 left

💡 Hint

Common Mistakes

Not using @Optional() decorator

Forgetting to assign default value

Using wrong variable names

The @Optional() decorator marks the provider as optional. If it's not injected, the code assigns a default instance new MyService() to defaultService and uses it as fallback.