ValidationPipe in depth in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when ValidationPipe is used globally with transform enabled?

Consider a NestJS app where ValidationPipe is applied globally with transform: true. What is the output type of a controller method receiving a DTO?

NestJS

app.useGlobalPipes(new ValidationPipe({ transform: true }));

@Post()
create(@Body() createDto: CreateDto) {
  return typeof createDto;
}

A "undefined"

B "object"

C "number"

D "string"

💡 Hint

Think about what transform: true does to the incoming data.

With transform: true, the incoming JSON is converted to an instance of the DTO class, so typeof createDto is "object".

📝 Syntax

intermediate

Which option correctly enables whitelist and forbids non-whitelisted properties?

Choose the correct way to configure ValidationPipe to remove properties not in the DTO and throw an error if extra properties are present.

A new ValidationPipe({ whitelist: true, forbidNonWhitelisted: true })

B new ValidationPipe({ whitelist: false, forbidNonWhitelisted: false })

C new ValidationPipe({ whitelist: true, forbidNonWhitelisted: false })

D new ValidationPipe({ whitelist: false, forbidNonWhitelisted: true })

💡 Hint

Whitelist removes extra properties. ForbidNonWhitelisted throws error on extra properties.

Setting whitelist: true removes extra properties. Setting forbidNonWhitelisted: true makes the pipe throw an error if extra properties are found.

🔧 Debug

advanced

Why does ValidationPipe not validate nested objects by default?

Given a DTO with a nested object property, why does ValidationPipe skip validating the nested object?

Example DTO:

class AddressDto {
  @IsString()
  street: string;
}

class UserDto {
  @ValidateNested()
  @Type(() => AddressDto)
  address: AddressDto;
}

A Because ValidationPipe needs <code>transform: true</code> to convert nested objects before validation

B Because <code>ValidateNested</code> and <code>Type</code> decorators are missing or misused

C Because ValidationPipe requires <code>enableImplicitConversion</code> to validate nested objects

D Because ValidationPipe does not support nested validation at all

💡 Hint

Think about how nested objects are converted before validation.

ValidationPipe requires transform: true to convert nested plain objects into class instances so that nested validation decorators work properly.

❓ state_output

advanced

What error message is returned when forbidNonWhitelisted is true and extra properties exist?

When ValidationPipe is configured with forbidNonWhitelisted: true and the request body contains extra properties not in the DTO, what is the typical error message returned?

A "Extra properties detected: extra"

B "Validation failed: extra is not allowed"

C "Unexpected property found: extra"

D "property 'extra' should not exist"

💡 Hint

Look for the exact wording from class-validator errors.

The error message from class-validator when forbidding non-whitelisted properties is typically: "property 'extra' should not exist".

🧠 Conceptual

expert

How does ValidationPipe interact with custom validators and async validation?

When using custom async validators in DTOs, how does ValidationPipe handle the validation process?

A ValidationPipe runs async validators but does not wait for their completion, causing possible false positives

B ValidationPipe ignores async validators and only runs synchronous ones

C ValidationPipe waits for all async validators to complete before returning the result

D ValidationPipe requires a special flag to enable async validation

💡 Hint

Think about how async validation integrates with the validation lifecycle.

ValidationPipe supports async validators and waits for all promises to resolve before returning validation results.