ValidationPipe setup in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when ValidationPipe rejects invalid input?

Consider a NestJS controller using ValidationPipe globally. What is the response behavior when a request body fails validation?

NestJS

import { Controller, Post, Body, UsePipes, ValidationPipe } from '@nestjs/common';
import { IsInt, Min } from 'class-validator';

class AgeDto {
  @IsInt()
  @Min(18)
  age: number;
}

@Controller('users')
export class UserController {
  @Post('age')
  @UsePipes(new ValidationPipe())
  setAge(@Body() ageDto: AgeDto) {
    return { message: `Age set to ${ageDto.age}` };
  }
}

A The server ignores validation and processes the request normally.

B The server responds with HTTP 400 and a detailed error message about validation failure.

C The server crashes with an unhandled exception.

D The server responds with HTTP 500 Internal Server Error.

💡 Hint

Think about what ValidationPipe does when input does not meet the rules.

ValidationPipe automatically checks the input against the rules defined in the DTO. If validation fails, it sends a 400 Bad Request with details.

📝 Syntax

intermediate

Which option correctly enables whitelist in ValidationPipe?

You want ValidationPipe to strip out any properties not defined in the DTO. Which code snippet correctly sets this up?

A new ValidationPipe({ whitelist: true })

B new ValidationPipe({ removeExtra: true })

C new ValidationPipe({ stripUnknown: true })

D new ValidationPipe({ filter: true })

💡 Hint

Check the official option name for removing extra properties.

The whitelist option removes properties not in the DTO. Other options are invalid.

❓ state_output

advanced

What is the output when transform is enabled in ValidationPipe?

Given this DTO and controller, what is the type of the parameter inside the method when ValidationPipe has transform enabled?

NestJS

import { Controller, Post, Body, UsePipes, ValidationPipe } from '@nestjs/common';
import { IsString } from 'class-validator';

class NameDto {
  @IsString()
  name: string;
}

@Controller('hello')
export class HelloController {
  @Post('name')
  @UsePipes(new ValidationPipe({ transform: true }))
  greet(@Body() nameDto: NameDto) {
    return typeof nameDto;
  }
}

A "undefined"

B "string"

C "number"

D "object"

💡 Hint

Transform converts plain JSON to class instance.

With transform: true, the input is converted to an instance of the DTO class, so typeof returns "object".

🔧 Debug

advanced

Why does ValidationPipe not validate nested objects by default?

Given this DTO with a nested object, why does ValidationPipe not validate the nested properties without extra setup?

NestJS

import { IsString, ValidateNested } from 'class-validator';
import { Type } from 'class-transformer';

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

class UserDto {
  @IsString()
  name: string;

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

A Because nested objects are ignored unless transform is false.

B Because ValidationPipe only validates primitive types by default.

C Because ValidationPipe requires the nested property to be decorated with @ValidateNested and Type() decorators.

D Because ValidationPipe does not support nested validation at all.

💡 Hint

Check how class-validator handles nested validation.

To validate nested objects, you must use @ValidateNested() and @Type(() => ClassName) decorators on the nested property.

🧠 Conceptual

expert

What is the effect of setting forbidNonWhitelisted to true in ValidationPipe?

If you configure ValidationPipe with whitelist: true and forbidNonWhitelisted: true, what happens when the request body contains extra properties not in the DTO?

A The request is rejected with HTTP 400 and an error listing the extra properties.

B The extra properties are silently removed and the request proceeds.

C The server logs a warning but processes the request normally.

D The extra properties are added to the DTO instance.

💡 Hint

Think about how forbidNonWhitelisted changes whitelist behavior.

Setting forbidNonWhitelisted: true causes ValidationPipe to reject requests with unknown properties instead of silently removing them.