Questions

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1What is a pipe in NestJS and what are its two primary uses?

2How does ValidationPipe work with class-validator and class-transformer in NestJS?

3What is the difference between applying a pipe at the param level vs handler level vs globally in NestJS?

4How do you write a custom pipe for parsing and validating a MongoDB ObjectId param in NestJS?

5What are all the built-in pipes NestJS ships with?

6What is class-transformer and how does it complement class-validator in NestJS?

7How do you write a custom pipe that sanitizes a string input (trim and lowercase) in NestJS?

8How do you write a pipe that uses metatype to skip validation for primitive types the same way ValidationPipe does?

9What are the two ways to register a global pipe in NestJS and what is the difference?

10What are all the significant ValidationPipe options and what does each do in NestJS?

11How do you unit test a custom pipe in NestJS?

12Can a pipe be asynchronous in NestJS and how do you implement one?

13How do you write an async custom validator that checks uniqueness against a database in NestJS?

14How do you validate arrays and conditional fields with class-validator in NestJS?

15How do you use validation groups with class-validator to apply different rules for create vs update operations in NestJS?

16What does @Exclude() and @Expose() do and how do you use them to strip sensitive fields from responses?

17What is the excludeExtraneousValues option in class-transformer and how does it differ from whitelist in ValidationPipe?

18How do you make a custom pipe configurable by accepting constructor options in NestJS?

19How do you customise the error response format produced by ValidationPipe in NestJS?

20How do you handle file upload validation with ParseFilePipe and custom file validators in NestJS?

21What interface must a custom pipe implement and what does the transform() method receive?

22What is the difference between a transformation pipe and a validation pipe in NestJS?

23How do you pass options to a built-in pipe like ParseIntPipe or ParseUUIDPipe in NestJS?

24What is class-validator and how does it integrate with NestJS ValidationPipe?

25What is the @ValidateNested() decorator and when must you pair it with @Type()?

26How do you write a custom class-validator decorator in NestJS?

27What does the @Transform() decorator do in class-transformer and when should you use it?

28How does @Type() interact with discriminated unions and polymorphic DTOs in class-transformer?

29How do you write a pipe that validates and parses a date string from a route parameter in NestJS?

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How does @Type() interact with discriminated unions and polymorphic DTOs in class-transformer?

For polymorphic nested objects where the correct class depends on a discriminator field, @Type() accepts a discriminator option. class-transformer reads the discriminator property from the incoming JSON and instantiates the matching subtype class. class-validator then runs on that specific subtype's constraints.

Discriminated union with @Type() discriminator

Polymorphic DTO validation rules:

1
The discriminator.property field must exist on the incoming JSON to select the correct subtype.
2
keepDiscriminatorProperty: true retains the type field in the instantiated object.
3
@ValidateNested() must be paired with the discriminator @Type() for nested validation to run.
4
Each subtype class (CatDto, DogDto) has its own class-validator decorators validated independently.
5
If the discriminator value does not match any subType, class-transformer returns a plain object.