What is class-validator and how does it integrate with NestJS ValidationPipe?
class-validator is a library that lets you declare validation rules as decorators on class properties. ValidationPipe uses it internally by first converting the plain JSON body into a typed DTO class instance, then running validate() from class-validator on that instance. Any failed constraint throws BadRequestException with a detailed errors array.
DTO with class-validator decorators and global setup
How ValidationPipe uses class-validator internally:
- class-transformer converts the plain JSON body to a CreateUserDto class instance.
- class-validator runs validate() on the instance, checking all decorator rules.
- If any rule fails, ValidationPipe throws BadRequestException with an errors array.
- Each error entry lists the property name and every violated constraint message.
Both class-validator and class-transformer must be installed — NestJS does not bundle them.