class-validator and class-transformer are complementary TypeScript libraries used extensively in NestJS to validate and transform incoming request data (such as JSON bodies, query parameters, and path parameters) before it reaches your controllers. class-validator provides decorator-based validation. It checks if property values meet specific constraints (e.g., @IsEmail(), @IsNumber(), @IsNotEmpty()) and returns validation errors if the data is invalid. It does not change the data type; it only ensures the data conforms to the expected rules.