class-validator

class-validator is a decorator-based validation library for TypeScript and JavaScript that uses class properties decorated with validation constraints to define validation rules. The library integrates naturally with class-based architectures like NestJS, TypeORM, and other frameworks that use TypeScript decorators extensively. Validation decorators are applied directly to class properties: @IsString(), @IsEmail(), @MinLength(3), @MaxLength(50), @IsInt(), @Min(0), @IsOptional(), @IsArray(), @ValidateNested(), and many more. The validate() function checks a class instance against its decorators and returns an array of validation errors with property paths, constraint names, and error messages. class-validator supports custom validation decorators through @ValidatorConstraint and @Validate, conditional validation with @ValidateIf, validation groups for applying different rules in different contexts, and whitelist validation that strips undecorated properties. The library works with class-transformer for converting plain objects to class instances before validation. In NestJS, class-validator integrates through the ValidationPipe, automatically validating request bodies, query parameters, and route parameters against DTO classes. class-validator is the standard validation approach in the NestJS ecosystem and any TypeScript project using decorator-based class definitions.