How do you write a custom class-validator decorator in NestJS?
Implement ValidatorConstraintInterface and register it with @ValidatorConstraint, then create a convenience decorator using registerDecorator. The constraint class contains the validate() logic and defaultMessage() for the error text.
Custom @IsStrongPassword() validator
Custom decorator creation steps:
- Create a class implementing ValidatorConstraintInterface with validate() and defaultMessage().
- Decorate it with @ValidatorConstraint({ name: 'uniqueName', async: false }).
- Create a convenience function that calls registerDecorator() with the constraint.
- ValidationArguments gives access to value, property, object, and constraints.
Set async: true on @ValidatorConstraint for validators that perform async operations like DB queries.