Methods and systems for managing property types, constraints, and other property validations in prototype-based dynamic programming languages, such as the JavaScript® programming language, are presented. A property definition is created for a target class by a programmer, and a properties class is automatically generated for the target class along with get and set methods to access and validate properties in the properties class. A properties class of a parent class can be automatically determined to exist and added such that the target class properties class inherits from the parent class properties class.