The described technology is directed towards an object-oriented programming (OOP) system and library that maps conventional object-oriented concepts such as class hierarchies, interfaces, and static methods and properties onto a prototypal object system such as JavaScript®. Various functions of the OOP system are called to create a class-like structure that is instantiated into an object instance upon request. The OOP system allows for use of a wrapper pattern to wrap selected objects with added functionality such as validation and tracing, and to provide enhanced functionality with respect to properties, such as to call a function associated with a property when the property's value changes, and/or to lazily generate property values only when accessed.