Triggering The Execution Of Code With The Setting Of Object Properties

Subject line would seem to say it all:

How does one trigger the execution of a method within an object or any
other code/function with the setting of an object property?


More elaboration for those who want it:

Suppose we have anObjectType, with a property .description, whose value
can be a string, and the value of .description actually describes the
structure of anObjectType as a string. (The details are not important
about how the structure is described, but if you want something concrete,
think of 'anObjectType' as a DOM Node of type NODE_ELEMENT, and
..description as the property .innerHTML.)

When an instance of anObjectType is created, the value of description is
created with the instance. Indeed, the value of description may not be
set with the construction of anObjectType, but it might be added to the
prototype of anObjectType.

But the property .description is not merely read-only. Fetching the value
of .description returns the string which describes the structure of the
object in a meaningful way.

When .description is set with a value of type string, the string is
checked to see if it properly describes a legally formed structured for
anObjectType, and the instance (and only that instance) of anObjectType is
completely re-built: it might possibly be destructed and then re-
constructed, or all properties (but not methods) which involve descendant
objects are changed to conform to the described structure.

The question is, the setting of a string value of the property
..description of object anObjectType does not automatically trigger a
method/methods (or an exception??) for doing something (such as
restructing the object) as a result of the setting of an object property.