Object Descriptors & Property Flags

Data properties store values, accessor properties run functions
Data properties are enumerable, accessor properties are not
Data properties work with primitives, accessor properties with objects
Data properties use 'value', accessor properties use 'get' and/or 'set'
The fundamental difference between data and accessor properties is in their descriptors: 1) Data properties use 'value' and 'writable' in their descriptors, 2) Accessor properties use 'get' and/or 'set' functions, 3) Data properties directly store values, while accessor properties compute values on-demand, 4) You cannot mix them - a property must be either data or accessor, not both, 5) Both types can be enumerable and configurable, 6) Accessor properties are computed each time they're accessed, 7) Accessor properties are ideal for derived values, validation, or side effects when properties are accessed.