JavaScript Runtime Semantics

Objects & Property Descriptors

JavaScript objects are collections of properties. Each property has a descriptor that defines whether it is a data property or accessor, and whether it is writable, enumerable, and configurable.

Objects are not just dictionaries
Property lookup checks own properties first, then prototypes
Descriptors control property behavior
Enumerability affects iteration and serialization
Freezing is shallow

Property descriptor fields
Field	Meaning	Applies to
value	Stored value	Data property
writable	Can value be changed?	Data property
get / set	Accessor functions	Accessor property
enumerable	Appears in enumeration?	Both
configurable	Can redefine/delete?	Both

Descriptor-defined property

const user = {}

Object.defineProperty(user, "id", {
  value: "u_123",
  writable: false,
  enumerable: true,
  configurable: false,
})

console.log(Object.keys(user)) // ["id"]

Descriptors explain behavior that normal property syntax hides.