Property Attributes and Descriptors

JavaScript properties have hidden attributes that control their behaviour. Most of the time you don’t need to think about them — but understanding they exist explains some otherwise puzzling behaviour.

The Three Attributes

Every data property has:

writable — can the value be changed? (true by default)
enumerable — does it show up in for-in and Object.keys? (true by default)
configurable — can the descriptor be modified or the property deleted? (true by default)

Properties you create with normal code — (obj :name "x"), bind, assignment — have all three set to true. Properties defined by the language itself (like Array.prototype.map) are typically non-enumerable, which is why they don’t show up when you iterate an object’s keys.

Inspecting and Defining

;; Inspect a property's descriptor
(bind desc (Object:get-own-property-descriptor obj :name))
(console:log desc)
;; → { value: "x", writable: true, enumerable: true, configurable: true }

;; Define a property with specific attributes
(Object:define-property obj :frozen-name
  (obj :value "immutable"
       :writable false
       :enumerable true
       :configurable false))

Assignment vs Definition

Assignment ((= obj:name value)) goes through the prototype chain and calls setters. Definition (Object:define-property) creates or modifies the property directly on the object, bypassing setters. The distinction rarely matters in daily code, but it explains why some patterns behave unexpectedly. Deep JavaScript covers the full semantics of assignment vs definition.

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