JS: Property Overview
A JavaScript object is a set of key/value pairs. Each pair is called a “property”.
〔►see JS: Object System Overview〕
2 Kinds of Properties
There are 2 kinds of properties.
- Data property → This is the property most of us are familiar with, and is the property used in 99% of code.
- Accessor property → Also known as Getter/Setter property. Accessor property generates its value dynamically when being accessed or set. (it calls a function implicitly).
〔►see JS: Getter/Setter Properties〕
Property Attributes
Each JavaScript property has associated info called attribute.
For Data Property, the attributes are: {value, writable, enumerable, configurable}.
For Accessor Property:
- For Getter Property, the attributes are: {get, enumerable, configurable}.
- For Setter Property, the attributes are: {set, enumerable, configurable}.
〔►see JS: Property Attributes〕
Own Property and Inherited Property
A property can be said to be a object's “own property” or “inherited property”.
- own property → a property of a object.
- inherited property → a property of the object's parent or ancestor. (parent/ancestor line is called prototype chain)
When a property is looked up (For example, x.color), JavaScript look at the object to see if it has that property, if not, it lookup its parent, and repeat, until a property is found or a parent is null. This is the technical meaning of inheritance.
〔►see JS: Prototype and Inheritance〕
This is the most important thing about properties. The behavior of many operations on properties, such as {accessing, setting, deleting, “for … in” loop, checking existence, …}, depend on whether the property is the object's own property.
〔►see JS: Check Property Existence〕
When writing to a property or deleting a property, parent object is never touched. Only when reading property, the prototype chain (parents) is looked up.
〔►see JS: Get Property, Set Property〕
Property Key Datatype, Property Value Datatype
Property key can be type string or type symbol. 〔►see JS: Property Key〕
Property value can be any type. 〔►see JS: Data Types〕
When a property value is a function, that property is often called a “method”.
Dot Notation and Bracket Notation
There are 2 syntax to access a object's property:
- Dot notation:
object_name.property_key. (For example,x.b) - Bracket notation:
object_name[property_key]. (For example,x["b"]) This is more general, but inconvenient to type.
〔►see JS: Property Dot Notation / Bracket Notation〕
Add / Remove Property
Property can be added or removed from object.
〔►see JS: Create/Delete Property〕
〔►see JS: Prevent Adding Property〕
Accessing (Reading, Writing, Listing, Check Existence) of Property
Property value can be changed anytime (unless the attribute “configurable” is false).
You can also check if a property exists for a object, in its parents, or list a object's own properties. See: