JS: Property Overview

By Xah Lee. Date: . Last updated: .

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.

  1. Data property → This is the property most of us are familiar with, and is the property used in 99% of code.
  2. 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:

[see JS: Property Attributes]

Own Property and Inherited Property

A property can be said to be a object's “own property” or “inherited property”.

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:

[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:

JS Object Property Topic

  1. JS: Property Overview
  2. JS: Property Key
  3. JS: Property Dot Notation / Bracket Notation
  4. JS: Create/Delete Property
  5. JS: Get Property, Set Property
  6. JS: Check Property Existence
  7. JS: Access Property, List Properties
  8. JS: Property Attributes
  9. JS: Getter/Setter Properties
  10. JS: Property Descriptor
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