JavaScript: Object Overview

By Xah Lee. Date: . Last updated: .

This page is a overview of JavaScript object system. It's called prototype-based object system.

If you don't know how to create object, read properties, first see: Object Basics .

What is JavaScript Object

A JavaScript Object is a collection of key/value pairs. e.g. {a:1, b:2}. Each key/value pair is called a Property .

Special Object and Data Object

JavaScript objects can be categorized into 2 kinds:

[see Object Type]

Prototype of an Object (Parent)

[see Prototype and Inheritance]

IsExtensible

Each object has a special true/false value attached called the [[isExtensible]] internal slot. It specifies whether new properties can be added to the object.

[see Prevent Adding Property]

Standard Object, Hosted Object, User-Created Object

It is useful to distinguish objects by where they came from. We have the following sources:

Standard Object
Objects from the JavaScript language. For example, arrays, functions, dates, regex, Math, etc. [see JavaScript Object Reference]
Hosted Object
Objects from the hosting environment. For example, in Browser, there's Browser's Window Object. In deno, there's deno object, with properties such as version etc.
User-defined Object (Data Object)
User defined. For example, let x = {a:1, b:2}.

Create Object

JavaScript: Create Object

JavaScript Object Property Overview

Property Overview

JavaScript Object and Inheritance

BUY
ΣJS
JavaScript in Depth

JavaScript in Depth

Basic Syntax

Value Types

Variable

String

Property

Object and Inheritance

Array

Function

Constructor/Class

Iterable 🌟

Misc