JS: Object Overview

What is JavaScript Object

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

• JS: Property Overview

Special Object and Data Object

JavaScript objects can be categorized into 2 kinds:

Object Object. (aka data object)

collection of key-and-value pairs. e.g. {a:1, b:2}

Specialized Object

• Special purpose and may have internal data.
• For example, function, date, regex, etc, are specialized objects.
• They have special properties and behaviors.
• But they still have the feature of holding key-and-value pairs.

• JS: Object Type

Prototype of an Object (Parent)

• Each object MAY have ONE parent object. This object is call its prototype.
• The chain of parents is called Prototype Chain. The significance of Prototype Chain is how object's properties are looked up: When a property is accessed, JavaScript will go up the parent object or parent of parent etc to look for the property. (this is called inheritance.)

This is called Prototype-Based Object System.

• JS: 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.

• JS: 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. e.g. arrays, functions, dates, regex, etc.

• JavaScript Object Reference

Hosted Object

Objects from the hosting environment. e.g. in Browser, there's Browser's Window Object. In deno, there is deno object, with properties such as version etc.

User-defined Object (Data Object)

User defined.

• JS: Create Object