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 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