JavaScript: Object Overview

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:

• Object Object. (aka data object) → collection of key value pairs. E.g. {a:1, b:2}
• Specialized Object → special purpose, and internal data. {Arrays, functions, date, regex, etc}, are specialized objects. They have special properties and behaviors. But they still has normal key/value pairs, and can be used like data object to hold collection of key value pairs.

[see Object Type]

Prototype of an Object (Parent)

• Each object has zero or one parent object, called its “prototype”.
• The line 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”.)

[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 is deno object, with properties such as version etc.

User-defined Object (Data Object)

User defined. For example, let x = {a:1, b:2}.