Table of Contents A note Gather state and behaviour : an object Invariant Polymorphism Templating performance issues

Object oriented is considered here to imply:

• objects
• message passing

There's not explicit referral to inheritance.

Alan Kay, who coined the OOP, that object-oriented programming is more about messages than objects. The key is that programs are designed as a set of objects that communicate through messages. In many languages a message is referred to as a polymorphic method call. When:

• a message is sent to an object;
• the object hasn't a method matching the message's signature;

then

• in a class-based language (eg. Smalltalk), check (recursively) if any of the object's base classes contains a method matching the method signature
• in prototype-based language (eg. JavaScript), a pattern known as delegation is used to transfer control to another object, known as a prototype.

Source

2 ways:

• manual: see here.
• language feature, see here

JS does not require such an abstract thing as a class: everything is an object !

In JS, an object include:

• behaviour, in function;
• state, in primitive values.

If you create more than on object, you may want these objects to:

• share behaviour (function)
• not to share state.

You can accomplish:

• this
• prevent object state corruption

with function only, see here.

However, if you want to store the behaviour (function) once, to save space in memory, you'll have to rely to language-dedicated feature:

• the new operator
• constructor functions.

Constructor functions are functions that are used to construct new object:

• specify behaviour
• describing state.

The new operator is used to create new instances based off a constructor function. Then, the constructor function that instantiated the object is called a prototype.

To implement prototype, see here

JavaScript classes, introduced in ECMAScript 2015, are primarily syntactical sugar over JavaScript's existing prototype-based inheritance. The class syntax does NOT introduce a new object-oriented inheritance model to JavaScript

Source: MSDN