OO

JavaScript provides functionality to describe classes via the class keyword after ES6.

But these classes are just syntactic sugar, designed to emulate how classes behave. These are built on top of javascript prototype model.

Prototype Model

Every JS object has a hidden property called Prototype (__proto__ in some environments). This might point to another object or maybe null.

Prototype Chain

This is kind of like an inheritance chain.

When we try to access a property on an object and its not found, JS checks the object's prototype and continues up the chain until the property is found or it reaches end of the chain and encounters null.

This allows inheritance.

OOP#Inheritance

Implemented by extends keyword

OOP#Encapsulation

JS enforces access control on members via prefixes. There are no special access modifiers as such.

• private: #privateMember
• protected: _protectedMember
• public: publicMember

OOP#Polymorphism

Since, JS is dynamically types all types are determined at runtime. So polymorphism is also runtime. Its not possible to assign an object of child class to reference of parent class since there is no static typing. It is only at runtime that type of any object or reference is determined.

It does support OOP#Overriding parent class methods.