oop-example.js

// This is called the object constructor function for the "object type" or class Person
function Person(firstName, lastName, age, eyeColor) {
  // Person will be the parent class or superclass for a child class called Employee (defined later)
  this.firstName = firstName; 
  this.lastName = lastName;
  this.age = age;
  this.eyeColor = eyeColor;
  this.changeLastName = function (name) {
    // The changeLastName() function assigns the value of name to the person's lastName property.
    // Since this function is defined in the construct, all Person objects will inherit it:
    this.lastName = name; 
  };
}

// Usage example of creating new Person objects:
var myMom = new Person("Sally", "Doe", 50, "blue");
var myDad = new Person("Victor", "Rally", 48, "green");

// Adding a new method to an existing object is easy: 
myDad.name = function () {
  return this.firstName + " " + this.lastName;
};
// The method will be added to myDad, not to myMom or any other person objects.

// Adding a new property to an existing object is also easy:
myDad.nationality = "English";
// The property will be added to myDad, not to myMom or any other person objects.

// Calling a method written inside a constructor is easy:
myMom.changeLastName("Doer");
// JavaScript knows which Person you are talking about by "substituting" this (line 11) with myMom.

function Employee (firstName, lastName, age, eyeColor, location) {
  // Child is the subclass or child class of Parent.
  Person.call(this, firstName, lastName, age, eyeColor); // Call super constructor of the parent class.
  this.officeLocation = officeLocation; // New property exclusive to the Employee class.
} // A child class extends the parent class with prototypal inheritance.

var myEmployee = new Employee("Jane", "Roe", 34, "brown", "123 Sesame Street 98765"); // subclass method

myEmployee.lastName = "Doe";
myEmployee.officeLocation = "245 Sesame Lane 98765";

// ES6 (ECMAScript 2015) introduced the class keyword as an alternative to create classes.
// Instead of using the keyword function to initiate it, we use the keyword class,
// then the properties are assigned inside a constructor() method as shown here:
class Car { // This is our Car class, which will be the parent class or superclass for a child class
  constructor(brandName = "Unknown") {
    // This example includes a default value for the function parameter, meaning
    // this.brand will be assigned "Unknown" if the value is undefined or not passed.
    this.brand = brandName;
  }
}
myCar = new Car("Toyota");
// This code is more reminiscent of other OOP languages, like C++ or Java.