gistfile1.js

function Polygon(height, width) { //class constructor
  this.name = 'Polygon';
  this.height = height;
  this.width = width;
}

Polygon.prototype = {
  sayName: function () { //class method
    console.log('Hi, I am a ' + this.name + '.');
  }
};

function Square(length) { //class constructor
  Polygon.call(this, length, length);
  this.name = 'Square';
}
// Square inherits from Polygon
Square.prototype = Object.create(Polygon.prototype);
// Set the "constructor" property to refer to Square
Square.prototype.constructor = Square;
// Add getter
Square.prototype.getArea = function() { //class method
  return this.height * this.width;
};

var p = new Polygon(5,5);
p.sayName(); // Hi, I am a Polygon.

var s = new Square(5);
s.sayName(); // Hi, I am a Square.
console.log(s.getArea()); // 25