function Rectangle(width, height) {

  this.width = width;

  this.height = height;

}

Rectangle.prototype.getArea = function() {

  return this.width * this.height;

};

​

Rectangle.prototype.toString = function() {

  return "[" + this.constructor.name + ' ' + this.width + "x" + this.height + "]";

};

​

function Square(size) {

  Rectangle.call(this, size, size);

}

​

Square.prototype = Object.create(Rectangle.prototype, {

  constructor: {

    configurable: true,

    enumerable: true,

    value: Square,

    writable: true

  }

});

​

var rect = new Rectangle(5, 10);

var square = new Square(6);

​

console.log(rect.getArea());

console.log(square.getArea());

​

console.log(rect.toString()); 

console.log(square.toString());