inheritsFrom.js

Function.prototype.inheritsFrom = function (base) {
    this.prototype = Object.create(base.prototype);
    this.prototype.constructor = this;
    this.prototype._super = base.prototype;
    return this;
};

var Base = function () {
    if (!(this instanceof Base))
        throw ('Constructor called without "new"');
};

var Shape = function () {
    Base.call(this);

    this.sides = 0;

}.inheritsFrom(Base);

Shape.prototype.sharp = function () {
    return true;
}

var Square = function () {
    Shape.call(this);

    this.sides = 4;

}.inheritsFrom(Shape);

Square.prototype.sharp = function () {
    return this._super.sharp.call(this);
}

var Circle = function () {
    Shape.call(this);

    this.radius = 3;
}.inheritsFrom(Shape);

var square = new Square();
console.log(square instanceof Square); // True
console.log(square instanceof Shape); // True
console.log(square.constructor === Square); // True
console.log(square.sides === 4); // True
console.log(square.sharp() === true); //True

var circle = new Circle();
console.log(circle.sides === 0); // True
console.log(circle.radius === 3); // True

Shape.prototype.color = "red";

console.log(circle.color); // Red
console.log(square.color); // Red

Shape(); // Error