Object-oriented JavaScript

dog.js

if (typeof Object.create !== "function") {
    Object.create = function (proto) {
        function Type() { }
        Type.prototype = proto;
        return new Type();
    };
}

function Animal(name) {
    var self = this;
    var energy = 100;  // Private
    self.name = name;  // Public
    
    // Private
    function log() {
        var status;
        if (energy >= 100) {
            status = "not tired";
        } else if (energy >= 50) {
            status = "somewhat tired";
        } else {
            status = "very tired";
        }
        console.log(self.name + " is " + status + ".");
    }
    
    // Privileged
    self.play = function () {
        console.log(self.name + " is playing.");
        energy -= 1;
        log();
    };
    
    // Privileged
    self.eat = function () {
        console.log(self.name + " is eating.");
        energy += 1;
        log();
        self.speak();
    };
}

// Public
Animal.prototype.speak = function () {
    console.log(this.name + " makes a noise.");
};

var alfa = new Animal("Alfa");
console.log(alfa.energy);
console.log(alfa.log);
console.log(alfa.name);
alfa.play();
alfa.eat();

function Dog(name) {
    var self = this;
    Animal.call(self, name);
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

Dog.prototype.speak = function () {
    console.log(this.name + " barks.");
};

var bravo = new Dog("Bravo");
console.log(bravo instanceof Animal);
console.log(bravo instanceof Dog);
bravo.play();
bravo.eat();

output.txt

undefined
undefined
Alfa
Alfa is playing.
Alfa is somewhat tired.
Alfa is eating.
Alfa is not tired.
Alfa makes a noise.
true
true
Bravo is playing.
Bravo is somewhat tired.
Bravo is eating.
Bravo is not tired.
Bravo barks.