How do I implement this java pattern in JavaScript (using inheritance)?

Question

Here is the working java code to provide a base engine class which handles the balance listener registration, for a number of engine implementations used by various games. eg there would be a demo engine which maintains a demo balance for a demo game and a cash version of the same engine which gets the balance from a back office etc. The crux here is not the actual java, but how to implement this kind of pattern in JavaScript. i have tried about 30 different ways to do it, including using John Resigs "simple JavaScript inheritance" and the extend() sugar defined in "JavaScript: The definitive Guide", using various module patterns, using that=this etc. none of which worked for this problem.

Here is the working java code:

File Engine.java:

package com.test;
public abstract class Engine {
    BalanceListener externalBalanceListener = null;
    double balance = 0;
    public void registerBalanceListener(BalanceListener balanceListener) {
        externalBalanceListener = balanceListener;
        balanceListener.update(balance);  // call once when first register
    }
    public double getBalance() {
        return balance;
    }
    protected void setBalance(double newBal) {
        if (newBal != balance) {
            balance = newBal;
            if (externalBalanceListener != null) {
                externalBalanceListener.update(newBal);
            }
        }       
    }
    public abstract double startGame(double stake, int numLines);
}

File BalanceListener.java

package com.test;
public interface BalanceListener {
    void update(double balance);
}

File DemoEngine.java

package com.test;
import java.util.Random;

public class DemoEngine extends Engine {
    public DemoEngine() {
        setBalance(10000);
    }
    public double startGame(double stake, int numLines) {
        double wonAmount;
        Random random = new Random();

        setBalance (getBalance() - (stake * numLines));

        // some game logic
        wonAmount = Math.round((random.nextDouble() * 10)) * stake;
        setBalance (getBalance() + wonAmount);          
        return wonAmount;
    }
}

File DemoGame.java

package com.test;

public class DemoGame {

    public class MyListener implements BalanceListener {
        public MyListener(){
        }
        public void update(double balance) {
            System.out.println("new balance: " + balance);
        }
    }

    public static void main(String[] args) {
        Engine engine = new DemoEngine();

        DemoGame demoGame = new DemoGame();

        BalanceListener balanceListener = demoGame.new MyListener();

        engine.registerBalanceListener(balanceListener);

        engine.startGame(10, 20);
    }
}

Here was a plain (failed) attempt to get the same thing working in JavaScript (see http://jsfiddle.net/fmX67/ )

function Engine() {
    this.balance = 0;
    this.externalBalanceListener;

    this.registerBalanceListener = function(l) {
            this.externalBalanceListener= l;
            this.externalBalanceListener(this.balance);
    };

    this.getBalance = function() {
        return this.balance;
    };

    this.setBalance = function (newBal) {
        if (newBal != this.balance) {
            this.balance = newBal;
            if (this.externalBalanceListener != undefined) {
                this.externalBalanceListener(newBal);
            }
        }
    };

};

function DemoEngine() {
    this.startGame = function(stake, numLines) {
        var won;

        setBalance(this.getBalance() - stake*numlines);
        won = Math.round(Math.random() * 10) * Stake;

        this.setBalance(this.getBalance() + won);

        return won;
    };
}

DemoEngine.prototype = Engine;

function DemoGame() {

    function balanceListener(balance) {
        console.log(balance);
    }

    var engine = new DemoEngine();

    engine.registerBalanceListener(balanceListener); // This throws an exception: Uncaught TypeError: Object [object Object] has no method 'registerBalanceListener'

    engine.startGame(10, 25);
}

var game = new DemoGame();

Obviously I have no idea what I am doing (despite reading several JS Books). I assume I could use composition instead of trying to inherit, but this limits the use of the language and what patterns can be implemented.

Edit: here is the working version with Shaun West's answer. see http://jsfiddle.net/fmX67/3/

function Engine() {
    this.balance = 0;
    this.externalBalanceListener;

    this.registerBalanceListener = function(l) {
            this.externalBalanceListener= l;
            this.externalBalanceListener(this.balance);
    };

    this.getBalance = function() {
        return this.balance;
    };

    this.setBalance = function (newBal) {
        if (newBal != this.balance) {
            this.balance = newBal;
            if (this.externalBalanceListener != undefined) {
                this.externalBalanceListener(newBal);
            }
        }
    };

};

function DemoEngine() {
    this.setBalance(1000);
    this.startGame = function(stake, numLines) {
        var won;

        this.setBalance(this.getBalance() - stake*numLines);
        won = Math.round(Math.random() * 10) * stake;

        this.setBalance(this.getBalance() + won);

        return won;
    };
}

DemoEngine.prototype = new Engine();

function DemoGame() {

    function balanceListener(balance) {
        console.log(balance);
    }

    var engine = new DemoEngine();

    engine.registerBalanceListener(balanceListener); // This throws an exception: Uncaught TypeError: Object [object Object] has no method 'registerBalanceListener'

    engine.startGame(10, 25);
}


var game = new DemoGame();

Answer 1

As odd as it may seem to someone coming from Java, in your attempt you'll want to change this:

DemoEngine.prototype = Engine;

To this:

DemoEngine.prototype = new Engine();

This answer is pretty good if you want more information: What is the 'new' keyword in JavaScript?