A solution to the problem posed by Mike Pennisi on http://weblog.bocoup.com/info-hiding-in-js/ - per-instance private variables and methods that don't forgo the benefits of prototypal inheritance or rely on obscurity (or ES6)

Player.js

var Player = (function () {
    
    var Player, generateGuid, maxCoins, secrets, get, set, cashIn;

    maxCoins = 100;

    // via http://stackoverflow.com/questions/105034/how-to-create-a-guid-uuid-in-javascript
    generateGuid = function () {
        return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
            var r, v;

            r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
            return v.toString(16);
        });
    };

    secrets = {};

    get = function ( player, property ) {
        var data = secrets[ player.guid ];
        return data[ property ];
    };

    set = function ( player, property, value ) {
        var data = secrets[ player.guid ];
        data[ property ] = value;
    };

    cashIn = function ( player ) {
        var coinCount, lifeCount;

        coinCount = get( player, 'coinCount' );
        lifeCount = get( player, 'lifeCount' );

        lifeCount += Math.floor( coinCount / maxCoins );
        coinCount %= maxCoins;

        set( player, 'coinCount', coinCount );
        set( player, 'lifeCount', lifeCount );
    };

    Player = function () {
        Object.defineProperty( this, 'guid', {
            value: generateGuid()
        });
        
        secrets[ this.guid ] = {
            coinCount: 0,
            lifeCount: 3
        };
    };

    Player.prototype = {
        addCoin: function () {
            coinCount = get( this, 'coinCount' );
            set( this, 'coinCount', ++coinCount );

            if ( coinCount >= maxCoins ) {
                cashIn( this );
            }
        }
    };

    return Player;

}());

As @lawnsea points out, this wasn't entirely foolproof - you can swap GUIDs between player instances. In all modern browsers you can use Object.defineProperty() to make the guid property read-only. Code updated