JavaScript traits implementation

I wanted to step up my JS game, and move on from mostly closure based scripting, so I decided to write an application in node. I don't have much experience with prototype based programming, and I guess it's a little too early for ES6 (well, maybe with traceur).

During planning of my app I saw that often I'll need container behaviour. So, I know I could implement it with a class, but I thought: why not use trait-like thingy instead?

Gamemode.js, concrete class

'use strict';

var Script = require('../Script');

var Gamemode = module.exports = function () {
    Script.apply(this, arguments);
};

Gamemode.prototype = Object.create(Script.prototype);
Gamemode.prototype.constructor = Gamemode;

Script.js, abstract class

'use strict';

var helper = require("./helper");
var _ = require("lodash");

var Script = module.exports = function () {
    if (this.constructor === Script) {
        throw new Error("Can't initialize abstract class!");
    }

    var defaultDefinitions = {
        MAX_PLAYERS: '(500)'
    };

    _.merge(this, helper.collection("definition", defaultDefinitions));
};

Script.prototype.build = function () {
    return this.definitions;
};

helper.js

'use strict';

var _ = require("lodash");

var capitalize = function (string) {
    return string.charAt(0).toUpperCase() + string.slice(1).toLowerCase();
};

var container = function (singular, mutable) {
    if (undefined === mutable) {
        mutable = false;
    }

    var accessors = {},
        methodSingular = capitalize(singular),
        plural = singular + 's',
        methodPlural = capitalize(plural);

    accessors['get' + methodPlural] = function () {
        return this[plural];
    };

    accessors['set' + methodPlural] = function (value) {
        if (undefined === value) {
            delete this[plural]; 
        }

        this[plural] = value;

        return this;
    };


    if (mutable) {
        accessors['add' + methodSingular] = function (item) {
            _.assign(this[plural], item);
        };

        accessors['remove' + methodSingular] = function (item) {
            delete this[plural][item];
        };
    }

    return accessors;
};

var collection = function (name, defaults) {
    if (undefined === defaults) {
        defaults = {};
    };

    var all = {};
    all[name + 's'] = _.defaults({}, defaults);

    return _.merge(all, container(name, true));
};

var helper = module.exports = {
    capitalize: capitalize,
    container: container,
    collection: collection
};

It works properly, now every instance of Gamemode has its own "definitions" property, with getDefinitions, setDefinitions, addDefinition and removeDefinition methods.

1. Am I violating any rules, like encapsulation?
2. Am I adding property somewhere else than in the constructor?
3. Would creating a "container" object instead be better?