one way to do a dictionary in ES5

dictionary.js

// as a nodejs module

module.exports = makeDictionary();

function makeDictionary (dict) {
    return function Dictionary () {
        !dict && (dict = Object.create(null));
        var Dictionary = {
            __proto__: null,
            get: function (name) {
                return dict[name];
            },
            set: function (name, value) {
                return dict[name] = value;
            },
            each: function (callback, thisObj) {
                var key;
                for (key in dict)
                    callback.call(thisObj, dict[key], key)
            },
            map: function (callback, thisObj) {
                var _dict = Object.create(null),
                    key;
                for (key in dict)
                    _dict[key] = callback.call(thisObj, dict[key], key);
                return makeDictionary(_dict)();
            },
            filter: function (comparator, thisObj) {
                var _dict = Object.create(null),
                    key;
                for (key in dict)
                    callback.call(thisObj, dict[key], key) && (_dict[key] = dict[key]);
                return makeDictionary(_dict)();

            }
        };
        Object.freeze(Dictionary);
        return Dictionary;
   }
}

/*
 // example:
 var dict = require('dictionary');
 var d = dict();
 
 d.set('foo', 'bar');
 d.get('foo'); // 'bar'

 d.foo = 'bar';
 d.foo; // undefined

 var D = d.map(function (val) { return val.toUpperCase() });
 D.get('foo'); // 'BAR'

// having .get() and .set() annoys me, though
// I think harmony proxies will make it possible to write PHP style __get and __set methods?

*/