I rewrote the examples from the stampit (https://github.com/stampit-org/stampit) readme in vanilla ES6. Great way to understand the inner machinations of Javascript!

stampit.js

// Use a closure for data privacy
const a = function() {
    const priv = 'a';
    return {
        getA() {
            return priv;
        }
    };    
};

a(); // Object
a().getA(); // "a"

// Another one - note that `priv` is named the same in `a` and `b`
const b = function() {
    const priv = 'b';
    return {
        getB() {
            return priv;
        }
    };
};

b(); // Object
b().getB(); // "b"

// Compose the two factories to create a third
const c = function() {
    return Object.assign({}, a(), b());
};

const foo = c();
foo.getA(); // "a"
foo.getB(); // "b"

// Another example: more private data
const availability = function() {
    var isOpen = false; // private

    return {
        open() {
            isOpen = true;
            return this;
        },
        close() {
            isOpen = false;
            return this;
        },
        isOpen() {
            return isOpen;
        }
    };
};

// A mixin with public methods and state
const membership = {
    add(member) {
        this.members[member.name] = member;
        return this;
    },
    getMember(name) {
        return this.members[name];
    },

    members: {}
};

// A mixin that represents some default values
const defaults = {
    name: 'The Saloon',
    specials: 'Whisky, Gin, Tequila'
};

// Compose our mixins to create a bar!
const bar = function() {
    return Object.assign({}, defaults, availability(), membership);
};

// Override defaults with Object.assign
const myBar = Object.assign({}, bar(), {name: 'Moe\'s'});

// Everything works!
myBar.add({name: 'Homer'}).open().getMember('Homer');