Private variables in ES6 Classes.

class.js

/**
 * So, obviously this is by no means a "new trick",
 * but encapsulation in classes works the same as
 * it does in "Modules" (IIFE pattern): You hide them
 * in a scope, and define the methods that need access
 * to them within that same scope. The constructor is
 * as good a place as any for that.
*/

import _ from 'underscore';
import { get, set } from '../lib/state';

export default class Model {
  constructor(initialState={}){
    let state = _.extend({}, this.defaults, initialState);
    
    this.get = get(state);
    this.set = set(state);
  }
}

state.js

export function get(state){
  return function(key){
    return state[key] || '';
  }
}

export function set(state){
  return function(key, value){
    state[key] = value;
    return this;
  }
}

I really like this design for private instance variables. I just have one question about the above, how is this.defaults defined?

Perhaps you meant Model.defaults, where the Model.defaults property is defined after the Model class?