JS model with mixins

models.js

(function() {
  'use strict';

  /** Helper methods */
  function merge(obj1, obj2) {
    Object.keys(obj2).forEach(function(key) {
      obj1[key] = obj2[key];
    });
    return obj1;
  }

  function mixinMethods(data, mixins) {
    mixins = mixins || [];
    return mixins.reduce(function(all, curr) {
      return merge(all, curr(data));
    }, {});
  }

  /* User model */
  function user(data, mixins) {
    function fullname() {
      return (data.firstname || ' - ') + ' ' + data.surname;
    }

    return Object.freeze(merge(mixinMethods(data, mixins), {
      fullname: fullname
    }));
  }

  /* Length model */
  function length(spec) {

    function ll() {
      var string = spec.firstname;
      if(!string) {
        return 0;
      }
      return string.length;
    }
    return Object.freeze({
      length: ll
    });
  }

  var user1 = user({
    firstname: 'Magnus',
    surname: 'Landerblom'
  }, [length]);
  console.log(user1.fullname(), user1.length());

}());