Poor man's persistent map in JS

define(['morearty/Util'], function (Util) {
  'use strict';

  var O, getIn, update, dissoc;

  O = function (value) {
    this.value = value
  };

  getIn = function (obj, keys) {
    return keys.reduce(function (node, key) { return node[key] }, obj)
  };

  update = function (obj, keys, update) {
    var keysButLast = keys.slice(0, -1);
    var lastKey = keys[keys.length - 1];

    var objCopy = Util.shallowCopy(obj);

    var lastNode = keysButLast.reduce(
      function (node, key) {
        var subNode = Util.shallowCopy(node[key]);
        node[key] = subNode;
        return subNode
      },
      objCopy
    );

    lastNode[lastKey] = update(lastNode[lastKey]);
    return objCopy
  };

  dissoc = function (obj, keys) {
    var keysButLast = keys.slice(0, -1);
    var lastKey = keys[keys.length - 1];

    return update(obj, keysButLast, function (node) { delete node[lastKey] })
  };

  O.prototype = Object.freeze({

    get: function (keys) {
      return getIn(this.value, keys)
    },

    update: function (keys, update) {
      this.value = update(this.value, keys, update)
    },

    assoc: function (keys, newValue) {
      this.value = update(this.value, keys, Util.constantly(newValue))
    },

    dissoc: function (keys) {
      dissoc(this.value, keys)
    }

  });

  return {

    createO: function (value) {
      return new O(value)
    }

  }

});