object.js

/**
  * Assign nested properties from `copy` to `obj`
  * @template O, C  object and copy
  * @param {O} obj The object to copy properties to
  * @param {C} copy The object to copy properties from
  * @returns {O & C} a new object
  */
function assign(obj, copy) {
  var draft = {};
  var O = Object;
  var objectKeys = O.keys;
  var isObject = object =>
    O.prototype.toString.call(object) === "[object Object]";

  objectKeys(obj).forEach(function(key) {
    draft[key] = obj[key];
  });

  objectKeys(copy).forEach(function(key) {
    var ck = copy[key];
    var dk = draft[key];
    draft[key] = isObject(ck) && isObject(dk) ? assign(dk, ck) : ck;
  });

  return draft;
}