code: function assignMergeValue(object, key, value) {
      if (value !== undefined && !eq(object[key], value) || typeof key == 'number' && value === undefined && !(key in object)) {
        object[key] = value;
      }
    }
docs: This function is like `assignValue` except that it doesn't assign
  `undefined` values.
  
  @private
  @param {Object} object The object to modify.
  @param {string} key The key of the property to assign.
  @param {*} value The value to assign.