typeMapGet = (parent, key) => {
  const val = parent._map.get(key);

  return val !== undefined && !val.deleted ? val.content.getContent()[val.length - 1] : undefined;
}
@param {AbstractType<any>} parent
@param {string} key
@return {Object<string,any>|number|null|Array<any>|string|Uint8Array|AbstractType<any>|undefined}

@private
@function