回答編集履歴

  const getMethodNameOf = obj => {

  const proto = Object.getPrototypeOf(obj);

  if ( proto === null ) return [ ...Object.getOwnPropertyNames(obj).filter( p => obj[p] instanceof Function) ];

  return [ ...Object.getOwnPropertyNames(obj).filter( p => obj[p] instanceof Function ), ...getMethodNameOf(proto)];

  }