ネストしたObjectからキーを全て取得するユーティリティ型

type Maybe<T> = T | null | undefined;
type NestedKeyOf<T extends Record<string, unknown>> = {
  [K in keyof T & string]: T[K] extends Maybe<Record<string, unknown>>
    ? T[K] extends Maybe<Record<string, unknown>>
      ? `${K}.${NestedKeyOf<NonNullable<T[K]>>}`
      : K
    : K;
}[keyof T & string];

以下の User 型を与えてみる

type User = {
  id: string;
  name: string;
  age?: number | null;
  category: string;
  email: string;
  address?: {
    city: string;
    country: string;
  } | null;
};

参考: https://medium.com/xgeeks/typescript-utility-keyof-nested-object-fa3e457ef2b2