A utility to flatten keys just like how react-hook-form does it

flatNestedKeys.ts

type FlattenKeys<T extends Record<string, any>, prefix extends string = ''> = {
	[key in keyof T]: key extends string
		? T[key] extends object
			? `${prefix}${key}` | FlattenKeys<T[key], `${prefix}${key}.`>
			: `${prefix}${key}`
		: never;
}[keyof T];

// and to use it:
type Data = {
	title: string;
	meta: {
		dateCreated: string;
		author: {
			username: string;
			userId: string;
		};
	};
};

// "title" | "meta" | "meta.author" | "meta.author.userId" | ...
type Keys = FlattenKeys<Data>;

// for arrays being inside, you need to remove their types from the union
type RemoveNonString<T> = T extends string ? T : never;
type FixedData = RemoveNonString<FlattenKeys<Data>>;