Recursively add a property in TypeScript

OrderedDeep.ts

import { type Primitive } from "type-fest";

export type BuiltIns = Primitive | Date | RegExp;

/**
 * Add's a numeric `order` property to all objects within arrays, recursively.
 * Used for creating nodes from nested arrays when we want to keep the same order
 * as the feed.
 *
 * Based on type-fest's `ReadonlyDeep`
 *
 * @see https://github.com/sindresorhus/type-fest/blob/main/source/readonly-deep.d.ts#L37
 */
export type OrderedDeep<O> = O extends BuiltIns | BuiltIns[]
	? O
	: O extends (infer U)[]
	? (OrderedDeep<U> & { order: number })[]
	: O extends object
	? { [K in keyof O]: OrderedDeep<O[K]> }
	: O;