TypeScript — all possible paths of an object

nestedObjectPaths.ts

/**
 * Object type that represents all possible paths of an object,
 * including optional members.
 * @template T The object type to get the paths from.
 * @see https://stackoverflow.com/a/76131375/10851645
 * @see https://gist.github.com/albertms10/5a8b83e436a1689aa4b425ec22058301
 * @example
 * interface Package {
 *   name: string;
 *   man?: string[];
 *   bin: { "my-program": string };
 *   funding?: { type: string; url: string }[];
 *   peerDependenciesMeta?: {
 *     "soy-milk"?: { optional: boolean };
 *   };
 * }
 *
 * const list: Paths<Package>[] = [
 *   "name", // OK
 *   "man", // OK
 *   "bin.my-program", // OK
 *   "funding", // OK
 *   "funding.type", // OK
 *   "peerDependenciesMeta.soy-milk", // OK
 *   "peerDependenciesMeta.soy-milk.optional", // OK
 *   "invalid", // ERROR
 *   "bin.other", // ERROR
 * ];
 */
export type Paths<T> = T extends Array<infer U>
  ? `${Paths<U>}`
  : T extends object
  ? {
      [K in keyof T & (string | number)]: K extends string
        ? `${K}` | `${K}.${Paths<T[K]>}`
        : never;
    }[keyof T & (string | number)]
  : never;