Awesome utility type copied from `native-base` that extracts the key paths to the leaves of a nested object type into a new type

LeafKeyPaths.ts

/**
 * Joins two types with a `.`.
 *
 * _Pure inspirational typescript wizardry!_
 * @note this was copied from the type system of `native-base`.
 */
export type Join<K, P> = K extends string | number ? P extends string | number ? `${K}${'' extends P ? '' : '.'}${P}` : never : never;

/**
 * Recursively flattens the keys in the object to key-paths of the leaf nodes.
 *
 * **Example**
 * ```typescript
 * const colors = {
 *     primary: {
 *         100: 'red',
 *         200: 'pink',
 *     },
 *     secondary: {
 *         100: 'blue',
 *         200: 'lightBlue',
 *     }
 * }
 *
 * type ColorKeys = LeafKeyPaths<typeof colors>;
 *
 * // the above is equivalent to this
 * type ColorKeys = 'primary.100' | 'primary.200' | 'secondary.100' | 'secondary.200';
 * ```
 *
 * _Pure inspirational typescript wizardry!_
 * @note this was copied from the type system of `native-base`.
 */
export type LeafKeyPaths<T> = T extends object ? {
    [K in keyof T]-?: Join<K, LeafKeyPaths<T[K]>>;
}[keyof T] : '';