objectify.ts

const objectify = <T, EXT extends Object>(
  f: (v: string) => T,
  ext?: EXT,
  root?: string
): Record<string, T> & EXT =>
  new Proxy(f, {
    get: (target, prop) =>
      typeof prop === 'string'
        ? ext?.[prop] || target(prop as string)
        : () => root,
  }) as any

const x = objectify(
  (v) => ({
    a: [v, 'bla'].join('/'),
  }),
  {
    b: 'test',
  },
  'x'
)

x['asdf'].a /*?*/
x.b /*?*/
x /*?*/