I need to document a function that returns an object with the same structure user passed, but fields types are different.

class CoolThing {
  constructor (id, settings) { }

  happyLittleMethod () { /* do something */ }
}

/**
 * This definition says that an object returns ALMOST the same structure that passed.
 * Feels close.
 *
 * @template T
 * @param {T} ABThingsAndStuff
 * @returns {T}
 */
function makeSomeThings ({ thingsA, thingsB, ...otherStuff }) {
  const a = {}
  const b = {}

  for (let key in thingsA) {
    a[key] = new CoolThing(key, thingsA[key])
  }

  for (let key in thingsB) {
    b[key] = new CoolThing(key, thingsB[key])
  }

  return { thingsA: a, thingsB: b, ...otherStuff }
}

Then someone can use a function like this:

const myThings = makeSomeThings({
  thingsA: {
    cat: 1,
    dog: 2,
  },
  thingsB: {
    bone: 3,
    apple: 4,
  },
  worldName: 'Pretty strange place',
})

myThings autocomplete everything from the user payload, however, some internals of the objects now has a different type.

myThings.thingsA.cat.happyLittleMethod()
//                   ^
//                   how can I autocomplete this happyLittleMethod?