Min fyrste rekursive type i Typescript

/**
 * An example API response where all fields are expressed
 * as strings. 
 */
const rawFishData = {
    id: "321",
    name: "Selmer Salmon",
    type: "wild salmon",
    details: {
        weight: "5.3",
        amountOfBlubb: "9001"
    },
    tracking: [{
        lat: "60.408773",
        lon: "5.287155",
        dateTime: '2020-03-23T19:27:20.942Z'
    }]
}

/**
 * In our program we want to parse the raw data into
 * this more manageable data structure. Instead of
 * wondering if we had remembered to convert the
 * dateTime properties to proper dates we should
 * enforce that the type has a proper date type.
 */
interface FishData {
    id: number
    name: string
    type: string
    details: {
        weight: number
        amountOfBlubb: number
    }
    tracking: {
        lat: number
        lon: number
        dateTime: Date
    }[]
}

/**
 * This conditional type will convert any type into a
 * type containing only strings.
 */
type Stringify<T> = {
    [K in keyof T]:
    // if it's an array, infer the contained type and recurse.
    T[K] extends (infer U)[] ? Stringify<U>[]
    // any dates will be described as strings from the API.
    : T[K] extends Date
    ? string
    // we'll need to traverse into any objects and process them 
    : T[K] extends object
    ? Stringify<T[K]>
    // anything else will be converted to a string
    : string
}

/**
 * With the Stringify<T> type we can now use our target type that the want
 * and express the rawApiResponse which is a FishData object containing only
 * strings. By enforcing these strict input and output types TS will help us
 * ensure that we in fact convert all necessary fields.
 */
function parseApiResponse(rawApiResponse: Stringify<FishData>): FishData {
    const { id, details: { weight, amountOfBlubb }, tracking } = rawApiResponse
    return {
        ...rawApiResponse,
        id: Number.parseInt(id),
        details: {
            weight: Number.parseFloat(weight),
            amountOfBlubb: Number.parseInt(amountOfBlubb)
        },
        tracking: tracking.map(({ lat, lon, dateTime }) => ({
            lat: Number.parseFloat(lat),
            lon: Number.parseFloat(lon),
            dateTime: new Date(dateTime)
        }))
    }
}

const parsedFishData = parseApiResponse(rawFishData)
// Result
// {
//   "id": 321,
//   "name": "Selmer Salmon",
//   "type": "wild salmon",
//   "details": {
//     "weight": 5.3,
//     "amountOfBlubb": 9001
//   },
//   "tracking": [
//     {
//       "lat": 60.408773,
//       "lon": 5.287155,
//       "dateTime": "2020-03-23T19:27:20.942Z"
//     }
//   ]
// }