Immutable.JS TypeScript 2.8 edition

immutable.ts

//Some example interface
interface IGeographicCoordinate {
    lat: number;
    lng: number;
}

interface IPlacemark {
    id: number;
    coordinate: IGeographicCoordinate;
    name: string;
}

interface ISearchResult {
    query: string;
    results: IPlacemark[];
}

// ============= The magic sauce ============== //
interface ImmutableObject<T> {
    get<P extends keyof T>(key: P): T[P] extends Array<infer U> ? ImmutableList<U> : T[P] extends object ? ImmutableObject<T[P]> : T[P]; 
}

interface ImmutableList<T> {
    count(): number;
    get(index: number): T extends object ? ImmutableObject<T> : T;
}

// The fromJS() API from immutable.js
declare function fromJS<T>(obj: T): ImmutableObject<T>;
// ============= End magic sauce =============== //

function test() {
    const pm1: IPlacemark = {
        id: 1,
        coordinate: {
            lat: 0,
            lng: 0
        },
        name: "Null Island"
    };
    const pm2: IPlacemark = {
        id: 2,
        coordinate: {
            lat: -37.815080,
            lng: 144.963886
        },
        name: "Melbourne"
    };
    const results: ISearchResult = {
        query: "Some example placemarks",
        results: [
            pm1,
            pm2
        ]
    }
    const imp1 = fromJS(pm1); //ImmutableObject<IPlacemark>
    const imp2 = fromJS(pm2); //ImmutableObject<IPlacemark>
    const iResults = fromJS(results); //ImmutableObject<ISearchResults>

    const a = imp1.get("id"); //number
    const b = imp1.get("coordinate"); //ImmutableObject<IGeographicCoordinate>
    const blat = b.get("lat"); //number
    const blng = b.get("lng"); //number
    const c = imp1.get("name"); //string
    const d = iResults.get("query"); //string
    const e = iResults.get("results"); //ImmutableList<IPlacemark>
    const f = e.get(0); //ImmutableObject<IPlacemark>
    const g = e.get(1); //ImmutableObject<IPlacemark>
    const h = f.get("coordinate"); //ImmutableObject<IGeographicCordinate>
    const i = g.get("coordinate"); //ImmutableObject<IGeographicCordinate>
    const hlat = h.get("lat"); //number
    const hlng = h.get("lng"); //number
    const ilat = i.get("lat"); //number
    const ilng = i.get("lng"); //number
}