Leveraging ixjs AsyncIterable to apply sync and async functions via map, filter...

ixjs.AsyncIterableTest.js

import { from as asyncIterableFrom } from 'ix/asynciterable/index.js';
import { filter as asyncFilter, map as asyncMap } from 'ix/asynciterable/operators/index.js';

function asyncSleep(interval){
    return new Promise(resolveFn => setTimeout(resolveFn, interval));
}

async function findCountryForCityAsync(city){
    let countries = [
        {
            name: "France",
            mainCities: ["Toulouse", "Paris", "Lyon", "Marseille", "Bordeaux"]
        },
        {
            name: "Belgium",
            mainCities: ["Bruxelles", "Antwerp", "Liege"]
        }
    ];
    console.log("finding country for " + city);
    await asyncSleep(2000);
    let country = countries.find(country => country.mainCities.findIndex(_city => _city == city) > -1);
    return country ? country.name : null;
}

let cities = ["Toulouse", "Paris", "Ghent", "Bruxelles"];

let transformedCitiesIterable = asyncIterableFrom(cities).pipe(
    asyncMap(findCountryForCityAsync),
    asyncFilter(city => city !== null),
    asyncMap(x => x.toLocaleUpperCase())
);
console.log("transformedCitiesIterable type: " + transformedCitiesIterable.constructor.name);
//we have an objects chain like this:
// MapAsyncIterable -> FilterAsyncIterable -> MapAsyncIterable -> FromAsyncIterable

//async main
(async () => {
    console.log("starting asyncIterableTest");
    for await (let city of transformedCitiesIterable){
        console.log(city);
    }
    console.log("ending asyncIterableTest");
})();