A collections of unique values

const set = new Set(['1', '2', '3', '4', '4']);

for (let item of set) {
  console.log(`${ item }`);
}
set.add('5')
set.size
set.has('4')
set.delete('4')
set.clear();
set.forEach(x => console.log(x))

It is also possible to use an object as dictionary, with a small helper function

let dictionary = Object.fromEntries(data.map(x => [x.id, x.country]));
console.log(dictionary["NL"])

329800cookie-checkJavascript Set object, Hashset, Dictionary (Typescript)