groupPermutations.js

const cities = ['Tokyo', 'London', 'Rome', 'Donlon', 'Kyoto', 'Paris'];

const result = groupPermutations(cities);

console.log(result);

function groupPermutations(words) {
  const map = words.reduce((acc, next) => {
    sortedName = next.toLowerCase().split('').sort().join('');
    var list = acc[sortedName];
    if (list == null) {
      list = [];
      acc[sortedName] = list;
    }
    list.push(next);
    return acc;
  }, {});

  return Object.values(map);
}