Hashmap / Set

function groupAnagrams(strs: string[]): string[][] {
  const groups = new Map<string, string[]>();
  for (const word of strs) {
    const key = [...word].sort().join(''); // canonical form = sorted letters
    const bucket = groups.get(key);
    if (bucket) bucket.push(word);
    else groups.set(key, [word]);
  }
  return [...groups.values()];
}

def group_anagrams(strs)
  strs.group_by { |word| word.chars.sort.join }.values # canonical key = sorted letters
end