phonenumber.js

var letterCombinations = function(digits) {
  if (digits.length === 0) return [];

  const result = [''];
  const letters = ' ||abc|def|ghi|jkl|mno|pqrs|tuv|wxyz'.split('|');
  for (const d of digits) {
    for (const r of result.splice(0)) {
      for (const l of letters[+d]) {
        result.push(r + l);
      }
    }
  }
  return result;
};