var letterCombinations = function(digits) {
  const mappings = {
    '2': ['a', 'b', 'c'],
    '3': ['d', 'e', 'f'],
    '4': ['g', 'h', 'i'],
    '5': ['j', 'k', 'l'],
    '6': ['m', 'n', 'o'],
    '7': ['p', 'q', 'r', 's'],
    '8': ['t', 'u', 'v'],
    '9': ['w', 'x', 'y', 'z']
  };
  if (!digits || digits.length === 0) return [];
  if (digits.length === 1) {
    return mappings[digits];
  }
  let result = [];
  let set1 = letterCombinations(digits.substr(0, 1));
  let set2 = letterCombinations(digits.substr(1));
  for (let i = 0; i < set1.length; i++) {
    for (let j = 0; j < set2.length; j++) {
    result.push(set1[i] + set2[j]);
    }
  }
  return result;
  };
console.log(letterCombinations('23'))
var letterCombinations1 = function(digits) {
 var map = {
 "2": ["a", "b", "c"],
 "3": ["d", "e", "f"],
 "4": ["g", "h", "i"],
 "5": ["j", "k", "l"],
 "6": ["m", "n", "o"],
 "7": ["p", "q", "r", "s"],
 "8": ["t", "u", "v"],
 "9": ["w", "x", "y", "z"]
 };
 var rtn = map[digits[0]];
 digits = digits.substr(1);
 digits.split("").forEach(function(digit) {
 var t = [];
 map[digit].forEach(function(letter) {
  t = t.concat(rtn.map(function(item) {
  return item + letter;
  }));
 });
 rtn = t;
 });
 return rtn === undefined ? [] : rtn;
};
console.log(letterCombinations1('32'))