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'))