LeetCode problem https://leetcode.com/problems/letter-combinations-of-a-phone-number/

phonenumber.js

const letters = c => ({
  0: " ",
  1: "",
  2: "abc",
  3: "def",
  4: "ghi",
  5: "jkl",
  6: "mno",
  7: "pqrs",
  8: "tuv",
  9: "wxyz"
}[c]);

var letterCombinations = function (digits) {
  if (digits.length === 0) return [];
  return traverse(letters, digits).map(s => s.join(""));
};