Letter Combinations of a Phone Number

/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function(digits) {
  if  (!digits) {
    return [];
  }
const result = [];
const map = {2: 'abc',
3: 'def',
4:'ghi',
5:'jkl',
6:'mno',
7:'pqrs',
8: 'tuv',
9:'wxyz',};
  computeCombinations(map, digits, 0, result, '');
  return result;
};

const computeCombinations = (map, digits, index, result, str) => {
  if (index === digits.length) {
    result.push(str);
  } else {
    for (const letter of map[digits[index]].split('')) {
       computeCombinations(map, digits, index + 1, result, str + letter);
    }
  }
};