Creating all possible combinations of array

var letterCombinations = function (digits) {
    let alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');
    let digitValues = [];
    let calculationArr = [];
    let resultSet = [];
    for (let i = 2; i < 10; i++) {
        if (i === 7 || i === 9) {
            digitValues[i] = alphabet.splice(0, 4)
        } else {
            digitValues[i] = alphabet.splice(0, 3)
        }
    }
    if (digits !== "" && digits.length) {
        for (const digit of digits) {
            calculationArr.push(digitValues[digit]);
        }
       resultSet = cartesian(...calculationArr);
    }
    return resultSet;
};



function cartesian(...args) {
    var r = [], max = args.length-1;

    function helper(arr, i) {
        for (var j=0, l=args[i].length; j<l; j++) {
            var a = arr.slice(0); // clone arr
            a.push(args[i][j]);
            if (i==max)
                r.push(a.join(''));
            else
                helper(a, i+1);
        }
    }
    helper([], 0);
    return r;
}


const digits = '792';

const result = letterCombinations(digits);

console.log('result', result);