Calculate Digit Sum of a String.js

// Runtime: 117 ms (Top 16.67%) | Memory: 42.4 MB (Top 76.67%)
var digitSum = function(s, k) {
    while (s.length > k) {
        let newString = "";
        for (let i = 0; i < s.length; i += k)
            newString += s.substring(i, i + k).split("").reduce((acc, val) => acc + (+val), 0);

        s = newString;
    }

    return s;
};