A Concise JavaScript Solution
\nvar digitSum = function(s, k) {\n    while (s.length > k) {\n        let newString = "";\n        for (let i = 0; i < s.length; i += k)\n            newString
```\nvar digitSum = function(s, k) {\n    while (s.length > k) {\n        let newString = "";\n        for (let i = 0; i < s.length; i += k)\n            newString += s.substring(i, i + k).split("").reduce((acc, val) => acc + (+val), 0);\n        \n        s = newString;\n    }\n    \n    return s;\n};\n```
['JavaScript']