JS 2 line solution!
\n/**\n * @param {string} s\n * @param {number} k\n * @return {string}\n */\nvar reverseStr = function(s, k) {\n    for(let i = 0; i<s.length; i=i+2*k) \n    s 
```\n/**\n * @param {string} s\n * @param {number} k\n * @return {string}\n */\nvar reverseStr = function(s, k) {\n    for(let i = 0; i<s.length; i=i+2*k) \n    s = s.slice(0,i) + s.substring(i,i+k).split(\'\').reverse().join(\'\') + s.slice(i+k); \n    return s;\n};\n```
['JavaScript']