var reverseStringRecursive = function(s) { return reverse(s, 0); }; var reverse = (s, left) => { let right = s.length - 1 - left; if(right <= left) return console.log(s); let temp = s[left]; s[left] = s[right]; s[right] = temp; reverse(s, left + 1); } reverseStringRecursive([1,2,3,4,5,6,7,8,9]);