reverstString(str)

reverse-string.js

function reverseString(str) {
  return !str ? '' : reverseString(str.substr(1)) + str.charAt(0);
}
reverseString("hello");

function reverse(str){
  for (var i = str.length - 1, s=""; i >= 0; i--) s += str[i];
  return s;
};
reverse("your string comes here")

// split to chunks, 4 characters, and reverse
function chunkReverse(str) {
  return (str.match(/.{1,4}/g) || []).reduce( (acc, el) => acc.push(reverse(el)) && acc, []).join('')
}
chunkReverse('1234567890') // 4321876509
chunkReverse('4321876509')