reverse-string.js

function reverse(string) {
  var len = string.length,
      halfIndex = Math.floor(len / 2),
      result = string.split(''),
      tmp;
  
  for (var i = 0; i < halfIndex; i++) {
    tmp = result[len - i - 1];
    result[len - i - 1] = result[i];
    result[i] = tmp;
  }
  
  return result.join('');
}

console.log(
  reverse('test')
);