String reverse.

reverse.js

arr = Array.from("Hello world!");
arrLen = arr.length;
arrMidLen = Math.floor(arrLen / 2);
arr.forEach(function(v, i) {
  if (arrMidLen > i) { return }
  var f = arr[i];
  var e = arr[arrLen - i - 1];
  arr[i] = e;
  arr[arrLen - i - 1] = f;
});
console.log(arr.join('')); // !dlrow olleH