isisogram.js

function isIsogram(s) {
  var map, str, c, i, l, e, h;
  if (typeof s !== "string") return false;
  if ((l = s.length) < 2) return true;
  c = (str = s.toLowerCase()).codePointAt();
  if ((h = c > 0xFFFF) && l < 4) return true;
  (map = {})[c] = true, e = l - 2, i = +h;
  do {
    if (map[c = str.codePointAt(++i)]) return false;
    if (c > 0xFFFF) if (i === e) return true; else ++i;
    map[c] = true;
  } while (i < l);
  return true;
}