function makeString(n, max = 256) {
  const str = [];
  for (let i = 0; i < n; i++) {
    str.push(String.fromCharCode(Math.floor(Math.random() * max)));
  }
  return str.join('');
}
Build a random string of the given length and range of characters.
@param {number} n Length of string
@param {number=} max Maximum charcode for each character
@return {string}