code: function isMixedCase(str) {
    let lower = false;
    let upper = false;
  
    for (let i = 0; i < str.length; i++) {
      const ch = str.charCodeAt(i);
  
      if (ch >= 0x30 && ch <= 0x39)
        continue;
  
      if (ch & 32) {
        assert(ch >= 0x61 && ch <= 0x7a);
        lower = true;
      } else {
        assert(ch >= 0x41 && ch <= 0x5a);
        upper = true;
      }
  
      if (lower && upper)
        return true;
    }
  
    return false;
  }
docs: /*
  Helpers