Conversion of the Delphi EnCrypta function to JavaScript

I have a problem converting this function, I would like someone to help me

Delphi code

function EnCripta(const InString:string; StartKey,MultKey,AddKey:Integer): string;
var I : Byte;
  begin
  Result := '';
  for I := 1 to Length(InString) do
  begin
    Result := Result + CHAR(Byte(InString[I]) xor (StartKey shr 8));
    StartKey := (Byte(Result[I]) + StartKey) * MultKey + AddKey;
  end;
end;

JavaScrypt code

var EnCripta = function (inString, startKey, multKey, addKey) {
            var I;
            var result = "";
            var vStartKey = startKey;
            for (I = 0; I < inString.length; I++) {
                var codigo = inString.charCodeAt(I) ^ (startKey >> 8);
                codigo = codigo & 0xff;
                var a = String.fromCharCode(codigo);
                result = result.toString( 'ASCII' ) + a;
                startKey = ((result.charCodeAt(I) + startKey) * multKey + addKey);
            }
            return result;
        }

Some words that work, but not others

Please, if you have someone help me