Why The Output is 4 Explain? [closed]

<html>
    <script>
        let user1=[342,4344,43];
        let user2=("424","100");
        let join=user1.concat(user2);
        document.writeln(join.length);
        </script>
 </html>

I M Trying to get length expected 5 but not get as i expect can you explain in deep why not happening and reason of coming 4