JavaScript using indexOf and lastIndexOf to put setattribuit

I want to use indexOf() and lastIndex() to add attributes by setAttribute after customizing the URL.

I try this code but not working:

<!DOCTYPE html>
<html>
    <head>
        <style>
          #two {
              position: absolute;
              width: 100%;
              height: 100%;
              left: 0px;
              top: 50px;
          }
        </style>
        
        <title></title>
    </head>
    <body>
        <select onchange="check()" id="selectbox" name="">
            <option hidden value="empty"></option>
            <option value="first">1</option>
            <option value="second">2</option>
        </select>
        <div id="two">
            <div id="de6854">
                <div style="width: 100%;height: 100%">
                    <iframe id="4526d" src="https://www.ask.com/wp-content/uploads/sites/3/2022/02/PowerOfDogPromo.jpeg?resize=200,100" style="width: 100%; height: 100%">
                    </iframe>
                    <iframe id="3ad34" src="https://www.ask.com/wp-content/uploads/sites/3/2022/02/PowerOfDogPromo.jpeg?resize=200,100" style="width: 100%; height: 100%">
                    </iframe>
                </div>
            </div>
        </div>

    </body>
    <script>
        function check(){
            var val = document.getElementById("selectbox").value
            var pic =  document.querySelector("#two iframe")
            var s = val.substring(0, val.indexOf('?resize='));
            var ss= val.substring(val.lastIndexOf(',10'));
            var f = s + ?resize= + '400' + ss;
            var s = s + ?resize= + '500' + ss;
            if(val==="first"){
            pic.setAttribute('src','f')
            } else if(val==="second"){
            pic.setAttribute('src','s')
            }
        }
    </script>
</html>

Any one can find solution of that problem:

I test write that code in JavaScript and running good:

var s = 'https://www.ask.com/wp-content/uploads/sites/3/2022/02/PowerOfDogPromo.jpeg?resize=200,100';
s = s.substring(0, s.indexOf('?resize='));
document.write(s);

document.write('<br>');
document.write('<br>');

var ss = 'https://www.ask.com/wp-content/uploads/sites/3/2022/02/PowerOfDogPromo.jpeg?resize=200,100';
ss = ss.substring(ss.lastIndexOf(',10'));
document.write(ss);

document.write('<br>');
document.write('<br>');

rr = s+"?resize="+"500"+ss
document.write(rr);

this code give result: https://www.ask.com/wp-content/uploads/sites/3/2022/02/PowerOfDogPromo.jpeg?resize=500,100

change 200 and add 500.