Add multiple select options to url paramter with Javascript | remove undefined

I want to create a url where I send select option values to the url.

I already saw that article, which helps a lot. Add multiple select options to url paramter with Javascript

The only thing with this code is, if I choose the second option and leave the first blank, I get a name=undefined in the url for the first value. But only for the first option value, the others are just not in the url if blank.

Can you please help to adjust the code, thank you. if there is no value selected, it should be not added to the url.

Another point is, if the option is preselected with html, it gets ignored and therefor an undefined.

<option value="2" selected>2</option>

thank you for your help.

Here is the code/answer from the other article:

<form action="" method="GET" id="myForm">
    <select name="channel" id="0" onChange="changeURL(0)">
            <option value="" selected disabled>Choose Channel</option>
            <option value="facebook-ads">Facebook ads</option>
            <option value="instagram-ads">Instagram ads</option>
    </select>
        <select name="brand" id="1" onChange="changeURL(1)">
            <option value="" selected disabled>Choose Brand</option>
            <option value="brand-1">brand 1</option>
            <option value="brand-2">brand 2</option>
            <option value="brand-3">brand 3</option>
        </select>
</form>
<p id="test"></p>
<script>

var filters = [,]; // create an array with empty values (only 2 in this case)

function changeURL(a) {
    var yourUrl = "https://yourdomain.com"; // your url
    filters[a] = document.getElementById(a).value; // set the value of the selected filter inside the array
    var preFilter = "?"; // blank url will need a "?" to start with your filters
    for(var i=0; i<filters.length; i++) {
        aName = document.getElementById(i).name; // get the name of the filter
        yourUrl += preFilter + aName + "=" + filters[i];
        preFilter = "&";  
    }
    document.getElementById("test").innerHTML = yourUrl; // test output of the new url
}
</script>