You can easily get the radio button being checked by use of the pseudo class :checked, no need for looping or searching.
In your function numberGroup you are not able to control whether the form is submitted or not. You need to do this in the onsubmit handler.
This code works for me:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<script type="text/javascript">
function usePage(frm, ev) {
// get radio button being checked
const checkedBtn = document.querySelector('input[type="radio"][name="number"]:checked');
// was a button checked?
if (checkedBtn) {
// let user confirm
const result = confirm("Link " + checkedBtn.value + " selected. Click OK to Confirm");
if (result) {
// user confirmed
frm.action = checkedBtn.value;
} else {
// user denied
// prevent form from being submitted
ev.preventDefault();
}
} else {
// no radio button checked
// notify user
alert("Please select a number");
// prevent form from being submitted
ev.preventDefault();
}
}
</script>
</head>
<body>
<div>
<form name="myNumber" onsubmit="usePage(this, event);" target="_blank">
<input type="radio" name="number" value="Link1">
<input type="radio" name="number" value="Link2"><br /><br />
<input type="submit" value="Go To Page" />
</form>
</div>
</body>
</html>
BTW: You better use code tags: [code]your code here[/code] when posting code. I edited your posting accordingly.
