yashka,
а радиокнопки никак?
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form name="sel">
<input type='checkbox' value='1'>
<input type='checkbox' value='2'>
<input type='checkbox' value='3'>
<input type='checkbox' value='4'>
<input type='checkbox' value='5'>
<input type='checkbox' value='6'>
<input type='checkbox' value='7'>
<textarea id="text" name=""></textarea></form>
<script>
var s = document.forms.sel;
s.onchange = function(event) {
var l = s.querySelectorAll('[type="checkbox"]:checked'),elem = event.target, i = 0;
if (elem.type == 'checkbox'){
[].forEach.call(l, function(el) {
el.checked = el == elem;
if (el.checked) i = el.value;
});
document.getElementById('text').value = i;
}
}
</script>
</body>
</html>