[Decode]

<form action="#">
	<input type="radio" value="result 1" name="a" />
	<input type="radio" value="result 2" name="a" />

	<input type="hidden" value="result 1" />
</form>

<script>
	document.forms[0].onchange = function(e) {
		if ( !e.target.closest('input[type=radio]') ) return;

		document.querySelector('input[type=hidden]').value = e.target.value;
	};
</script>