var rad = document.myForm.myRadios;
var prev = null;
for (var i = 0; i < rad.length; i++) {
rad[i].addEventListener('change', function() {
(prev) ? console.log(prev.value): null;
if (this !== prev) {
prev = this;
}
console.log(this.value)
});
}
<form name="myForm">
<input type="radio" name="myRadios" value="1" />
<input type="radio" name="myRadios" value="2" />
</form>
Here’s a JSFiddle demo: https://jsfiddle.net/crp6em1z/