OnChange event handler for radio button (INPUT type="radio") doesn't work as one value

Question

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/

I would make two changes:

<input type="radio" name="myRadios" onclick="handleClick(this);" value="1" />
<input type="radio" name="myRadios" onclick="handleClick(this);" value="2" />

1. Use the onclick handler instead of onchange - you're changing the "checked state" of the radio input, not the value, so there's not a change event happening.
2. Use a single function, and pass this as a parameter, that will make it easy to check which value is currently selected.

ETA: Along with your handleClick() function, you can track the original / old value of the radio in a page-scoped variable. That is:

var currentValue = 0;
function handleClick(myRadio) {
    alert('Old value: ' + currentValue);
    alert('New value: ' + myRadio.value);
    currentValue = myRadio.value;
}

var currentValue = 0;
function handleClick(myRadio) {
    alert('Old value: ' + currentValue);
    alert('New value: ' + myRadio.value);
    currentValue = myRadio.value;
}

<input type="radio" name="myRadios" onclick="handleClick(this);" value="1" />
<input type="radio" name="myRadios" onclick="handleClick(this);" value="2" />

As you can see from this example: http://jsfiddle.net/UTwGS/

HTML:

<label><input type="radio" value="1" name="my-radio">Radio One</label>
<label><input type="radio" value="2" name="my-radio">Radio One</label>

jQuery:

$('input[type="radio"]').on('click change', function(e) {
    console.log(e.type);
});

both the click and change events are fired when selecting a radio button option (at least in some browsers).

I should also point out that in my example the click event is still fired when you use tab and the keyboard to select an option.

So, my point is that even though the change event is fired is some browsers, the click event should supply the coverage you need.

What about using the change event of Jquery?

$(function() {
    $('input:radio[name="myRadios"]').change(function() {
        if ($(this).val() == '1') {
            alert("You selected the first option and deselected the second one");
        } else {
            alert("You selected the second option and deselected the first one");
        }
    });
});

jsfiddle: http://jsfiddle.net/f8233x20/

You can add the following JS script

<script>
    function myfunction(event) {
        alert('Checked radio with ID = ' + event.target.id);
    }
    document.querySelectorAll("input[name='myRadios']").forEach((input) => {
        input.addEventListener('change', myfunction);
    });
</script>

I don't think there is any way other then storing the previous state. Here is the solution with jQuery

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script> 
<script type="text/javascript">
    var lastSelected;
    $(function () {
        //if you have any radio selected by default
        lastSelected = $('[name="myRadios"]:checked').val();
    });
    $(document).on('click', '[name="myRadios"]', function () {
        if (lastSelected != $(this).val() && typeof lastSelected != "undefined") {
            alert("radio box with value " + $('[name="myRadios"][value="' + lastSelected + '"]').val() + " was deselected");
        }
        lastSelected = $(this).val();
    });
</script>

<input type="radio" name="myRadios" value="1" />
<input type="radio" name="myRadios" value="2" />
<input type="radio" name="myRadios" value="3" />
<input type="radio" name="myRadios" value="4" />
<input type="radio" name="myRadios" value="5" />

After thinking about it a bit more, I decided to get rid of the variable and add/remove class. Here is what I got: http://jsfiddle.net/BeQh3/2/