How to count checked checkbox with Javascript?

I would like to count the checkboxes that are checked and display the count in the Div.

Here is my HTML :

<form name="liste_figurine" method="post">

<input type="checkbox" id="radio-1" class="z" name="chck1[]" onclick="updateCount()" />
<input type="checkbox" id="radio-2" class="z" name="chck2[]" onclick="updateCount()" />

</form>

<div id="y"></div>

Here is my JS :

function updateCount {
    var x = $(".z:checked").size();
    document.getElementById("y").innerHTML = x;
};

Here is an exemple : http://ift.tt/2oA3PN4

Sorry, I'm not really used to JS... What is wrong with my code ?