Create javascript variable with button click no jQuery

I am wondering how I would create a variable on the click of a button, so I could make a random name generator, like this:

    <input class="varValue" placeholder="Name to be generated">
    <button class="createVar></button>
    <script>
        var createVar = document.querySelector(".createVar");
        var varValue = document.querySelector(".varValue");
        var nameNumber = 0;
        var newVar = function(){
            varValue.value = //new var with ascending values or something;
            nameNumber = nameNumber + 1;
        };
        createValue.addEventListener("click", getValue(), false");
        //then some Math.random using, nameNumber, anyway I'll figure it out
    </script>

So how would I make the variables on the click of the button?

Also no jQuery please.