var btnBuy = document.getElementById("btnBuy");
var btnPreOrder = document.getElementById("btnPreOrder");
var btnBuyEbook = document.getElementById("btnBuyEbook");

if (btnBuy !== null)
{
    btnBuy.onmouseover = handleButton;
    btnBuy.onmouseout = handleButton;
    btnBuy.onmousedown = handleButton;
    btnBuy.onmouseup = handleButton;
}
if (btnPreOrder !== null)
{
    btnPreOrder.onmouseover = handleButton;
    btnPreOrder.onmouseout = handleButton;
    btnPreOrder.onmousedown = handleButton;
    btnPreOrder.onmouseup = handleButton;
}

if (btnBuyEbook !== null)
{
    btnBuyEbook.onmouseover = handleButton;
    btnBuyEbook.onmouseout = handleButton;
    btnBuyEbook.onmousedown = handleButton;
    btnBuyEbook.onmouseup = handleButton;
}