There is no need to manage your own variable (either an incrementing counter or a boolean); you can see if the built-in ItemIndex
property is divisible by two, and use that to set a css class:
class="<%# Container.ItemIndex % 2 == 0 ? "" : "alternate" %>"
This has the benefit of being completely based in your UI code (ascx or aspx file), and doesn’t rely on JavaScript.