use varStatus to get the index c:forEach varStatus properties
<c:forEach var="categoryName" items="${categoriesList}" varStatus="loop">
<li><a onclick="getCategoryIndex(${loop.index})" href="#">${categoryName}</a></li>
</c:forEach>
use varStatus to get the index c:forEach varStatus properties
<c:forEach var="categoryName" items="${categoriesList}" varStatus="loop">
<li><a onclick="getCategoryIndex(${loop.index})" href="#">${categoryName}</a></li>
</c:forEach>