record.raw.[attributeFieldName] can be used for accessing data client-side.
For a click handler defined as below:
.Listeners(l =>
{
l.ItemClick.Handler = "myClickHandler(item,record,node,index,e)";
}
Set attribute in server-side:
section01.AttributesObject = new { nodeType = "Section" };
Access custom set attribute in client-side:
function myClickHandler(item, record, node, index, e) {
var nodeType = record.raw.nodeType;
console.log(nodeType); //prints "Section"
}