How to add + (extended to each node)

Hi,
I want to add + sign for each node no matter if the node have a son or not and only when I click on the + Icon I will ask from server the new node ?

image

The Incremental Tree sample does exactly what I think you are asking for, except that the layout in that diagram uses ForceDirectedLayout instead of TreeLayout as I assume you want to use.
https://gojs.net/latest/samples/incrementalTree.html

Basically, it depends on a flag in the data to indicate whether it needs to find out what the children are.