jstree change data attribute (li_attr)

Question

I try to change a data-attribute on an existing node in jstree, but no answer i find worked for me. I found this, didnt work: How to update data attribute in JSTREE?

.jstree(true).get_node(item_id).li_attr.data-opened-before = true

I get the error: "Uncaught SyntaxError: Invalid left-hand side in assignment" Can anybody help?