fix genderef of jsgen (#5274)

tests/js/tderef.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''true
+'''
+"""
+
+import tables
+
+type EventStore = Table[string, seq[proc ()]]
+
+proc newEventStore(): EventStore =
+  initTable[string, seq[proc ()]]()
+
+proc register(store: var EventStore, name: string, callback: proc ()) =
+  if not store.hasKey(name):
+    store[name] = @[]
+  store[name].add(callback)
+
+var store = newEventStore()
+store.register("test", proc () = echo "true")
+store["test"][0]()