How to use the add_timeline_item client side function

Hello

I’m trying to figure out how to use the add_timeline_item function in a client script
As can be seen in the screen shot, what is the nature of the “item” parameter? Is it a string, an object, etc?

In general, how can I use the browser’s DevTools to figure out the nature of any function’s parameters?

Screenshot from 2023-12-23 13-17-13

I figured out some of the parameters, and the following works well, creating a new entry in the Timeline.

frm.timeline.add_timeline_item({content:"meep", creation:frappe.datetime.now_datetime()},false);

However, even if I save the document, the Timeline entry is not retained, but lost should I refresh the document. How can I persist the Timeline entry? It appears that

frm.timeline.insert_comment();

is no longer available in v15

Some further investigation revealed that directly after a

cur_frm.timeline.add_timeline_item({content:“Custom Content”}, false);

any of these commands actually removes the timeline item

cur_frm.timeline.refresh();
cur_frm.timeline.render_timeline_items();
cur_frm.save();

Yet I have no idea how to persist the timeline item so that it actually renders again upon a form refresh.