In Layout > Topic Page Buttons, RH provides a couple of default buttons and their respective actions to be performed when the user clicks the button in the output
I would like to know where are these functions defined in RH2019. For example, if I select the Expand/Collapse button, in the OnClick field, RH shows rh.model.publish(rh.consts('EVT_EXPAND_COLLAPSE_ALL'), 'Expand');
I added a Share button but I am not able to find the function for it so that I can add it in the OnClick field.
Does RH have the command for it already available as we have it for the other buttons such as Expand/Collapse, Favourites, etc?
I think the only "list" of pre-defined button functions is the list of buttons included in the skins by default. I tried deleting them, but then there was no way to add back in the javascriipt function - I ended up creating a new skin and copying the functions back across to my test skin.
In Frameless Skin > Topic Page > Buttons, RH provides 8 predefiend buttons and for each one of them the icon and icon applied fields are already populated.
By User Assets are you refering to Frameless Skin > User Assets?
Hmm, sorry, I only got as far as the Topic Page Buttons section under Layout and given there's no help on that, I assume there's no help for any other features either. The Azure Blue responsive skin has a Social Media section, which seems like the most obvious target for a Share button, but I couldn't find anything similar for Frameless, although I only had a quick poke around. The Robohelp Reimagined project mentions being able to edit *.ejs files, so perhaps you could find those and see if anything interesting shows up.
This feature is now available in RH2020. Open the About Robohelp project and you will see a Share button at the right top corner.