Page scripts.
The Composer's "Scripts & events" drawer runs your own JavaScript on a page at "Page load", "Click", "Submit", "Becomes visible", "Scroll depth", "Toggle open" or "Hover", with an optional event to capture. "Click", "Submit", "Becomes visible" and "Hover" need a CSS target. It is carried by the "Script & event binding" section, and it publishes with the page.