It gets called on all clicks (line 1, it’s a window-level click handler), then finds the link the click target is inside (line 2), and does nothing if it wasn’t

It gets called on all clicks (line 1, it’s a window-level click handler), then finds the link the click target is inside (line 2), and does nothing if it wasn’t inside a link (line 9). (You might think that it should use .closest("a[href]") instead of .closest("a"), since a:not([href]) is not a link, but in that case, link.href === "", and so it fails the line 10 is-it-eligible-for-client-side-routing test. Note also that except for the “no href attribute” case, HTMLAnchorElement#href gives a full resolved URL, so <a href=""> will produce the document base URL.) Fundamentally, there’s no such thing as an event handler that triggers only on links—you instead have to use a global event handler that starts by checking whether it’s being triggered on a link.