The newState read-only property of the ToggleEvent interface is a string representing the state the element is transitioning to.
A string. Possible values are "open" (the popover is being shown) or "closed" (the popover is being hidden).
const popover = document.getElementById("mypopover");
// …
popover.addEventListener("beforetoggle", (event) => {
if (event.newState === "open") {
console.log("Popover is being shown");
} else {
console.log("Popover is being hidden");
}
});