The slotchange event is fired on an HTMLSlotElement instance (<slot> element) when the node(s) contained in that slot change.

In order to trigger a slotchange event, one has to set or remove the slot attribute.

This event is not cancelable.

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("slotchange", (event) => {});

onslotchange = (event) => {};

A generic Event.

element.setAttribute("slot", slotName);
// element.assignedSlot = $slot
element.removeAttribute("slot");
// element.assignedSlot = null

The following snippet is taken from our slotchange example (see it live also).

let slots = this.shadowRoot.querySelectorAll("slot");
slots[1].addEventListener("slotchange", (e) => {
  let nodes = slots[1].assignedNodes();
  console.log(
    `Element in Slot "${slots[1].name}" changed to "${nodes[0].outerHTML}".`,
  );
});

Here we grab references to all the <slot>s, then add a slotchange event listener to the template's second slot — which is the one which has its contents changed in the example.

Every time the element inserted in the slot changes, we log a report to the console saying which slot has changed, and what the new node inside the slot is.