The onslotchange property of the GlobalEventHandlers mixin is an event handler that processes slotchange events.

The slotchange event is fired on HTMLSlotElement instances (<slot> elements) when the node(s) contained in the slot change.

The following snippet is a slightly modified version of our slotchange example which uses onslotchange rather than adding a listener for the slotchange event.

First the code gets an array of all the <slot>s and then assigns a handler function to the onslotchange property on the template's second slot — this second slot is the one that has its contents changed in the example. Every time the element 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.

let slots = this.shadowRoot.querySelectorAll('slot');
slots[1].onslotchange = function(e) {
  let nodes = slots[1].assignedNodes();
  console.log('Element in Slot "' + slots[1].name + '" changed to "' + nodes[0].outerHTML + '".');
};

We might alternatively handle the event at a higher level. The snippet below shows the equivalent code if the handler is assigned to onslotchange on the shadowRoot. Every time the element in any slot changes, we log a report to the console saying which slot has changed, and what the new node inside the slot is.

this.shadowRoot.onslotchange = function(e) {
  const nodes = e.originalTarget.assignedNodes();
  console.log(`Element in Slot "${e.originalTarget.name}" changed to "${nodes[0].outerHTML}".`);
};

• Using templates and slots
• slotchange event
• HTMLSlotElement