HTMLSlotElement.assign()

The assign() method of the HTMLSlotElement interface sets the slot's manually assigned nodes to an ordered set of slottables. The manually assigned nodes set is initially empty until nodes are assigned using assign().

...nodes

A set of Element or Text nodes.

Undefined.

In the below example, the assign() method is used to display the correct tab in a tabbed application. The function is called and passed the panel to show, which is then assigned to the slot.

function UpdateDisplayTab(elem, tabIdx) {
  const shadow = elem.shadowRoot;
  const slot = shadow.querySelector("slot");
  const panels = elem.querySelectorAll('tab-panel');
  if (panels.length && tabIdx && tabIdx <= panels.length ) {
    slot.assign(panels[tabIdx-1]);
  } else {
    slot.assign();
  }
}