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().

Please note that you cannot mix declarative and imperative slot assignment. Therefore, for this to work, the shadow tree needs to have been created with the slotAssignment: "manual" option.

assign(node1)
assign(node1, node2)
assign(node1, node2, /* … ,*/ nodeN)

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();
  }
}