The read-only slotAssignment property of the ShadowRoot interface returns the slot assignment mode for the shadow DOM tree. Nodes are either automatically assigned (named) or manually assigned (manual). The value of this property defined using the slotAssignment option when calling Element.attachShadow().

A string that can be one of:

named

Elements are automatically assigned to <slot> elements within this shadow root. Any descendants of the host with a slot attribute which matches the name attribute of a <slot> within this shadow root will be assigned to that slot. Any top-level children of the host with no slot attribute will be assigned to a <slot> with no name attribute (the "default slot") if one is present.

manual

Elements are not automatically assigned to <slot> elements. Instead, they must be manually assigned with HTMLSlotElement.assign().

In the example below, 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. We test if the slotAssignment is named to prevent an exception to be raised when HTMLSlotElement.assign() is called.

function UpdateDisplayTab(elem, tabIdx) {
  const shadow = elem.shadowRoot;

  // This test is usually not needed, but can be useful when debugging
  if (shadow.slotAssignment === "named") {
    console.error(
      "Trying to manually assign a slot on an automatically-assigned (named) slot",
    );
  }
  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();
  }
}

• Element.attachShadow()
• HTMLSlotElement.assign()