getExtent() {
    return /** @type {import("../extent.js").Extent|undefined} */ (
      this.get(LayerProperty.EXTENT)
    );
  }
Return the {@link module:ol/extent~Extent extent} of the layer or `undefined` if it
will be visible regardless of extent.
@return {import("../extent.js").Extent|undefined} The layer extent.
@observable
@api