The shadowRoot read-only property of the ElementInternals interface returns the ShadowRoot for this element.

A ShadowRoot if the element has a shadow root, otherwise null.

The following example prints the value of shadowRoot to the console, immediately after calling HTMLElement.attachInternals(). At this point the value is null. After calling Element.attachShadow() the element has a Shadow Root, and shadowRoot returns the object representing it.

class MyCustomElement extends HTMLElement {
  constructor() {
    super();
    this.internals_ = this.attachInternals();

    console.log(this.internals_.shadowRoot); // null

    this.attachShadow({ mode: "open" });

    console.log(this.internals_.shadowRoot); // a ShadowRoot object
  }
}