The ShadowRoot interface of the Shadow DOM API is the root node of a DOM subtree that is rendered separately from a document's main DOM tree.

You can retrieve a reference to an element's shadow root using its Element.shadowRoot property, provided it was created using Element.attachShadow() with the mode option set to open.

ShadowRoot.activeElement Read only

Returns the Element within the shadow tree that has focus.

ShadowRoot.adoptedStyleSheets

Add an array of constructed stylesheets to be used by the shadow DOM subtree. These may be shared with other DOM subtrees that share the same parent Document node, and the document itself.

ShadowRoot.delegatesFocus Read only

Returns a boolean that indicates whether delegatesFocus was set when the shadow was attached (see Element.attachShadow()).

ShadowRoot.fullscreenElement Read only

The element that's currently in full screen mode for this shadow tree.

ShadowRoot.host Read only

Returns a reference to the DOM element the ShadowRoot is attached to.

ShadowRoot.innerHTML

Sets or returns a reference to the DOM tree inside the ShadowRoot.

ShadowRoot.mode Read only

The mode of the ShadowRoot, either open or closed. This defines whether or not the shadow root's internal features are accessible from JavaScript.

ShadowRoot.pictureInPictureElement Read only

Returns the Element within the shadow tree that is currently being presented in picture-in-picture mode.

ShadowRoot.pointerLockElement Read only

Returns the Element set as the target for mouse events while the pointer is locked. null if lock is pending, pointer is unlocked, or if the target is in another tree.

ShadowRoot.slotAssignment Read only

Returns a string containing the type of slot assignment, either manual or named.

ShadowRoot.styleSheets Read only

Returns a StyleSheetList of CSSStyleSheet objects for stylesheets explicitly linked into, or embedded in a shadow tree.

ShadowRoot.getAnimations()

Returns an array of all Animation objects currently in effect, whose target elements are descendants of the shadow tree.

ShadowRoot.getSelection() Non-standard

Returns a Selection object representing the range of text selected by the user, or the current position of the caret.

ShadowRoot.elementFromPoint() Non-standard

Returns the topmost element at the specified coordinates.

ShadowRoot.elementsFromPoint() Non-standard

Returns an array of all elements at the specified coordinates.

The following events are available to ShadowRoot via event bubbling from HTMLSlotElement:

HTMLSlotElement slotchange event

An event fired when the node(s) contained in that slot change.

The following snippets are taken from our life-cycle-callbacks example (see it live also), which creates an element that displays a square of a size and color specified in the element's attributes.

Inside the <custom-square> element's class definition we include some life cycle callbacks that make a call to an external function, updateStyle(), which actually applies the size and color to the element. You'll see that we are passing it this (the custom element itself) as a parameter.

connectedCallback() {
  console.log('Custom square element added to page.');
  updateStyle(this);
}

attributeChangedCallback(name, oldValue, newValue) {
  console.log('Custom square element attributes changed.');
  updateStyle(this);
}

In the updateStyle() function itself, we get a reference to the shadow DOM using Element.shadowRoot. From here we use standard DOM traversal techniques to find the <style> element inside the shadow DOM and then update the CSS found inside it:

function updateStyle(elem) {
  const shadow = elem.shadowRoot;
  const childNodes = shadow.childNodes;
  for (const node of childNodes) {
    if (node.nodeName === "STYLE") {
      node.textContent = `
div {
  width: ${elem.getAttribute("l")}px;
  height: ${elem.getAttribute("l")}px;
  background-color: ${elem.getAttribute("c")};
}
      `;
    }
  }
}