AShadowRoot object instance, ornull if the associatedshadow root was attached with itsmode set toclosed. (SeeElement.attachShadow() for further details).

Some built-in elements, such as<input> and<img>, have user-agent shadow roots that are closed to script. Therefore, theirshadowRoot property is alwaysnull.

The following snippets are taken from ourlife-cycle-callbacksexample (see it live also), which creates an element that displays a square of a size and colorspecified in the element's attributes.

Inside the<custom-square> element's class definition we includesome 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 itthis (the custom element itself) as aparameter.

class Square extends HTMLElement {  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 theupdateStyle() function itself, we get a reference to the shadow DOMusingElement.shadowRoot. From here we use standard DOM traversaltechniques to find the<style> element inside the shadow DOM and thenupdate the CSS found inside it:

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