Element: shadowRoot property

A ShadowRoot object instance, or null if the associated shadow root was attached with its mode set to closed. (See Element.attachShadow() for further details).

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

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.

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 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 = 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")}; } `; } }); }