Element: attachShadow() method

The Element.attachShadow() method attaches a shadow DOM tree to the specified element and returns a reference to its ShadowRoot.

Elements you can attach a shadow to

Note that you can't attach a shadow root to every type of element. There are some that can't have a shadow DOM for security reasons (for example <a>).

The following is a list of elements you can attach a shadow root to:

Syntax

attachShadow(options)

Parameters

options

An object which contains the following fields:

mode

A string specifying the encapsulation mode for the shadow DOM tree. This can be one of:

open

Elements of the shadow root are accessible from JavaScript outside the root, for example using Element.shadowRoot: 

element.attachShadow({ mode: "open" });
element.shadowRoot; // Returns a ShadowRoot obj
closed

Denies access to the node(s) of a closed shadow root from JavaScript outside it: 

element.attachShadow({ mode: "closed" });
element.shadowRoot; // Returns null
delegatesFocus Optional

A boolean that, when set to true, specifies behavior that mitigates custom element issues around focusability. When a non-focusable part of the shadow DOM is clicked, the first focusable part is given focus, and the shadow host is given any available :focus styling. Its default value is false.

slotAssignment Optional

A string specifying the slot assignment mode for the shadow DOM tree. This can be one of:

named

Elements are automatically assigned to <slot> elements within this shadow root. Any descendants of the host with a slot attribute which matches the name attribute of a <slot> within this shadow root will be assigned to that slot. Any top-level children of the host with no slot attribute will be assigned to a <slot> with no name attribute (the "default slot") if one is present.

manual

Elements are not automatically assigned to <slot> elements. Instead, they must be manually assigned with HTMLSlotElement.assign(). Its default value is named.

Return value

Returns a ShadowRoot object.

Exceptions

InvalidStateError DOMException

The element you are trying to attach to is already a shadow host.

NotSupportedError DOMException

You are trying to attach a shadow root to an element outside the HTML namespace, the element cannot have a shadow attached to it, or the static property disabledFeatures has been given a value of "shadow" in the element definition.

Examples

The following example is taken from our word-count-web-component demo (see it live also). You can see that we use attachShadow() in the middle of the code to create a shadow root, which we then attach our custom element's contents to. 

// Create a class for the element
class WordCount extends HTMLParagraphElement {
  constructor() {
    // Always call super first in constructor
    super();

    // count words in element's parent element
    const wcParent = this.parentNode;

    function countWords(node) {
      const text = node.innerText || node.textContent;
      return text
        .trim()
        .split(/\s+/g)
        .filter((a) => a.trim().length > 0).length;
    }

    const count = `Words: ${countWords(wcParent)}`;

    // Create a shadow root
    const shadow = this.attachShadow({ mode: "open" });

    // Create text node and add word count to it
    const text = document.createElement("span");
    text.textContent = count;

    // Append it to the shadow root
    shadow.appendChild(text);

    // Update count when element content changes
    setInterval(() => {
      const count = `Words: ${countWords(wcParent)}`;
      text.textContent = count;
    }, 200);
  }
}

// Define the new element
customElements.define("word-count", WordCount, { extends: "p" });

Specifications

Specification
DOM Standard
# ref-for-dom-element-attachshadow①

Browser compatibility

BCD tables only load in the browser

See also