Element.attachShadow()

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>), and more besides. The following is a list of elements you can attach a shadow root to:

Any autonomous custom element with a valid name
<article>
<aside>
<blockquote>
<body>
<div>
<footer>
<h1>
<h2>
<h3>
<h4>
<h5>
<h6>
<header>
<main>
<nav>
<p>
<section>
<span>

Syntax

var shadowroot = element.attachShadow(shadowRootInit);

Parameters

shadowRootInit

A ShadowRootInit dictionary, which can contain 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.shadowRoot; // Returns a ShadowRoot obj
```
closed: Denies access to the node(s) of a closed shadow root from JavaScript outside it:
```
element.shadowRoot; // Returns null
```

delegatesFocus

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.

Return value

Returns a ShadowRoot object.

Exceptions

Exception	Explanation
`InvalidStateError`	The element you are trying to attach to is already a shadow host.
`NotSupportedError`	You are trying to attach a shadow root to an element outside the HTML namespace, or the element cannot have a shadow attached to it (see above).

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
    var wcParent = this.parentNode;

    function countWords(node){
      var text = node.innerText || node.textContent
      return text.trim().split(/\s+/g).length;
    }

    var count = 'Words: ' + countWords(wcParent);

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

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

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

    // Update count when element content changes
    setInterval(function() {
      var count = 'Words: ' + countWords(wcParent);
      text.textContent = count;
    }, 200)
  }
}

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

Specifications

Specification	Status	Comment
DOM The definition of 'attachShadow()' in that specification.	Living Standard

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`attachShadow`	Chrome Full support 53	Edge Full support 79	Firefox Full support 63 Full support 63 No support 59 — 63 Disabled Disabled From version 59 until version 63 (exclusive): this feature is behind the `dom.webcomponents.shadowdom.enabled` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	IE No support No	Opera Full support 40	Safari Full support 10	WebView Android Full support 53	Chrome Android Full support 53	Firefox Android Full support 63 Full support 63 No support 59 — 63 Disabled Disabled From version 59 until version 63 (exclusive): this feature is behind the `dom.webcomponents.shadowdom.enabled` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	Opera Android Full support 41	Safari iOS Full support 10	Samsung Internet Android Full support 6.0
`delegatesFocus` option of `ShadowRootInit` dictionary. ExperimentalNon-standard	Chrome Full support Yes	Edge Full support 79	Firefox No support No	IE No support No	Opera ?	Safari ?	WebView Android Full support Yes	Chrome Android Full support Yes	Firefox Android No support No	Opera Android ?	Safari iOS ?	Samsung Internet Android Full support Yes

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
Experimental. Expect behavior to change in the future.: Experimental. Expect behavior to change in the future.
Non-standard. Expect poor cross-browser support.: Non-standard. Expect poor cross-browser support.
User must explicitly enable this feature.: User must explicitly enable this feature.