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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 any element. There are some really obvious ones 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:



var shadowroot = element.attachShadow(shadowRootInit); 


ShadowRootInit dictionary, which can contain the following field:

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

Return value

Returns a ShadowRoot object or null.


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).


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

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

    function countWords(node){
      var text = node.innerText || node.textContent
      return text.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

    // 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' });


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

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support53 No1


592 3

FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support5353 No1


592 3

4010 ?

1. Under consideration

2. See bug 1205323

3. From version 59: this feature is behind the dom.webcomponents.shadowdom.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

Etiquetas do documento e colaboradores

Última atualização por: ExE-Boss,