---
title: CustomElementRegistry
slug: Web/API/CustomElementRegistry
tags:
  - API
  - CustomElementRegistry
  - Experimental
  - Interface
  - Landing
  - NeedsTranslation
  - Reference
  - TopicStub
  - Web Components
  - custom elements
translation_of: Web/API/CustomElementRegistry
---
<p>{{DefaultAPISidebar("Web Components")}}</p>

<p><span class="seoSummary">The <strong><code>CustomElementRegistry</code></strong> interface provides methods for registering custom elements and querying registered elements. To get an instance of it, use the {{domxref("window.customElements")}} property. </span></p>

<h2 id="Methods">Methods</h2>

<dl>
 <dt>{{domxref("CustomElementRegistry.define()")}}</dt>
 <dd>Defines a new <a href="/en-US/docs/Web/Web_Components/Custom_Elements">custom element</a>.</dd>
 <dt>{{domxref("CustomElementRegistry.get()")}}</dt>
 <dd>Returns the constuctor for the named custom element, or <code>undefined</code> if the custom element is not defined.</dd>
 <dt>{{domxref("CustomElementRegistry.upgrade()")}}</dt>
 <dd>Upgrades a custom element directly, even before it is connected to its shadow root.</dd>
 <dt>{{domxref("CustomElementRegistry.whenDefined()")}}</dt>
 <dd>Returns an empty {{jsxref("Promise", "promise")}} that resolves when a custom element becomes defined with the given name. If such a custom element is already defined, the returned promise is immediately fulfilled.</dd>
</dl>

<h2 id="Examples">Examples</h2>

<p>The following code is taken from our <a href="https://github.com/mdn/web-components-examples/tree/master/word-count-web-component">word-count-web-component</a> example (<a href="https://mdn.github.io/web-components-examples/word-count-web-component/">see it live also</a>). Note how we use the {{domxref("CustomElementRegistry.define()")}} method to define the custom element after creating its class.</p>

<pre class="brush: js">// 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.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' });</pre>

<div class="note">
<p>Note: The CustomElementRegistry is available through the {{domxref("Window.customElements")}} property.</p>
</div>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName("HTML WHATWG", "custom-elements.html#customelementregistry", "CustomElementRegistry")}}</td>
   <td>{{Spec2("HTML WHATWG")}}</td>
   <td>Initial definition.</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p> </p>



<p>{{Compat("api.CustomElementRegistry")}}</p>

<p> </p>