The CustomElementRegistry interface provides methods for registering custom elements and querying registered elements. To get an instance of it, use the window.customElements (en-US) property.


Defines a new custom element.
CustomElementRegistry.get() (en-US)
Returns the constuctor for the named custom element, or undefined if the custom element is not defined.
CustomElementRegistry.upgrade() (en-US)
Upgrades a custom element directly, even before it is connected to its shadow root.
CustomElementRegistry.whenDefined() (en-US)
Returns an empty 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.


The following code is taken from our word-count-web-component example (see it live also). Note how we use the CustomElementRegistry.define() method to define the custom element after creating its class.

// 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' });

Note: The CustomElementRegistry is available through the Window.customElements (en-US) property.


Specification Status Comment
HTML Living Standard
La definición de 'CustomElementRegistry' en esta especificación.
Living Standard Initial definition.

Browser compatibility

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