CustomElementRegistry: define() method

The define() method of the CustomElementRegistry interface defines a new custom element.

There are two types of custom elements you can create:

  • Autonomous custom element: Standalone elements; they don't inherit from built-in HTML elements.
  • Customized built-in element: These elements inherit from — and extend — built-in HTML elements.


define(name, constructor)
define(name, constructor, options)



Name for the new custom element. Note that custom element names must contain a hyphen.


Constructor for the new custom element.

options Optional

Object that controls how the element is defined. One option is currently supported:


String specifying the name of a built-in element to extend. Used to create a customized built-in element.

Return value

None (undefined).


NotSupportedError DOMException

Thrown if the CustomElementRegistry already contains an entry with the same name or the same constructor (or is otherwise already defined), or extends is specified and it is a valid custom element name, or extends is specified but the element it is trying to extend is an unknown element.

SyntaxError DOMException

Thrown if the provided name is not a valid custom element name.


Thrown if the referenced constructor is not a constructor.

Note: You'll often get NotSupportedErrors thrown that seem like define() is failing, but instead it is likely a problem with Element.attachShadow().


Autonomous custom element

The following code is taken from our popup-info-box-web-component example (see it live also).

// Create a class for the element
class PopUpInfo extends HTMLElement {
  constructor() {
    // Always call super first in constructor

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

    // Create spans
    const wrapper = document.createElement("span");
    wrapper.setAttribute("class", "wrapper");

    const icon = document.createElement("span");
    icon.setAttribute("class", "icon");
    icon.setAttribute("tabindex", 0);

    const info = document.createElement("span");
    info.setAttribute("class", "info");

    // Take attribute content and put it inside the info span
    const text = this.getAttribute("data-text");
    info.textContent = text;

    // Insert icon
    const img = document.createElement("img");
    img.src = this.hasAttribute("img")
      ? this.getAttribute("img")
      : "img/default.png";

    // Create some CSS to apply to the shadow dom
    const style = document.createElement("style");

    style.textContent = `
      .wrapper {
        position: relative;
      .info {
        font-size: 0.8rem;
        width: 200px;
        display: inline-block;
        border: 1px solid black;
        padding: 10px;
        background: white;
        border-radius: 10px;
        opacity: 0;
        transition: 0.6s all;
        position: absolute;
        bottom: 20px;
        left: 10px;
        z-index: 3;
      img {
        width: 1.2rem;
      .icon:hover + .info, .icon:focus + .info {
        opacity: 1;

    // Attach the created elements to the shadow dom

// Define the new element
customElements.define("popup-info", PopUpInfo);
  data-text="Your card validation code (CVC) is an extra security feature — it is the last 3 or 4 numbers on the back of your card."></popup-info>

Note: Constructors for autonomous custom elements must extend HTMLElement.

Customized built-in element

The following code is taken from our word-count-web-component example (see it live also).

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

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

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

    // 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" });
<p is="word-count"></p>

Creating an element which disables the ability to attach a shadow root

If the class used for the element contains the static property disabledFeatures returning the string `shadow` this will cause Element.attachShadow() to return a NotSupportedError DOMException.

class PopUpInfo extends HTMLElement {
  static get disabledFeatures() {
    return ["shadow"];

  constructor() {

    const shadow = this.attachShadow({ mode: "open" });
    // this will cause an error to be thrown when the element is defined.


HTML Standard
# dom-customelementregistry-define-dev

Browser compatibility

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