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.

Syntax

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

Parameters

name

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

constructor

Constructor for the new custom element.

options Optional

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

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

Exceptions

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.
TypeError: 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().

Examples

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
    super();

    // 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";
    icon.appendChild(img);

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

    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
    shadow.appendChild(style);
    console.log(style.isConnected);
    shadow.appendChild(wrapper);
    wrapper.appendChild(icon);
    wrapper.appendChild(info);
  }
}

// Define the new element
customElements.define("popup-info", PopUpInfo);

html

<popup-info
  img="img/alt.png"
  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
    super();

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

    function countWords(node) {
      const text = node.innerText || node.textContent;
      return text
        .trim()
        .split(/\s+/g)
        .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
    shadow.appendChild(text);

    // 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" });

html

<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() {
    super();

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

Specifications

Specification
HTML Standard # dom-customelementregistry-define-dev

Browser compatibility

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