CustomElementRegistry.define()

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

customElements.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.

Return value

Void.

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 DOMException

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
    var shadow = this.attachShadow({mode: 'open'});

    // Create spans
    var wrapper = document.createElement('span');
    wrapper.setAttribute('class','wrapper');
    var icon = document.createElement('span');
    icon.setAttribute('class','icon');
    icon.setAttribute('tabindex', 0);
    var info = document.createElement('span');
    info.setAttribute('class','info');

    // Take attribute content and put it inside the info span
    var text = this.getAttribute('text');
    info.textContent = text;

    // Insert icon
    var imgUrl;
    if(this.hasAttribute('img')) {
      imgUrl = this.getAttribute('img');
    } else {
      imgUrl = 'img/default.png';
    }
    var img = document.createElement('img');
    img.src = imgUrl;
    icon.appendChild(img);

    // Create some CSS to apply to the shadow dom
    var 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

    shadow.appendChild(style);
    shadow.appendChild(wrapper);
    wrapper.appendChild(icon);
    wrapper.appendChild(info);
  }
}

// Define the new element
customElements.define('popup-info', PopUpInfo);
<popup-info img="img/alt.png" 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.">

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
    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' });
<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 DOMException NotSupportedError.

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

  constructor() {
    super();

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

Specifications

Specification
HTML Standard (HTML)
# dom-customelementregistry-define-dev

Browser compatibility

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