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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

Exception Description
NotSupportedError 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 The provided name is not a valid custom element name.
TypeError The referenced constructor is not a constructor.

Note: You'll often get NotSupportedErrors thrown that seem like define() is failing, but in actual fact it is a problem with Element.attachShadow(). Check that page too if you are coming up against problems.

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>

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'customElements.define()' in that specification.
Living Standard Initial definition.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support

661

542

No3

594

Yes — 595

No

531

412

10.16
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support

661

542

661

542

No3

594

Yes — 595

531

412

10.166.02

1. Support for 'Customized built-in elements' as well.

2. Support for 'Autonomous custom elements' only.

3. Under consideration

4. From version 59: this feature is behind the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

5. Until version 59 (exclusive): this feature is behind the dom.webcomponents.enabled preference (needs to be set to true) and the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

6. Supports 'Autonomous custom elements' but not 'Customized built-in elements'

Etiquetas y colaboradores del documento

Última actualización por: jpmedley,