Custom Elements is a capability for creating your own custom HTML elements. They can have their own scripted behavior and CSS styling. They are a part of web components, but can also be used by themselves.

Note: Custom elements have only recently stabilized, and parts of MDN still contain documentation for outdated APIs from old specification drafts.

It may be unclear as to why the new custom elements capability was created, as it was already possible to create a tag name like <mytag> and style it with CSS, then use scripting to attach behaviors to it.  An advantage that custom elements have are their lifecycle reactions, which allow attaching behaviors to different parts of the new element's "lifecycle." For example, a certain behavior can be attached for when the element is inserted into the DOM ("connected"), and a different behavior when it is removed from the DOM ("disconnected"), or when its attributes change.

The key enabler of v1 custom elements is the CustomElementRegistry.define() method, which can be used to define a new custom element. The new element will then use the supplied class for any instances, instead of the default HTMLUnknownElement. Custom elements can also be based on a native element like, <button> by using the syntax; <button is="my-button"> these are called customized built-in elements.

Custom element methods

Custom elements have the following methods that govern how they behave:

constructor()
Called when the element is created or upgraded
connectedCallback()
Called when the element is inserted into a document, including into a shadow tree
disconnectedCallback()
Called when the element is removed from a document
attributeChangedCallback(attributeName, oldValue, newValue, namespace)
Called when an attribute is changed, appended, removed, or replaced on the element. Only called for observed attributes.
adoptedCallback(oldDocument, newDocument)
Called when the element is adopted into a new document

Examples

Custom elements require using the class syntax introduced in modern versions of JavaScript.

HTML file:

If nothing appeared below, then your browser does not support Custom Elements yet.
<x-product data-name="Ruby" data-img="https://s3-us-west-2.amazonaws.com/s.cdpn.io/4621/ruby.png" data-url="http://example.com/1"></x-product>
<x-product data-name="JavaScript" data-img="https://s3-us-west-2.amazonaws.com/s.cdpn.io/4621/javascript.png" data-url="http://example.com/2"></x-product>
<x-product data-name="Python" data-img="https://s3-us-west-2.amazonaws.com/s.cdpn.io/4621/python.png" data-url="http://example.com/3"></x-product>

JS file:

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

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

    // Create a standard img element and set it's attributes.
    var img = document.createElement('img');
    img.alt = this.getAttribute('data-name');
    img.src = this.getAttribute('data-img');
    img.width = '150';
    img.height = '150';
    img.className = 'product-img';

    // Add the image to the shadow root.
    shadow.appendChild(img);

    // Add an event listener to the image.
    img.addEventListener('click', () => {
      window.location = this.getAttribute('data-url');
    });

    // Create a link to the product.
    var link = document.createElement('a');
    link.innerText = this.getAttribute('data-name');
    link.href = this.getAttribute('data-url');
    link.className = 'product-name';

    // Add the link to the shadow root.
    shadow.appendChild(link);
  }
}

// Define the new element
customElements.define('x-product', XProduct);

CSS file:

body {
  background: #F7F7F7;
}

x-product {
  display: inline-block;
  float: left;
  margin: 0.5em;
  border-radius: 3px;
  background: #FFF;
  box-shadow: 0 1px 3px rgba(0,0,0,0.25);
  font-family: Helvetica, arial, sans-serif;
  -webkit-font-smoothing: antialiased;
}

x-product::slotted(.product-img) {
  cursor: pointer;
  background: #FFF;
  margin: 0.5em;
}

x-product::slotted(.product-name) {
  display: block;
  text-align: center;
  text-decoration: none;
  color: #08C;
  border-top: 1px solid #EEE;
  font-weight: bold;
  padding: 0.75em 0;
}

Below is the live example of the above:

Observed attributes

To be notified when attributes change, a list of observed attributes must be defined when initializing the element, by placing an static observedAttributes getter on the element's class that returns an array of attribute names.

JS file:

class HelloElement extends HTMLElement {
  // Monitor the 'name' attribute for changes.
  static get observedAttributes() {return ['name']; }

  // Respond to attribute changes.
  attributeChangedCallback(attr, oldValue, newValue) {
    if (attr == 'name') {
      this.textContent = `Hello, ${newValue}`;
    }
  }
}

// Define the new element
customElements.define('hello-element', HelloElement);

HTML file:

<hello-element name="Anita"></hello-element>

Below is the live example of the above:

Specifications

Custom Elements are defined in the following specification:

Specification Status Comment
The HTML Standard: Custom elements LS  

Browser compatibility

Feature Firefox (Gecko) Chrome Internet Explorer Opera Safari
Basic support No support 59.0 No support 47.0 10.1
Customized built-in elements No support 59.0 No support 47.0 No support
Feature Firefox Mobile (Gecko) Chrome for Android IE Mobile Opera Mobile Safari Mobile
Basic support No support 56.0 No support 47.0 10.1
Customized built-in elements No support 56.0 No support 47.0 No support

1. Firefox has "dom.webcomponents.enabled" preference in about:config but even when set to true the custom elements feature is not available.

Resources