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Web Components is a suite of different technologies allowing you to create reusable custom elements — with their functionality encapsulated away from the rest of your code — and utilize them in your web apps.

Concepts and usage

As developers, we all know that reusing code as much as possible is a good idea. This has traditionally not been so easy for custom markup structures — think of the complex HTML (and associated style and script) you've sometimes had to write to render custom UI controls, and how using them multiple times can turn your page into a mess if you are not careful.

Web Components aims to solve such problems — it consists of three main technologies, which can be used together to create versatile custom elements with encapsulated functionality that can be reused wherever you like without fear of code collisions.

  • Custom elements: A set of JavaScript APIs that allow you to define custom elements and their behaviour, which can then be used as desired in your user interface.
  • Shadow DOM: A set of JavaScript APIs for attaching an encapsulated "shadow" DOM tree to an element — which is rendered separately from the main document DOM — and controlling associated functionality. In this way you can keep an element's features private, so they can be scripted and styled without the fear of collision with other parts of the document.
  • HTML templates: The <template> and <slot> elements enable you to write markup templates that are not displayed in the rendered page. These can then be reused multiple times as the basis of a custom element's structure.

The basic approach for implementing a web component generally looks something like this:

  1. Create a class in which to specify your web component functionality using the ECMAScript 2015 class syntax (see Classes for more information).
  2. Register your new custom element using the CustomElementRegistry.define() method, passing it the element name to be defined, class in which its functionality is specified, and optionally, what element it inherits from.
  3. If required, attach a shadow DOM to the custom element using Element.attachShadow() method. Add child elements, event listeners, etc., to the shadow DOM using regular DOM methods.
  4. If required, define an HTML template using <template> and <slot>. Again use regular DOM methods to clone the template and attach it to your shadow DOM.
  5. Use your custom element wherever you like on your page, just like you would any regular HTML element.

Tutorials

Using custom elements
A guide showing how to use the features of custom elements to create simple web components, as well as looking into life cycle callbacks and some other more advanced features.
Using shadow DOM
A guide that looks at shadow DOM fundamentals, showing how to attach a shadow DOM to an element, add to the shadow DOM tree, style it, and more.
Using templates and slots
A guide showing how to define a reusable HTML structure using <template> and <slot> elements, and then use that structure inside your web components.

Reference

Custom elements

CustomElementRegistry
Contains functionality related to custom elements, most notably the CustomElementRegistry.define() method used to register new custom elements so they can then be used in your document.
Window.customElements
Returns a reference to the CustomElementRegistry object.
Life cycle callbacks
Special callback functions defined inside the custom element's class definition, which affect its behavior:
  • connectedCallback: Invoked when the custom element is first connected to the document's DOM.
  • disconnectedCallback: Invoked when the custom element is disconnected from the document's DOM.
  • adoptedCallback: Invoked when the custom element is moved to a new document.
  • attributeChangedCallback: Invoked when one of the custom element's attributes is added, removed, or changed.
Extensions for creating custom built-in elements
  • The is global HTML attribute: Allows you to specify that a standard HTML element should behave like a registered custom built-in element.
  • The "is" option of the Document.createElement() method: Allows you to create an instance of a standard HTML element that behaves like a given registered custom built-in element.
CSS pseudo-classes
Pseudo-classes relating specifically to custom elements:
  • :defined: Matches any element that is defined, including built in elements and custom elements defined with CustomElementRegistry.define()).
  • :host: Selects the shadow host of the shadow DOM containing the CSS it is used inside.
  • :host(): Selects the shadow host of the shadow DOM containing the CSS it is used inside (so you can select a custom element from inside its shadow DOM) — but only if the selector given as the function's parameter matches the shadow host.
  • :host-context(): Selects the shadow host of the shadow DOM containing the CSS it is used inside (so you can select a custom element from inside its shadow DOM) — but only if the selector given as the function's parameter matches the shadow host's ancestor(s) in the place it sits inside the DOM hierarchy.

Shadow DOM

ShadowRoot
Represents the root node of a shadow DOM subtree.
DocumentOrShadowRoot
A mixin defining features that are available across document and shadow roots.
Element extensions
Extensions to the Element interface related to shadow DOM:
  • The Element.attachShadow() method attaches a shadow DOM tree to the specified element.
  • The Element.shadowRoot property returns the shadow root attached to the specified element, or null if there is no shadow root attached.
Relevant Node additions
Additions to the Node interface relevant to shadow DOM:
  • The Node.getRootNode() method returns the context object's root, which optionally includes the shadow root if it is available.
  • The Node.isConnected property returns a boolean indicating whether or not the Node is connected (directly or indirectly) to the context object, e.g. the Document object in the case of the normal DOM, or the ShadowRoot in the case of a shadow DOM.
Event extensions
Extensions to the Event interface related to shadow DOM:
  • Event.composed: Returns a Boolean which indicates whether the event will propagate across the shadow DOM boundary into the standard DOM (true), or not  (false).
  • Event.composedPath: Returns the event’s path (objects on which listeners will be invoked). This does not include nodes in shadow trees if the shadow root was created with ShadowRoot.mode closed.

HTML templates

<template>
Contains an HTML fragment that is not rendered when a containing document is initially loaded, but can be displayed at runtime using JavaScript, mainly used as the basis of custom element structures. The associated DOM interface is HTMLTemplateElement.
<slot>
A placeholder inside a web component that you can fill with your own markup, which lets you create separate DOM trees and present them together. The associated DOM interface is HTMLSlotElement.
The slot global HTML attribute
Assigns a slot in a shadow DOM shadow tree to an element.
Slotable
A mixin implemented by both Element and Text nodes, defining features that allow them to become the contents of an <slot> element. The mixin defines one attribute, Slotable.assignedSlot, which returns a reference to the slot the node is inserted in.
Element extensions
Extensions to the Element interface related to slots:
  • Element.slot: Returns the name of the shadow DOM slot attached to the element.
CSS pseudo-elements
Pseudo-elements relating specifically to slots:
  • ::slotted: Matches any content that is inserted into a slot.
The slotchange event
Fired on an HTMLSlotElement instance (<slot> element) when the node(s) contained in that slot change.

Examples

We are building up a number of examples in our web-components-examples GitHub repo. More will be added as time goes on.

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'template element' in that specification.
Living Standard  
HTML Living Standard
The definition of 'custom elements' in that specification.
Living Standard  
DOM
The definition of 'shadow trees' in that specification.
Living Standard  
Shadow DOM Obsolete  

Browser support

In general, web components are supported well in Chrome and Opera — most features have been enabled by default for a while. Firefox has experimental support for most features, disabled by default, which looks to be enabled by default by about version 63. Safari supports a number of features, but less than the above browsers, and Edge is working on an implementation.

For browser support of specific features, you'll have to consult the reference pages listed above.

Notes

  • To enable custom elements and shadow DOM in Firefox, set the dom.webcomponents.customelements.enabled and dom.webcomponents.shadowdom.enabled preferences to true.

See also

  • webcomponents.org — site featuring web components examples, tutorials, and other information.
  • Polymer — Google's web components framework — a set of polyfills, enhancements, and examples. Currently the easiest way to use web components cross-browser.

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