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• Using slots means that I would be doing just passing HTML and I don't think there is an easy way to iterate through HTML list items. Component I can keep on using components but now I have to convert my old list to an object and add some conditionals to manage to prepare the component for handling HTML formating.
• Some browsers are still in the process of updating to support the standards for Web Components. In the mean time, polyfills simulate the missing browser capabilities as closely as possible.

A slot is a placeholder for markup that a parent component passes into a component’s body. Slots are part of the Web Component specification. To define a slot in markup, use the slot tag, which has an optional name attribute. # Unnamed Slots. In the playground, click slotDemo.html to see an unnamed slot.

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.

Lightning Web Components Slots

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

1. Create a class in which you 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, the class or function 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

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.

CSS pseudo-elements

Pseudo-elements relating specifically to custom elements:

• ::part: Represents any element within a shadow tree that has a matching part attribute.

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

Web Components Nested Slots

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	The definition of <template>.
HTML Living Standard The definition of 'custom elements' in that specification.	Living Standard	The definition of HTML Custom Elements.
DOM The definition of 'shadow trees' in that specification.	Living Standard	The definition of Shadow DOM.
HTML Imports	Working Draft	Initial HTML Imports definition.
Shadow DOM	Obsolete	Initial Shadow DOM definition.

Browser compatibility

In general:

• Web components are supported by default in Firefox (version 63), Chrome, and Opera.
• Safari supports a number of web component features, but less than the above browsers.
• Edge is working on an implementation.

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

See also

• webcomponents.org — site featuring web components examples, tutorials, and other information.
• DataFormsJS — Open source web components library — Set of Web Components that can be used to build Single Page Apps (SPA), Display JSON data from API’s and Web Services, and bind data to different elements on screen. All Web Components are plain JavaScript and require no build process.
• FAST is a web component library built by Microsoft which offers several packages to leverage depending on your project needs. Fast Element is a lightweight means to easily build performant, memory-efficient, standards-compliant Web Components. Fast Foundation is a library of Web Component classes, templates, and other utilities built on fast-element intended to be composed into registered Web Components.
• Hybrids — Open source web components library, which favors plain objects and pure functions over class and this syntax. It provides a simple and functional API for creating custom elements.
• Polymer — Google's web components framework — a set of polyfills, enhancements, and examples. Currently the easiest way to use web components cross-browser.
• Snuggsi — Easy Web Components in ~1kB Including polyfill — All you need is a browser and basic understanding of HTML, CSS, and JavaScript classes to be productive.
• Slim.js — Open source web components library — a high-performant library for rapid and easy component authoring; extensible and pluggable and cross-framework compatible.
• Stencil — Toolchain for building reusable, scalable design systems in web components.

In my previous post I described how to send data in form of attributes to a web component. Since this mechanism is only applicable for simple values, there also is a separate mechanism for inserting complex values.

Let me explain the goal first.If I have a web component which consists of the following internal structure.

To use this component, you have to insert it into the DOM and it will render a visual representation.If I now want to fill those empty placeholders with content I have to use HTML elements within the component.

With this definition, the browser no longer knows where to place the header line or the content line within my control.

To tackle that issue, the spec proposes the use of “slot” elements, which will represent placeholders which can later be filled by content.

Slot elements can be placed anywhere within the component. It is preferred that those slots are named (if only one slot exists, the name can be omitted).

This is what a web component with slots would look like.

Web Components Slots Free

To use those predefined slots, the attached element has to name the slot it wants to be placed in.

Web Components Slots Online

Now the control gets rendered with the header and content part at the right places.