Web Components Everywhere

A B O U T M E
{
"name": "Ilia Idakiev",
"experience": [
"Developer & Founder of HNS/HG",
"Lecturer in Advanced JS @ Sofia University",
"Contractor / Consultant",
"Public / Private Courses"
],
"involvedIn": [
"Angular Sofia", "SofiaJS", "BeerJS",
]
}

WEB COMPONENTS EVERYWHERE
SEPARATION OF CONCERNS (SOC)
▸ Design principle for separating a computer program into distinct sections, such
that each section addresses a separate concern. (Modularity)

S.O.L.I.D PRINCIPLES OF OBJECT-ORIENTED PROGRAMMING
▸ Single Responsibility Principle
▸ Open / Close Principle
▸ Liskov Substitution Principle
▸ Interface Segregation Principle
▸ Dependency Inversion Principle
http://aspiringcraftsman.com/2011/12/08/solid-javascript-single-responsibility-principle/

WEB COMPONENTS
▸ Introduced by Alex Russell (Chrome team @ Google)  
at Fronteers Conference 2011
▸ A set of features currently being added by the W3C to
the HTML and DOM speciﬁcations that allow the creation of
reusable widgets or components in web documents and web applications.
▸ The intention behind them is to bring component-based software
engineering to the World Wide Web.

WEB COMPONENTS FEATURES:
▸ HTML Templates - an HTML fragment is not rendered, but stored until it is
instantiated via JavaScript.
▸ Shadow DOM - Encapsulated DOM and styling, with composition.
▸ Custom Elements - APIs to deﬁne new HTML elements.
▸ HTML Imports - Declarative methods of importing HTML documents into other
documents. (Replaced by ES6 Imports).

DEFINE CUSTOM ELEMENT
(function () {
const template = createTemplate('<div>Hello World<div>');
class Counter extends HTMLElement {
constructor() {
super();
const shadowRoot = this.attachShadow({ mode: 'open' });
shadowRoot.appendChild(template.content.cloneNode(true));
}
}
customElements.define('hg-counter', Counter);
}());
Create an isolated scope
counter.js

(function () {
constructor() {
super();
}
}
}());
DEFINE CUSTOM ELEMENT Create a new class that extends HTMLElement
counter.js

(function () {
constructor() {
super();
}
}
}());
DEFINE CUSTOM ELEMENT Register the new custom element.
counter.js

HTML TEMPLATES
▸ The <template> tag holds its content hidden from the client.
▸ Content inside a <template> tag will be parsed but not rendered.
▸ The content can be visible and rendered later by using JavaScript.

WAYS TO CREATE A TEMPLATE
<template id="template">
<h2>Hello World</h2>
</template>
const template =
document.createElement('template');
template.innerHTML =
'<h2>Hello World</h2>';
Using HTML Using JavaScript

CREATE TEMPLATE HELPER FUNCTION
function createTemplate(string) {
const template = document.createElement('template');
template.innerHTML = string;
return template;
}
utils.js

CREATE THE TEMPLATE
(function () {
constructor() {
super();
}
}
}());
Use the create template helper function.
counter.js

SHADOW DOM
▸ Isolated DOM - The component's DOM is self-contained
(e.g. document.querySelector() won't return nodes in the component's shadow DOM).
▸ Scoped CSS - CSS deﬁned inside shadow DOM is scoped to it. Style rules
don't leak out and page styles don't bleed in.
▸ Composition - done with the <slot> element. 
(Slots are placeholders inside your component that users can ﬁll with their own markup).

DEFINE CUSTOM ELEMENT
(function () {
constructor() {
super();
}
}
}());
counter.js
Utilise the class constructor.

ATTACH SHADOW DOM
(function () {
constructor() {
super();
}
}
}());
Attach the shadow DOM.
counter.js

(function () {
constructor() {
super();
}
}
}());
CREATE THE TEMPLATE Attach the template contents to the shadow root.
counter.js

USE OUR CUSTOM ELEMENT
<body>
<hg-counter></hg-counter>
<script src="./util.js"></script>
<script src="./counter.js"></script>
</body>
index.html

EXTEND OUR CUSTOM ELEMENT
index.html
(function () {
const template = createTemplate(`
<div name="value"></div>
<button data-type=“dec">-</button>
<button data-type="inc">+</button>
`);
constructor() {
super();

index.html
constructor() {
super();
shadowRoot.addEventListener('click', ({ target }) => {
const type = target.getAttribute('data-type');
if (type === 'dec') {
this.counter--;
} else if (type === 'inc') {
this.counter++;
}
});

UPDATING THE DOM
utils.js
function updateDOM(root, updates) {
updates.forEach(item => {
root.querySelectorAll(`[name=${item.name}]`).forEach(element =>
element.textContent = item.value
);
});
}

index.html
set counter(value) {
this._counter = value;
}
get counter() {
return this._counter;
}
constructor() {
super();
this.counter = 0;

index.html
constructor() {
super();
this.counter = 0;
this._update = () => {
updateDOM(shadowRoot, [{
name: 'value',
value: this.counter
}]);
}

index.html
set counter(value) {
this._counter = value;
this._update();
}
get counter() {
return this._counter;
}
constructor() {
super();
this.counter = 0;

CUSTOM COMPONENT ATTRIBUTES
index.html
<body>
<hg-counter value="10"></hg-counter>
<script src="./util.js"></script>
<script src="./counter.js"></script>
</body>

CUSTOM ELEMENTS LIFECYCLE CALLBACKS
▸ connectedCallback - Invoked each time the custom element is appended into a
document-connected element. This will happen each time the node is moved, and
may happen before the element's contents have been fully parsed.
▸ disconnectedCallback - Invoked each time the custom element is disconnected
from the document's DOM.
▸ attributeChangedCallback - Invoked each time one of the custom element's
attributes is added, removed, or changed.
▸ adoptedCallback - Invoked each time the custom element is moved to a new
document.

index.html
static get observedAttributes() {
return ['value'];
}
attributeChangedCallback(name, oldValue, newValue) {
if (name === 'value') {
this.counter = newValue;
}
}
constructor() {
Handle attribute changes

index.html
static get observedAttributes() {
return ['value'];
}
attributeChangedCallback(name, oldValue, newValue) {
if (name === 'value') {
this.counter = newValue;
}
}
constructor() {
Deﬁne which attributes should be watched

CUSTOM COMPONENT STYLES
index.html
Apply scoped styles to our component
(function () {
const template = createTemplate(`
<style>
:host {
display: flex;
}
div[name="value"] {
min-width: 30px;
}
</style>
<div name="value"></div>
<button data-type="dec">-</button>
<button data-type="inc">+</button>
`);

WEB COMPONENT CSS
▸ :host - selects the shadow host of the shadow DOM
▸ :host() - match only if the selector given as the function's parameter matches
the shadow host.
▸ :host-context() - match 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.
▸ ::slotted() - represents any element that has been placed into a slot inside an
HTML template

WHAT ABOUT
DISPATCHING EVENTS?

CUSTOM EVENTS Dispatching custom event
const test = shadowRoot.getElementById('element-button');
test.addEventListener('click', () => {
this.dispatchEvent(new CustomEvent('toggle', {
detail: {
value: true
}
}))
});
custom-element.js

CUSTOM EVENTS Listening for custom event
• CustomEvent {isTrusted: false, detail: {value: true}, type: "toggle", …}
const el = document.getElementById('my-custom-element');
el.addEventListener('toggle', e => {
console.log(e);
});
main.js
Console

FURTHER READING
▸ Extending different HTML Elements 
(e.g. HTMLButton)

BENEFITS OF USING CUSTOM COMPONENTS
▸ Framework agnostic - Written in JavaScript and native to the browser.
▸ Simpliﬁes CSS - Scoped DOM means you can use simple CSS selectors, more
generic id/class names, and not worry about naming conﬂicts.
• Productivity - Think of apps in chunks of DOM rather than one large (global) page.
▸ Productivity - Think of apps in chunks of DOM rather than one large (global)
page.

BROWSER SUPPORT

COSTS OF USING CUSTOM COMPONENTS
▸ Template Generation - manually construct the DOM for our templates  
(No JSX features or Structural Directives)
▸ DOM Updates - manually track and handle changes to our DOM 
(No Virtual DOM or Change Detection)

LIT HTML
▸ Library Developed by the Polymer Team @ GOOGLE
▸ Efﬁcient - lit-html is extremely fast. It uses fast platform features like
HTML <template> elements with native cloning.
▸ Expressive - lit-html gives you the full power of JavaScript and functional programming
patterns.
▸ Extensible - Different dialects of templates can be created with additional features for setting
element properties, declarative event handlers and more.
▸ It can be used standalone for simple tasks, or combined with a framework or component model,
like Web Components, for a full-featured UI development platform.
▸ It has an awesome VSC extension for syntax highlighting and formatting.

TAG FUNCTIONS
function myTagFn(str, ...expr) {
console.log(str, expr);
return str.reduce((acc, curr, i) => acc + curr + (expr[i] || ''), '');
}
myTagFn`1+1 equals ${1+1} and 3 + 3 equals ${3+3} ${3+2}`
> (4) ["1+1 equals ", " and 3 + 3 equals ", " ", ""] (3) [2, 6, 5]
> “1+1 equals 2 and 3 + 3 equals 6 5"
Console

RESOURCES
▸ LitHTML demo - https://github.com/Polymer/lit-html/blob/master/demo/
clock.js

FRAMEWORKS
▸ StencilJS - a simple library for generating Web Components and
progressive web apps (PWA).  
(built by the Ionic Framework team for its next generation of performant mobile and desktop Web
Components)
▸ Polymer - library for creating web components. 
(built by Google and used by YouTube, Netﬂix, Google Earth and others)
▸ SkateJS - library providing functional abstraction over web
components.
▸ Angular Elements

WEB COMPONENTS SERVER SIDE RENDERING
▸ SkateJS SSR - @skatejs/ssr is a web component server-side
rendering and testing library. (uses undom)
▸ Rendertron - Rendertron is a headless Chrome rendering solution
designed to render & serialise web pages on the ﬂy.
▸ Domino - Server-side DOM implementation based on Mozilla's
dom.js

ANGULAR ELEMENTS
▸ A part of Angular Labs set.
▸ Angular elements are Angular components
packaged as Custom Elements.

ANGULAR ELEMENTS
▸ Angular’s createCustomElement() function transforms an Angular component,
together with its dependencies, to a class that is conﬁgured to produce a self-
bootstrapping instance of the component.
▸ It transforms the property names to make them compatible with custom
elements.
▸ Component outputs are dispatched as HTML Custom Events, with the name
of the custom event matching the output name.
▸ Then customElements.deﬁne() is used to register our custom element.
Transformation

NEW ANGULAR PROJECT
ng new angular-elements // Create new Angular CLI project
cd angular-elements
ng add @angular/elements // Add angular elements package
ng g c counter // Generate a new component
// Navigate to our new project

ANGULAR ELEMENTS The generated component
@Component({
selector: 'app-counter',
templateUrl: './counter.component.html',
styleUrls: ['./counter.component.css'],
encapsulation: ViewEncapsulation.Native
})
export class CounterComponent {
@Input() counter = 0;
constructor() { }
inc() { this.counter++; }
dec() { this.counter--; }
}
src/app/counter/counter.component.ts

ANGULAR ELEMENTS Add View Encapsulation via Shadow DOM (Native)
@Component({
})
constructor() { }
}

ANGULAR ELEMENTS Create a counter input property
@Component({
})
constructor() { }
}

ANGULAR ELEMENTS Create counter handlers
@Component({
})
constructor() { }
}

ANGULAR ELEMENTS Present the counter value
<div>{{counter}}</div>
<button (click)="dec()">Dec</button>
<button (click)="inc()">Inc</button>
src/app/counter/counter.component.html

ANGULAR ELEMENTS Create the manipulation buttons and connect the to the handlers
<div>{{counter}}</div>
<button (click)="dec()">Dec</button>
<button (click)="inc()">Inc</button>
src/app/counter/counter.component.html

NEW ANGULAR MODULE
ng g m counter // Create new counter module

ANGULAR ELEMENTS Create a module for out custom elements
@NgModule({
imports: [
BrowserModule
],
declarations: [],
entryComponents: [CounterComponent]
})
export class CounterModule {
constructor(private injector: Injector) {}
ngDoBootstrap() {
const el = createCustomElement(CounterComponent, { injector: this.injector });
customElements.define('app-counter', el);
}
}
src/app/counter/counter.module.ts

ANGULAR ELEMENTS Create a module for out custom elements
@NgModule({
imports: [
BrowserModule
],
declarations: [CounterComponent],
entryComponents: [CounterComponent]
})
export class CounterModule {
constructor(private injector: Injector) {}
ngDoBootstrap() {
const el = createCustomElement(CounterComponent, { injector: this.injector });
customElements.define('app-counter', el);
}
}
src/app/counter/counter.module.ts

IVY - THE NEW RENDERING ENGINE FOR ANGULAR
▸ Incremental builds and uses monomorphic data structures internally (Fast)
▸ Tree Shaking (Efﬁcient)
https://github.com/angular/angular/blob/master/packages/compiler/design/architecture.md

ANGULAR ELEMENTS Using the Ivy hopefully we will have something like:
@Component({
customElement: true
})
constructor() { }
}

FAQ
▸ Can we use Dependency Injection?
▸ What about Content Projection?
▸ Can we still use Slots?

BENEFITS OF USING ANGULAR ELEMENTS
▸ All components can be reused across JavaScript applications.
▸ Creating Dynamic Components.
▸ Hybrid Rendering - Server Side Rendering with Custom Elements that don’t
wait for the application to bootstrap to start working.
▸ Micro Frontends Architecture - Vertical Application Scaling (Working with
multiple small applications instead of a big monolith.)
https://micro-frontends.org

ADDITIONAL RESOURCES
▸ https://custom-elements-everywhere.com - This project runs a suite of tests
against each framework to identify interoperability issues, and highlight
potential ﬁxes already implemented in other frameworks.

CONNECT
GitHub > https://github.com/iliaidakiev (/slides/ - list of future and past events)
Twitter > @ilia_idakiev

Web Components Everywhere

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