Custom Elements Everywhere

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Polymer Summit 2017 Rob Dodson @rob_dodson Custom Elements Everywhere

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I've been at Google for 3+ years

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Custom Elements should work everywhere.

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Custom Elements should work everywhere. So… why don't they?

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Did we put the before the unihorse

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TWO PARTS PART no. 1 What is a "good" Custom Element? PART no. 2 How should frameworks act?

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What is a "good" Custom Element?

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Do you know if there are any reference Custom Elements I could look at which live up to this standard? Not that I'm aware of.

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SCIENCE!

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HowTo: Components A collection of educational custom elements that demonstrate best practices. Examples are meant to be read, interpreted, and ported to your own elements.

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super(); this.attachShadow({mode: 'open'}); this.shadowRoot.appendChild(template.conte } connectedCallback() { if (!this.hasAttribute('role')) this.setAttribute('role', 'checkbox'); if (!this.hasAttribute('tabindex')) this.setAttribute('tabindex', 0); this._upgradeProperty('checked'); this._upgradeProperty('disabled'); this.addEventListener('keydown', this._onK The construtor is a good place to create Shadow DOM, though you should avoid touching any attributes or Light DOM children as they may not be available yet. connectedCallback ﬁres when the element is inserted into the DOM. It's a good place to set the initial role, tabindex, internal state, and install event listeners. A user may set a property on an instance of an element, before its prototype has been connected to this class. The `_upgradeProperty` method

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HowTo: Components are a work in progress.

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Check it out @ bit.ly/howto-components

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Shadow DOM Attributes & Properties Events

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Avoid absolutism

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Shadow DOM

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Does your element need Shadow DOM?

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You gotta use Shadow DOM! You gotta use Shadow DOM!

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howto-checkbox.js + howto-checkbox.css

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If someone puts your element inside another shadow root, you'll have to ﬁgure out how to get your styles in there.

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w3c/webcomponents #468

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Create a shadow root if you want to self-apply styles.

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We have CSS-in-JS for scoping styles, so why do we need Shadow DOM?

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const template = document.createElement('template'); template.innerHTML = `

Count:

`; Polymer Summit 2017

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export class CountWithShadow extends HTMLElement { constructor () { super(); this.attachShadow({ mode: 'open' }); this.shadowRoot.appendChild(template.content.cloneNode(true)); } } customElements.define('x-count-with-shadow', CountWithShadow); Polymer Summit 2017

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export class CountWithoutShadow extends HTMLElement { constructor () { super(); } connectedCallback() { this.appendChild(template.content.cloneNode(true)); } } customElements.define('x-count-without-shadow', Count); Polymer Summit 2017

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// Meanwhile, in a React component… render () { const { count } = this.state; // increments every second return (

{count} {count}

); } Polymer Summit 2017

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Count: 1 1 Count:

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Count: 1 1 Count: 1

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Count: 1 1 Count: 1

Count:

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Count: 1 1 Count: 1

Count:

ummm…

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2 Count: 2 2

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Place any children you create into a shadow root.

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These children are your implementation and should be hidden.

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Does your element need Shadow DOM?

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Yeah, it probably does.

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Without Shadow DOM you lose all guarantees of safety.

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Create a shadow root if you want to self-apply styles. Place any children you create into a shadow root. Shadow DOM

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Attributes & Properties

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Hey! HTML has a spec!

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But HTML is also kinda gnarly

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Accept primitive data as either attributes or properties; ideally, both.

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Reﬂect primitive data from attribute to property, and vice versa.

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class CustomVideo extends HTMLElement { get preload() { const value = this.getAttribute('preload'); return value === null ? 'auto' : value; } set preload(value) { this.setAttribute('preload', value); } } Polymer Summit 2017

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Only accept rich data as properties.

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Do NOT reﬂect rich data properties to attributes.

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It's expensive to reﬂect.

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A serialized object will lose its identity.

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Accept primitive data as either attributes or properties. Reﬂect primitive data from attribute to property, and vice versa. Only accept rich data as properties. Do NOT reﬂect rich data from property to attribute. Attributes & Properties

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Events

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It's superﬂuous and may cause inﬁnite loops.

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Do NOT dispatch events in response to the host setting a property (downward data ﬂow).

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Do dispatch events in response to internal element activity.

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The element knows something changed, but the host does not.

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Do NOT dispatch events in response to the host setting a property (downward data ﬂow). Do dispatch events in response to internal element activity. Events

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What is a "good" Custom Element?

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Resize This Window Building Components. A guide to Custom Element best practices.

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Check it out @ bit.ly/building-components

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How should frameworks act?

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Billions and billions…

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Custom Elements Everywhere Making sure frameworks and custom elements can be BFFs

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custom-elements-everywhere.com

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Rob waves his arms around and apologizes [ ]

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known unknowns

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Custom Element test scores

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Attributes & Properties Events

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Attributes & Properties

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Manual The developer tells the framework how to pass data. Automated The framework determines how to pass data.

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Angular

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Angular set the foo property equal to bar

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Angular i.e. myElement.foo = bar

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Angular set the baz attribute equal to squid

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Angular i.e. myElement.setAttribute('baz', squid)

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Vue set the foo attribute equal to bar

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Vue set the foo property equal to bar

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React set the foo attribute equal to bar

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React

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React You can work around this by imperatively grabbing a reference to the element and setting the property on it.

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React Resize This Window facebook/react #10399

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Preact

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Preact set the foo property if it's available

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Preact if ('foo' in myElement) myElement.foo = bar else myElement.setAttribute('foo', bar)

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Preact if ('foo' in myElement) myElement.foo = bar else myElement.setAttribute('foo', bar)

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Preact if ('foo' in myElement) myElement.foo = bar else myElement.setAttribute('foo', bar)

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Preact if ('foo' in myElement) myElement.foo = bar else myElement.setAttribute('foo', bar)

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Events

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Angular

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Angular myElement.addEventListener('foochanged', onFoo)

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Angular lowercase

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Angular CAPSCase

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Angular camelCase

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Angular kebab-case

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Angular PascalCase

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Angular AsShOlEcAsE

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Vue

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Vue same behavior as Angular

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React

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React unfortunately won't work.

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React You can work around this by imperatively grabbing a reference to the element and adding an event listener.

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React Resize This Window facebook/react #7901

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Preact

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Preact TOTES WORKS!

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Preact TOTES WORKS! kinda. sorta. mostly…

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Preact always calls .toLowerCase() myElement.addEventListener('foochanged', onFoo)

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So…

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So… it doesn't support AsShOlEcAsE?!?!

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Preact Resize This Window developit/preact #788

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So, what did we learn?

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There are best practices we can all follow

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What's broken today seems easy to ﬁx

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amazing people Reviewers HowTo: Components team Sean Rob Jason Evan Dan Domenic Steve Dominic Surma Monica Ewa

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Thank You! Polymer Summit 2017 Rob Dodson @rob_dodson Images by Simon Child, Eucalyp, Edward Boatman from the Noun Project.