@lume/element

v0.14.0

Published

2 months ago

Create Custom Elements with reactivity and automatic re-rendering.

Downloads

331

@lume/element

Easily create Custom Elements with simple templates and reactivity. This is an alternative to Lit, Stencil, and Fast.

:bulb:Tip:
If you are new to Custom Elements, first learn about the Custom Element APIs available natively in browsers. Lume Element simplifies the creation of Custom Elements compared to writing them with vanilla APIs, but sometimes vanilla APIs are all you need.

Live demos

Cliché Usage Example

Define a <click-counter> element:

import {Element, element, numberAttribute} from '@lume/element'
import html from 'solid-js/html'
import {createEffect} from 'solid-js'

@element('click-counter')
class ClickCounter extends Element {
	@numberAttribute count = 0

	template = () => html`<button onclick=${() => this.count++}>Click! (count is: ${() => this.count})</button>`

	css = `
		button {
			border: 2px solid deeppink;
			margin: 5px;
		}
	`

	connectedCallback() {
		super.connectedCallback()

		// Log the `count` any time it changes:
		createEffect(() => {
			console.log('count is:', this.count)
		})
	}
}

Use the <click-counter> in a plain HTML file:

<body>
	<click-counter></click-counter>

	<!-- Manually set the `count` value in HTML: -->
	<click-counter count="100"></click-counter>

	<script type="module">
		import './click-counter.js'

		// Manually set the `count` value in JS:
		document.querySelector('click-counter').count = 200
	</script>
</body>

Example on CodePen (without decorators)

[!Note] Once decorators land in browsers, the above example will work out of the box as-is without compiling, but for now a compile step is needed for using decorators.
You can also use JSX for the template, but that will always require compiling:
template = () => <button> Click! (count is: {this.count}) </button>
Further examples below show how to define elements without decorators or JSX, which works today without a compiler.

Use the <click-counter> in another element's template,

import {Element, element} from '@lume/element'
import html from 'solid-js/html'
import {signal} from 'classy-solid'

@element('counter-example')
class CounterExample extends Element {
	@signal count = 50 // Not an attribute, only a signal.

	template = () => html`<click-counter count=${() => this.count}></click-counter>`
}

document.body.append(new CounterExample())

Use <click-counter> in a plain function component (i.e. a Solid.js component):

// At this point this, this boils down to plain Solid.js code (`@lume/element` comes
// with `solid-js`)

import {createSignal} from 'solid-js'
import html from 'solid-js/html'

function CounterExample() {
	const [count, setCount] = createSignal(50)

	return html`<click-counter count=${count()}></click-counter>`
}

document.body.append(CounterExample())

Intro

Custom Elements (also known as Web Components are a feature of browsers that allow us to define new HTML elements that the browser understands in the same way as built-in elements like <div> or <button>. They are very useful for organizaing your web app into re-usable pieces (elements).

If that flew over your head then you might first want to try a beginner HTML tutorial. You will also need to some basic knowledge of JavaScript.

@lume/element provides a set of features that make it easier to manipulate elements and to define new custom elements and easily compose them together into an application.

With @lume/element we can create custom elements that have the following features:

Reactive instance properties that receive values from element attributes of the same name (but dash-cased).
Declarative templates, written with JSX or html template tag, that automatically update when reactive instance properties are used in the templates.
Scoped styling with or without a ShadowRoot.
Decorators for concise element definitions.

Element attributes are defined with @attribute decorators on class fields.
- Class fields decorated with @attribute receive values from HTML attributes (with the same name but dash-cased) when the HTML attribute values change.
- Deorators are powered by classy-solid: utilities for using Solid.js patterns on classes, such as the @signal decorator for making class fields reactive (backed by Solid signals). Decorators from @lume/element compose the @signal decorator to make properties be reactive.
- As decoraators are not out in browser yet, an alternative non-decorator API can be used, which does not require a build.
Each custom element can have an HTML template that automatically updates the DOM when any reactive variable used in the template changes.
- Templates can be written in the form of HTML-like markup inside JavaScript called JSX, specifically the JSX flavor from Solid.js. This requires a build step.
- Templates can also be written using Solid's html template string tag, which does not require a build.
- When a template updates, the whole template does not re-run, only the part of the template where a variable changed is updated, and only that particular piece of DOM gets modified. There is no virtual DOM diffing.
- Because changes to HTML attributes on an element map to properties backed by signals on the element instance, this will cause the custom element's template to update if its template uses those properties.
@lume/element can be used to create and manipulate trees of DOM elements without necessarily creating new custom elements, using function-style components. This can be especially useful for breaking pieces of a custom element into a few smaller parts without necessarily making new custom elements for each part if they don't need to be new elements.
Custom element styles are automatically scoped, similar to Vue, Svelte, and other systems with style scoping.
- If you're familiar with custom elements, you know that the browser gives this to us for free when using ShadowDOM.
- If you opt an element out of having a ShadowRoot, @lume/element applies its own style scoping for the element at the nearest root node.

STUB: This section needs expansion, but should be enough for anyone familiar with common build tooling in the webdev/JS ecosystem. Contributions very welcome!

Follow the guide on installing lume from CDN, but simply replace lume with @lume/element. The process is otherwise the same.

The examples here in the README follow the CDN approach to keep things simple, for example.

[!Note] Decorator syntax and JSX syntax are both not supported with this install method as it does not use a build step. In the near future, decorators will be out natively in browsers and JS engines (but not JSX).

This assumes some familiarity with command lines and JavScript build tools.

First make sure you've installed Node.js so that we have the npm package manager avaiable.

Install the @lume/element package using the following in a terminal:

npm install @lume/element

If you want to use decorators today (recommended), you'll either need to compile them with TypeScript 5 or higher (recommended, even if writing JS and not TS, as decorator syntax works out of the box with TypeScript), or use the Babel compiler with @babel/plugin-proposal-decorators.

npm install --save-dev typescript
# or
npm install --save-dev @babel/cli @babel/core @babel/plugin-proposal-decorators

If using TypeScript, set allowJs in tsconfig.json to allow compiling JS files, f.e.:

{
	"compilerOptions": {
		"allowJs": true,
		"outDir": "dist"
	},
	"include": ["./src/**/*"]
}

and running npx tsc. See the TypeScript section below for configuring JSX types for various frameworks (Solid, React, Preact, etc).

If using Babel, add the decorators plugin to .babelrc, f.e.

{
	"plugins": ["@babel/plugin-proposal-decorators"]
}

and running npx babel src --out-dir dist.

If you want to use the HTML-like markup inside JavaScript known as "JSX", instead of Solid's html template tag, you will need to additionally install babel-preset-solid:

npm install --save-dev babel-preset-solid

Configure Babel to use the preset inside your project's .babelrc file or in your Webpack babel-loader config:

{
	"plugins": ["@babel/plugin-proposal-decorators"],
	"presets": ["solid"]
}

[!Note] If you compile decorators with TypeScript, you still need Babel for Solid JSX because TypeScript does not compile JSX into Solid.js format, only into React format. You can either compile decorators with TypeScript and have Babel compile JSX in a second step, or you can compile both decorators and JSX with Babel in a single step.

Basic Usage

Create custom elements

A great way to create re-usable components is to create Custom Elements. The advantage of custom elements is that they follow web standards, and therefore they can be used in any web application and manipulated by any DOM manipulation libraries like jQuery, React, Vue, Svelte, or Angular, Solid.js, and all the rest.

The following is a custom element definition with a reactive property firstName that also accepts values from an attribute named first-name (the property name is converted to dash-case for the attribute name).

[!Note] Deorators and JSX are not required. The non-decorator and non-JSX forms are shown further below.

import {
	Element, // A base class for LUME custom elements
	element, // A decorator for defining elements, required for reactive JS properties.
	attribute, // A property decorator to map attributes to properties, and that makes properties reactive
	css, // A no-op identity template tag function (useful to enable CSS syntax highlighting in various text editors)
} from '@lume/element'

@element('greeting-card') // defines the element tag name
class GreetingCard extends Element {
	// The firstName property will be a reactive variable, and any value
	// from an attribute named 'first-name' will be mapped back to this property (the attribute
	// name is the dash-case version of the property name).
	@attribute firstName = 'Roger'

	// Define a DOM tree that we want rendered on screen by providing a
	// `template`. The `template` should be a function that returns a DOM
	// element or array of DOM elements (which we can create with JSX, or with
	// an `html` template tag, or with plain JS). The DOM content will be, by
	// default, appended into the ShadowRoot of our custom element.
	//
	// To take advantage of reactivity in our template, simply interpolate
	// properties that were decoratored with an attribute decorator or defined
	// with `static observedAttributeHandlers` into the template.
	//
	// Here, any time the `.firstName` property's value changes, the DOM will be
	// automatically updated.
	template = () => (
		<div>
			<span>
				Hello <i>{this.firstName}</i>
			</span>
			{/* Children of a <greeting-card> element get rendered here. */}
			<slot></slot>
		</div>
	)

	// Apply styling to your element and its content with the static `css` property.
	// Because the property is static, this style is re-used across all instances of the element.
	// Styles are by default scoped to your element's content.
	static css = css`
		:host {
			background: skyblue;
		}
		div {
			color: pink;
		}
	`

	// If you need instance-specific styling, use a non-static `css` property.
	// This style has higher precedence over styles in the `static css` property.
	// In this example, the divs in each instance of this element will have borders of random sizes.
	// Note, `css` is currently not reactive, it runs once initially, so using a
	// reactive property in the css will currently not update the style.
	css = css`
		div {
			border: ${Math.random() * 5}px solid teal;
		}
	`

	// connectedCallback is a method that fires any time this custom element is
	// connected into a web site's live DOM tree.
	connectedCallback() {
		super.connectedCallback() // Don't forget to call the super method!

		// Once the element is connected, let's update the `.firstName` prop after a
		// couple of seconds, and we'll see the change on screen.
		setTimeout(() => (this.firstName = 'Zaya'), 2000)

		// And show that it works by setting HTML attributes too, two seconds later.
		setTimeout(() => this.setAttribute('first-name', 'Raquel'), 4000)
	}

	// Use the disconnectedCallback to clean anything up when the element is removed from the DOM.
	disconnectedCallback() {
		super.disconnectedCallback()
		// ... clean up ...
	}
}

Now we can use it in the HTML of a web site, or in the template of another component:

<greeting-card first-name="Raynor"></greeting-card>

Example on CodePen (without decorators, with html template tag instead of JSX)

Inside an element's template() method we can assign bits and pieces of DOM to variables, and we can also use other custom elements and functional components. Similary, the css property can also be a method:

@element('greeting-card')
class GreetingCard extends Element {
	// ... same as before ...

	// This time 'template' is a method that has some logic, and refers to pieces of DOM using variables.
	template() {
		const greeting = (
			<span>
				Hello <i>{this.firstName}</i>
			</span>
		)

		console.log(greeting instanceof HTMLSpanElement) // true

		// One piece of DOM can be composed into another:
		const result = <div>{greeting}</div>

		console.log(result instanceof HTMLDivElement) // true

		return result
	}

	// ... same as before ...

	css() {
		const thickness = Math.random() * 5

		return css`
			div {
				border: ${thickness}px solid teal;
			}
		`
	}

	// ... same as before ...
}

Easily create and manipulate DOM

The following is just plain Solid.js at the top level of a module. This sort of code can be useful in the template of a custom element, or the body of a functional component.

import {createSignal} from 'solid-js'

// Make a signal with an initial value of 0.
const [count, setCount] = createSignal(0)

// Increment the value of count every second.
setInterval(() => setCount(count() + 1), 1000)

// Create a <div> element with a child <h1> element. The data-count attribute
// and the text content of the <h1> element will automatically be updated whenever
// the count variable changes.
const el = (
	<div>
		<h1 data-count={count()}>The count is: {count()}</h1>
	</div>
)

// The result stored in the `el` variable a <div> element! For example,
// you can call regular DOM APIs like setAttribute on it.
el.setAttribute('foo', 'bar')

// Append the element to the body of the page, and now you'll see a
// continually-updating message on your screen.
document.body.append(el)

Example on CodePen (with Solid's html template tag instead of JSX)

Create functional components

Continuing with the same count variable from the previous example, here's how to compose DOM trees using "functional components". This is also just plain Solid.js, but can be useful in a custom element's template.

A functional component is a function that simply returns one or more DOM elements. JSX expressions and the html template string tag both return the top level elements defined in the markup.

// This is just plain Solid.js code. See https://solidjs.com for more on writing
// functional components.

// This Label functional component uses the empty <></> tag to contain more than
// one root-level child, and the return value will be an array of DOM nodes.
const Label = props => (
	<>
		<div>{props.greeting}</div>
		{props.children}
	</>
)

// This Greeting functional component nests the content of the Label component
// in its template, and the <div> inside the <Label> gets distributed to the
// part of the Label component where we see `{props.children}`.
const Greeting = () => (
	<section>
		<Label greeting={'hello (' + count() + ')'}>
			<div>John</div>
		</Label>
	</section>
)

// You only need to call this once, and you get a reference to an element or
// multiple elements. You do NOT need to call it over and over to re-render like
// you do in some other libraries. That's what makes all of this simple and
// clean. The reactivity inside the component templates takes care of updating
// content of the created DOM tree.
// Here elem is a reference to an actual `<section>` element.
const elem = Greeting()

// It's just DOM! Use regular DOM APIs to append the element to the body.
document.body.append(elem)

Example on CodePen

Writing function components can sometimes be simpler, but functional components do not have features that custom elements have like style scoping (requires an additional a Solid.js library or compiler plugin).

In contrast to custom elements, functional components only work within the context of other functional components made with Solid.js or custom elements made with @lume/element. Functional components will no longer be compatible with React, Vue, Angular, Svelte, or all the other web libraries and frameworks. For portability across applications and frameworks, this is where custom elements accel.

Custom elements are also debuggable in a browser's element inspector out of the box, while functional components are not (functional components require devtools plugins for each browser, if they even exist). See Lume's Debugging guide for an example.

Use functional components inside custom elements

Continuing from above, here's a custom element that re-uses the Greeting component. This shows that any regular Solid.js component can be used in the template of a custom element made with @lume/element:

@element('cool-element')
class CoolElement extends Element {
	template = () => (
		<>
			<h2>Here's a greeting:</h2>
			<Greeting />
		</>
	)
}

document.body.insertAdjacentHTML('beforeend', `<cool-element></cool-element>`)

Example on CodePen (without decorators)

TypeScript

With Solid.js JSX

Load the required JSX types in one of two ways:

Import the types locally within particular files where JSX is used (this is useful for preventing type conflicts if you have other files that use React JSX types or other JSX types):
```
/* jsxImportSource solid-js */
```

Place the jsxImportSource in your tsconfig.json to have it apply to all files (this works great if you use only one form of JSX types in your project, but if you have files with different types of JSX, you'll want to use option 1 instead).

{
	"compilerOptions": {
		/* Solid.js Config */
		// Note, you need to use an additional tool such as Babel, Vite, etc, to
		// compile Solid JSX. `npm create solid` will scaffold things for you.
		"jsx": "preserve",
		"jsxImportSource": "solid-js"
	}
}

In TypeScript, all JSX expressions have the type JSX.Element. But Solid's JSX expressions return actual DOM nodes, and we want the JSX expression types to reflect that fact. For this we have a set of convenience helpers to cast JSX expressions to DOM element types in the @lume/element/dist/type-helpers.js module.

Modifying the very first example from above for TypeScript, it would look like the following.

/* @jsxImportSource solid-js */
// ^ Alternatively, configure this in tsconfig.json instead of per-file.

import {createSignal} from 'solid-js'
import {div} from '@lume/element/dist/type-helpers.js'

const [count, setCount] = createSignal(0)

setInterval(() => setCount(count() + 1), 1000)

const el = div(
	<div>
		<h1 data-count={count()}>The count is: {count()}</h1>
	</div>,
)

el.setAttribute('foo', 'bar')

document.body.appendChild(el)

The main differences from plain JS are

Use of the @jsxImportSource comment to place JSX types into scope. This is required, or TypeScript will not know what the types of elements in JSX markup are. Alternative to comments, configure it in tsconfig.json's compilerOptions.
The div() helper function explicitly returns the type HTMLDivElement so that the el variable will be typed as HTMLDivElement instead of JSX.Element. Under the hood, the div() function is an identity function at runtime, it simply returns whatever you pass into it, and serves only as a convenient type cast helper.

[!Warning] Keep in mind to use the correct type helper depending on what the root element of the JSX expression is. For for example, if the root of a JSX expression is a <menu> element then we need to use the menu() helper like follows.

/* @jsxImportSource solid-js */
import {createSignal} from 'solid-js'
import {menu} from '@lume/element/dist/type-helpers.js'

// ...

// The type of `el` will be `HTMLMenuElement`.
const el = menu(
	<menu>
		<h1 data-count={count()}>The count is: {count()}</h1>
	</menu>,
)

If the wrong helper is used, then it will effectively cast the expression to the wrong type. For example, in the next snippet the el variable will be of type HTMLDivElement despite the fact that at runtime we will be have an HTMLMenuElement instance.

/* @jsxImportSource solid-js */
import {div, button} from '@lume/element/dist/type-helpers.js'

// GOOD.
const el = button(<button>...</button>)

// BAD! Don't do this! Remember to double check, because the helpers are not
// type safe, you will not get an error here, and el2 will incorrectly be type
// HTMLDivElement.
const el2 = div(<menu>...</menu>)

Without the type helpers, we would need to write more verbose code like the following to have the proper types, but note that the following is also not type safe:

/* @jsxImportSource solid-js */

// GOOD.
const el = (<menu>...</menu>) as any as HTMLMenuElement

// BAD! Don't do this! Remember to double check, because the helpers are not
// type safe, you will not get an error here.
const el2 = (<menu>...</menu>) as any as HTMLDivElement

Type definitions for custom elements

With Solid JSX

To give your Custom Elements type checking for use with DOM APIs, and type checking in Solid JSX, we can add the element type definition to JSX.IntrinsicElements:

/* @jsxImportSource solid-js */

// We already use @jsxImportSource above, but if you need to reference JSX
// anywhere in non-JSX parts of the code, you also need to import it from
// solid-js:
import {Element, element, stringAttribute, numberAttribute, /*...,*/ JSX} from 'solid-js'

// Define the attributes that your element accepts
export interface CoolElementAttributes extends JSX.HTMLAttributes<CoolElement> {
	'cool-type'?: 'beans' | 'hair'
	'cool-factor'?: number
	// ^ NOTE: These should be dash-case versions of your class's attribute properties.
}

@element('cool-element')
class CoolElement extends Element {
	@stringAttribute coolType: 'beans' | 'hair' = 'beans'
	@numberAttribute coolFactor = 100
	// ^ NOTE: These are the camelCase equivalents of the attributes defined above.

	// ... Define your class as described above. ...
}

export {CoolElement}

// Add your element to the list of known HTML elements. This makes it possible
// for browser APIs to have the expected return type. For example, the return
// type of `document.createElement('cool-element')` will be `CoolElement`.
declare global {
	interface HTMLElementTagNameMap {
		'cool-element': CoolElement
	}
}

// Also register the element name in the JSX types for TypeScript to recognize
// the element as a valid JSX tag.
declare module 'solid-js' {
	namespace JSX {
		interface IntrinsicElements {
			'cool-element': CoolElementAttributes
		}
	}
}

:bulb:TIP:
To make code less redundant, use the ElementAttributes helper to pluck the types of properties directly from your custom element class for the attribute types:

import type {ElementAttributes} from '@lume/element'

// This definition is now shorter than before, automatically maps the property
// names to dash-case, and automatically picks up the property types from the
// class.
export type CoolElementAttributes = ElementAttributes<CoolElement, 'coolType' | 'coolFactor'>

// The same as before:
declare module 'solid-js' {
	namespace JSX {
		interface IntrinsicElements {
			'cool-element': CoolElementAttributes
		}
	}
}

Now when you use <cool-element> in Solid JSX, it will be type checked:

return (
	<cool-element
		cool-type={123} // Type error: number is not assignable to 'beans' | 'hair'
		cool-factor={'foo'} // Type error: string is not assignable to number
	></cool-element>
)

With React JSX

Defining the types of custom elements for React JSX is similar as for Solid JSX above, but with some small differences for React JSX:

// tsconfig.json
{
  "compilerOptions": {
		/* React Config */
    "jsx": "react-jsx",
    "jsxImportSource": "react" // React >=19 (Omit for React <=18)
  }
}

import type {HTMLAttributes} from 'react'

// Define the attributes that your element accepts, almost the same as before:
export interface CoolElementAttributes extends HTMLAttributes<CoolElement> {
	coolType?: 'beans' | 'hair'
	coolFactor?: number
	// ^ NOTE: These are the names of the class's properties verbatim, not
	// dash-cased as with Solid. React works differently than Solid's: it will
	// map the exact prop name to the JS property.
}

// Add your element to the list of known HTML elements, like before.
declare global {
	interface HTMLElementTagNameMap {
		'cool-element': CoolElement
	}
}

// Also register the element name in the React JSX types, which are global in
// the case of React.
declare global {
	namespace JSX {
		interface IntrinsicElements {
			'cool-element': CoolElementAttributes
		}
	}
}

:bulb:TIP:
To make code less redundant, use the ReactElementAttributes helper to pluck the types of properties directly from your custom element class for the attribute types:

import type {ReactElementAttributes} from '@lume/element/dist/react'

// This definition is now shorter than before, and automatically maps the property names to dash-case.
export type CoolElementAttributes = ReactElementAttributes<CoolElement, 'coolType' | 'coolFactor'>

// The same as before:
declare global {
	namespace JSX {
		interface IntrinsicElements {
			'cool-element': CoolElementAttributes
		}
	}
}

Now when you use <cool-element> in React JSX, it will be type checked:

return (
	<cool-element
		coolType={123} // Type error: number is not assignable to 'beans' | 'hair'
		coolFactor={'foo'} // Type error: string is not assignable to number
	></cool-element>
)

[!Note] You may want to define React JSX types for your elements in separate files, and have only React users import those files if they need the types, and similar if you make JSX types for Vue, Svelte, etc (we don't have helpers for those other fameworks yet, but you can manually augment JSX as in the examples above on a per-framework basis, contributions welcome!).

With Preact JSX

It works the same as the previous section for React JSX. Define the element types with the same ReactElementAttributes helper as described above. In your TypeScript compilerOptions make sure you link to the React compatibility layer:

{
	"compilerOptions": {
		/* Preact Config */
		"jsx": "react-jsx",
		"jsxImportSource": "preact",
		"paths": {
			"react": ["./node_modules/preact/compat/"],
			"react-dom": ["./node_modules/preact/compat/"]
		}
	}
}

API

`Element`

A base class for custom elements made with @lume/element.

[!Note] The Element class from @lume/element extends from HTMLElement.
Safari does not support customized built-ins, and neither does @lume/element, so at the moment we do not support extending from other classes such as HTMLButtonElement, etc.

The Element class provides:

`template`

A subclass can define a .template that returns a DOM node, and this DOM node will be appened into the element's ShadowRoot by default, or to the element itself if .hasShadow is false.

One way to write a template is using Solid JSX syntax (this will always require a build step).

import {Element} from '@lume/element'
import {createSignalFunction} from 'classy-solid' // a small wrapper around Solid's createSignal that allows reading and writing from the same function.

class CoolElement extends Element {
	count = createSignalFunction(100)

	template = () => (
		<div>
			<span>The count is: {this.count()}!</span>
		</div>
	)
	// ...
}

customElements.define('cool-element', CoolElement)

Another way to write a template is using Solid's html template string tag (which does not require a build step). Using the following template, the example can run in a browser without a compile step (note, we're not using decorators yet):

// ...
template = () => html`
	<div>
		<span>The count is: ${this.count}!</span>
	</div>
`
// ...

Example on CodePen (with html template tag instead of JSX)

[!Note] When count changes, the template updates automatically.

We can also manually create DOM any other way, for example here we make and return a DOM tree using DOM APIs, and using a Solid effect to update the element when count changes (but you could have used React or jQuery, or anything else!):

// ...same...

import {createEffect} from 'solid-js'

// ...same...

// Replace the previous `template` with this one:
template = () => {
	const div = document.createElement('div')
	const span = document.createElement('span')
	div.append(span)

	createEffect(() => {
		// Automatically set the textContent whenever `count` changes (this is a
		// conceptually-simplified example of what Solid JSX compiles to).
		span.textContent = `The count is: ${this.count()}!`
	})

	return div
}

// ...same...

Example on CodePen

`static css`

Use the static css field to define a CSS string for styling all instances of the given class. A static property allows @lume/element to optimize by sharing a single CSSStyleSheet across all instances of the element, which could be beneficial for performance if you have many thousands of instances.

import {Element} from '@lume/element'

class CoolElement extends Element {
	template = () => <span>This is some DOM!</span>

	// Style is scoped to our element, this will only style the <span> inside our element.
	static css = `
    span { color: violet; }
  `
}

customElements.define('cool-element', CoolElement)

Example on CodePen (with html template tag instead of JSX)

The static css property can also be a function:

// ...

class CoolElement extends Element {
	// ...
	static css = () => {
		const color = 'limegreen'

		return `
      span { color: ${color}; }
    `
	}
	// ...
}

Example on CodePen (with html template tag instead of JSX)

:bulb:Tip:
Use the css identity template tag to enable syntax highlighting and code formatting in some IDEs:

import {css} from '@lume/element'
// ...

class CoolElement extends Element {
	// ...
	static css = css`
		span {
			color: cornflowerblue;
		}
	`
	// ...
}

`css`

Use the non-static css property to define styles that are applied per instance of the given element. This is useful for style that should differ across instances. This will not be as optimized as static css will be because it will create one stylesheet per element instance, but the performance difference will not matter for most use cases.

import {Element, css} from '@lume/element'

class CoolElement extends Element {
	template = () => <span>This is some DOM!</span>

	// A random color per instance.
	#color = `hsl(calc(${Math.random()} * 360) 50% 50%)`

	// Style is scoped to our element, this will only style the <span> inside our element.
	css = css`
		span {
			color: ${this.#color};
		}
	`
}

Example on CodePen (with html template tag instead of JSX)

`connectedCallback`

Nothing new here, this is simply a part of the browser's native Custom Elements connectedCallback API. It is triggered when the element is connected into the document. Use it to create things.

import {Element} from '@lume/element'

class CoolElement extends Element {
	connectedCallback() {
		// Don't forget to call the super method from the Element class!
		super.connectedCallback()

		// ...Create things...
	}
	// ...
}

`disconnectedCallback`

Nothing new here, this is simply a part of the browser's native Custom Elements disconnectedCallback API. It is triggered when the element is disconnected from the document. Use it to clean things up.

import {Element} from '@lume/element'

class CoolElement extends Element {
	disconnectedCallback() {
		// Don't forget to call the super method from the Element class!
		super.disconnectedCallback()

		// ...Clean things up...
	}
	// ...
}

`adoptedCallback`

Nothing new here, this is simply a part of the browser's native Custom Elements adoptedCallback API. It is triggered when the element is adopted into a new document (f.e. in an iframe).

import {Element} from '@lume/element'

class CoolElement extends Element {
	adoptedCallback() {
		// Don't forget to call the super method from the Element class!
		super.adoptedCallback()

		// ...Do something when the element was transferred into another window's or iframe's document...
	}
	// ...
}

`attributeChangedCallback`

Nothing new here, this is simply a part of the browser's native Custom Elements attributeChangedCallback API. It is triggered when an observed attribute of the element is added, modified, or removed.

import {Element} from '@lume/element'

class CoolElement extends Element {
	static observedAttributes = ['foo', 'bar']

	attributeChangedCallback(attributeName, oldValue, newValue) {
		// Don't forget to call the super method from the Element class!
		super.attributeChangedCallback(attributeName, oldValue, newValue)

		// Attribute name is the name of the attribute change changed.
		// If `oldValue` is `null` and `newValue` is a string, it means the attribute was added.
		// If `oldValue` and `newValue` are both strings, it means the value changed.
		// If `oldValue` is a string and `newValue` is `null`, it means the attribute was removed.
	}
	// ...
}

[!Warning] The static observedAttributes property is required for observing attributes, and specifies which attributes will trigger attributeChangedCallback. attributeChangedCallback will not be triggered for any attributes that are not listed in static observedAttributes!

`static observedAttributes`

Nothing new here, this is simply a part of the browser's native Custom Elements static observedAttributes API. It defines which attributes will be observed. From the previous example:

class CoolElement extends Element {
	static observedAttributes = ['foo', 'bar']
	// ...
}

`static observedAttributeHandlers`

As an alternative to static observedAttributes, and mainly for non-decorator users (because not all JS engines support them yet at time of writing this), observed attributes can be defined with static observedAttributeHandlers, a map of attribute names to attribute handlers. This requires using the @element decorator (calling it as a plain function for non-decorator usage). This will map attributes to JS properties and make the JS properties reactive.

Each value in the observedAttributeHandlers object has the following shape:

/**
 * Defines how values are mapped from an attribute to a JS property on a custom
 * element class.
 */
export type AttributeHandler<T = any> = {
	// TODO `to` handler currently does nothing. If it is present, then prop
	// changes should reflect back to the attribute. This will add a performance
	// hit.
	to?: (propValue: T) => string | null

	/**
	 * Define how to deserialize an attribute string value on its way to the
	 * respective JS property.
	 *
	 * If not defined, the attribute string value is passed to the JS property
	 * untouched.
	 */
	from?: (AttributeValue: string) => T

	/**
	 * The default value that the respective JS property should have when the
	 * attribute is removed.
	 *
	 * If not initially defined, this will be defined to whatever the initial JS
	 * property value is.
	 *
	 * When explicitly defined, an attribute's respective JS property will be set to this
	 * value when the attribute is removed, even if that is different than the JS property's initial value.
	 *
	 * If this is not explicitly defined, and the JS property has no initial
	 * value, then the JS property will receive `undefined` when the attribute is
	 * removed which matches the initial value of the JS property (this is not
	 * ideal, especially in TypeScript, you should provide initial JS property
	 * values so that shapes of your elements are well defined), just like
	 * `attributeChangedCallback` does.
	 */
	default?: T
}

Here's an example of an element definition with no decorators, with HTML attributes mapped to same-name JS properties:

import {Element, element} from '@lume/element'

element('cool-element')(
	class CoolElement extends Element {
		static observedAttributeHandlers = {
			foo: {from: Number},
			bar: {from: Boolean},
		}

		// Due to the `observedAttributeHandlers` definition, any time the `"foo"` attribute
		// on the element changes, the attribute string value will be converted into a
		// `Number` and assigned to the JS `.foo` property.
		// Not only does `.foo` have an initial value of `123`, but when the element's
		// `"foo"` attribute is removed, `.foo` will be set back to the initial value
		// of `123`.
		foo = 123

		// Due to the `observedAttributeHandlers` definition, any time the `"bar"` attribute
		// on the element changes, the attribute string value will be converted into a
		// `Boolean` and assigned to the JS `.bar` property.
		// Not only does `.bar` have an initial value of `123`, but when the element's
		// `"bar"` attribute is removed, `.bar` will be set back to the initial value
		// of `false`.
		bar = false

		// ...
	},
)

Example on CodePen

@lume/element comes with a set of basic handlers available out of the box, each of which are alternatives to a respective set of included decorators:

import {Element, element, attribute} from '@lume/element'

element('cool-element')(
	class CoolElement extends Element {
		static observedAttributeHandlers = {
			lorem: {}, // Effectively the same as attribute.string()
			foo: attribute.string(), // Effectively the same as the @stringAttribute decorator. Values get passed to the JS property as strings.
			bar: attribute.number(), // Effectively the same as the @numberAttribute decorator. Values get passed to the JS property as numbers.
			baz: attribute.boolean(), // Effectively the same as the @booleanAttribute decorator. Values get passed to the JS property as booleans.

			// Here we define an attribute with custom handling of the string value, in this case making it accept a JSON string that maps it to a parsed object on the JS property.
			bespoke: {from: value => JSON.parse(value)}, // f.e. besoke='{"b": true}' results in the JS property having the value `{b: true}`
		}

		// The initial values of the JS properties define the values that the JS properties get reset back to when the corresponding attributes are removed.
		lorem = 'hello'
		foo = 'world'
		bar = 123
		baz = false
		bespoke = {n: 123}

		// ...
	},
)

Example on CodePen

If you have decorator support (either with a build, or natively in near-future JS engines), defining attributes with decorators is simpler and more concise:

import {Element, element, numberAttribute, booleanAttribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	// Due to the `@numberAttribute` decorator, any time the `"foo"` attribute
	// on the element changes, the attribute string value will be converted into a
	// `Number` and assigned to the JS `.foo` property.
	// Not only does `.foo` have an initial value of `123`, but when the element's
	// `"foo"` attribute is removed, `.foo` will be set back to the initial value
	// of `123`.
	@numberAttribute foo = 123

	// Due to the `@booleanAttribute` decorator, any time the `"bar"` attribute
	// on the element changes, the attribute string value will be converted into a
	// `Boolean` and assigned to the JS `.bar` property.
	// Not only does `.bar` have an initial value of `true`, but when the element's
	// `"bar"` attribute is removed, `.bar` will be set back to the initial value
	// of `true`.
	@booleanAttribute bar = true

	// ...
}

[!Note] Not only do decorators make the definition more concise, but they avoid surface area for human error: the non-decorator form requires defining the same-name property in both the observedAttributeHandlers object and in the class fields, and if you miss one or the other then things might not work as expected.

Each of the available decorators are detailed further below.

Decorators, and the observedAttributeHandlers object format, both work with getter/setter properties as well:

import {Element, element, numberAttribute, booleanAttribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	#foo = 123

	// Like with class fields, the initial value is 123, so when the "foo"
	// attribute is removed the setter will receive 123.
	@numberAttribute
	get foo() {
		return this.#foo
	}
	set foo(v) {
		this.#foo = v
	}
	// ...
}

Auto accessors are not supported yet. If there is enough demand for them, we'll add support and they will look like so:

@element('cool-element')
class CoolElement extends Element {
	// The same rules with initial values and attribute removal will apply.
	@numberAttribute accessor foo = 123
	@booleanAttribute accessor bar = false

	// ...
}

`createEffect`

The createEffect method is a wrapper around Solid's createEffect with some differences for convenience:

createRoot is not required in order to dispose of effects created with this.createEffect()
Effects created with this.createEffect() will automatically be cleaned up when the element is disconnected.
Besides being useful for re-running logic on signals changes, this.createEffect() is useful as an alternative to disconnectedCallback when paired with Solid's onCleanup.

import {Element} from '@lume/element'
import {createSignal, onCleanup} from 'solid-js'

const [count, setCount] = createSignal(0)

setInterval(() => setCount(n => ++n), 1000)

class CoolElement extends Element {
	connectedCallback() {
		super.connectedCallback()

		// Log `count()` any time it changes.
		this.createEffect(() => console.log(count()))

		this.createEffect(() => {
			const interval1 = setInterval(() => console.log('interval 1'), 1000)
			onCleanup(() => clearInterval(interval1))

			const interval2 = setInterval(() => console.log('interval 2'), 1000)
			onCleanup(() => clearInterval(interval2))
		})
	}
}

customElements.define('cool-element', CoolElement)

// After removing the element, onCleanup fires and cleans up the intervals created in connectedCallback (not the count interval outside the element)
setTimeout(() => {
	const el = document.querySelector('cool-element')
	el.remove()
}, 2000)

Example on CodePen

Compare that to using disconnectedCallback:

import {Element} from '@lume/element'
import {createSignal, onCleanup} from 'solid-js'

const [count, setCount] = createSignal(0)

setInterval(() => setCount(n => ++n), 1000)

class CoolElement extends Element {
	#interval1 = 0
	#interval2 = 0

	connectedCallback() {
		super.connectedCallback()

		// Log `count()` any time it changes.
		this.createEffect(() => console.log(count()))

		this.#interval1 = setInterval(() => console.log('interval 1'), 1000)
		this.#interval2 = setInterval(() => console.log('interval 2'), 1000)
	}

	disconnectedCallback() {
		super.disconnectedCallback()

		clearInterval(this.#interval1)
		clearInterval(this.#interval2)
	}
}

customElements.define('cool-element', CoolElement)

:bulb:Tip:
Prefer onCleanup instead of disconnectedCallback because composition of logic will be easier while also keeping it co-located and easier to read. That example is simple, but when logic grows, having to clean things up in disconnectedCallback can get more complicated, especially when each piece of creation logic and cleanup logic is multiple lines long and interleaving them would be harder to read. Plus, putting them in effects makes them creatable+cleanable if signals in the effects change, not just if the element is connected or disconnected. For example, the following element cleans up the interval any time the signal changes, not only on disconnect:

import {Element} from '@lume/element'
import {createSignal, onCleanup} from 'solid-js'

const [count, setCount] = createSignal(0)

setInterval(() => setCount(n => ++n), 1000)

class CoolElement extends Element {
	connectedCallback() {
		super.connectedCallback()

		// Log `count()` any time it changes.
		this.createEffect(() => console.log(count()))

		this.createEffect(() => {
			// Run the interval only during moments that count() is an even number.
			// Whenever count() is odd, the running interval will be cleaned up and a new interval will not be created.
			// Also, when the element is disconnected (while count() is even), the interval will be cleaned up.
			if (count() % 2 !== 0) return
			const interval = setInterval(() => console.log('interval'), 100)
			onCleanup(() => clearInterval(interval))
		})
	}
}

customElements.define('cool-element', CoolElement)

// After removing the element, onCleanup fires and cleans up any interval currently created in connectedCallback (not the count interval outside the element)
setTimeout(() => {
	const el = document.querySelector('cool-element')
	el.remove()
}, 2500)

Example on CodePen

The beauty of this you can write logic based on signals, and not worry about disconnectedCallback, you'll rest assured things clean up properly.

`static elementName`

The default tag name of the elements this class instantiates. When using the @element decorator, this property will be set to the value defined by the decorator.

@element
class SomeEl extends LumeElement {
	static elementName = 'some-el'
}

SomeEl.defineElement() // defines <some-el> with the SomeEl class

Example on CodePen

`static defineElement`

Define this class for the given element name, or using its default name (TheClass.elementName) if no was name given and the element was not already defined using the @element decorator. Defaults to using the global customElements registry unless another registry is provided (for example a ShadowRoot-scoped registry) as a second argument.

@element('some-el') // defines <some-el> with the decorated class
class SomeEl extends LumeElement {}

const OtherEl = SomeEl.defineElement('other-el') // defines <other-el> with an empty subclass of SomeEl
console.log(OtherEl === SomeEl) // false

@element // without a name, the decorator does not perform the element definition
class AnotherEl extends LumeElement {}

const El = AnotherEl.defineElement('another-el') // defines <another-el>
console.log(El === AnotherEl) // true
const El2 = AnotherEl.defineElement('yet-another-el') // defines <yet-another-el>
console.log(El2 === AnotherEl) // false

If the class is already registered with another name, then the class will be extended with an empty subclass so that a new class is used for the new name, because a CustomElementRegistry does not allow the same class reference to be used more than once regardless of the name.

Returns the defined element class, which may be a different subclass of the class this is called on if the class this is called on is already associated with another name, otherwise returns the same class this is called on.

Example on CodePen

`hasShadow`

When true, the custom element will have a ShadowRoot. Set to false to not use a ShadowRoot. When false, styles will not be scoped via the built-in ShadowRoot scoping mechanism, but by a much more simple shared style sheet placed at the nearest root node, with :host selectors converted to tag names.

@element('some-el')
class SomeEl extends Element {
	hasShadow = false

	template = () => html`<div>hello</div>`
}

The template content will be appended to the SomeEl instance directly, with no ShadowRoot:

<some-el id="el"></some-el>
<script>
	const el = document.getElementById('el')
	console.log(el.shadowRoot) // null
	console.log(el.children[0]) // <div>hello</div>
</script>

Example on CodePen

[!Note] Note that without a ShadowRoot, <slot> no longer works because it must be inside a ShadowRoot, therefore going without a ShadowRoot is useful moreso for elements that are leafs at the end of your DOM tree branches and elements that will not slot accept any children and will only have template content as their children.

`root`

Deprecated, renamed to templateRoot.

`templateRoot`

Subclasses can override the templateRoot property to provide an alternate Node for template content to be placed into (f.e. a subclass can set it to this to have template content appended to itself regardless of the value of hasShadow).

A primary use case for this is customizing the ShadowRoot:

@element('some-el')
class SomeEl extends Element {
	// Create the element's ShadowRoot with custom options for example:
	templateRoot = this.attachShadow({
		mode: 'closed',
	})

	template = () => html`<div>hello</div>`
}

Example on CodePen

`shadowOptions`

Define a shadowOptions property to specify any options for the element's ShadowRoot. These options are passed to attachShadow(). This is a simpler alternative to overriding templateRoot in the previous example.

@element('some-el')
class SomeEl extends Element {
	shadowOptions = {mode: 'closed'}

	template = () => html`<div>hello</div>`
}

`styleRoot`

Similar to the previous templateRoot, this defines which Node to append style sheets to when hasShadow is true. This is ignored if hasShadow is false. It defaults to this.templateRoot, which in turn defaults to the element's ShadowRoot.

When hasShadow is true, an alternate styleRoot is sometimes desired so that styles will be appended elsewhere than the templateRoot. To customize this, override it in your class:

@element('some-el')
class SomeEl extends Element {
	styleRoot = document.createElement('div')

	template = () => html`
		<div>
			<div>${this.styleRoot}</div>

			<span>hello</span>
		</div>
	`
}

Example on CodePen

This can be useful for fixing issues where the default append location of an element's style sheet into the ShadowRoot conflicts with how DOM is created in template (f.e. if the user's DOM creation in template clears the ShadowRoot content, or etc, then the user may want to place the stylesheet somewhere else).

Decorators

Using decorators (if available in your build, or natively in your JS engine) instead of observedAttributeHandlers is more concise and less error prone. Here's the list of included attribute decorators and the attribute handler equivalents:

Use @stringAttribute foo in place of foo: {}
Use @stringAttribute foo in place of foo: attribute.string()
Use @numberAttribute foo in place of foo: attribute.number()
Use @booleanAttribute foo in place of foo: attribute.boolean()

[!Warning] When using attribute decorators, the @element decorator is also required on the class, or the attribute decorators won't work.

Below are more details on each decorator:

`@element`

The star of the show, a decorator for defining a custom element.

When passed a string, it will be the element's tag name:

import {Element, element} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	// ...
}

[!Note] Make sure you extend from the Element base class from @lume/element when using the @element decorator.

When not passed a string, the element will not be defined (while reactivity features will still be applied), and customElements.define should be used manually, which can be useful for upcoming scoped registries:

import {Element, element} from '@lume/element'

@element
class CoolElement extends Element {
	// ...
}

customElements.define('cool-element', CoolElement)
// or
const myRegistry = new CustomElementRegistry()
myRegistry.define('cool-element', CoolElement)

Finally, even if passed a string for the element name, a second boolean option can disable automatic definition, and in this case the constructor's .defineElement() method can be used to trigger the definition using the given name:

import {Element, element} from '@lume/element'

const autoDefine = false

@element('cool-element', autoDefine)
class CoolElement extends Element {
	// ...
}

CoolElement.defineElement() // defines <cool-element>

A custom name can be passed to .defineElement() too:

CoolElement.defineElement('other-element') // defines <other-element> (even if `<cool-element>` is already defined)

`@attribute`

A decorator for defining a generic element attribute. The name of the property is mapped from camelCase to dash-case.

The @attribute decorator is effectively the same as the @stringAttribute decorator.

import {Element, element, attribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@attribute firstName = null // the attribute name is first-name
	// ...
}

When an attribute is removed, the JS property will receive the default value determined by the initial value of the JS property.

Sample usage of the attribute from the outside:

const el = document.querySelector('cool-element')

el.setAttribute('first-name', 'Superman')
console.log(el.firstName) // logs "Superman"

el.removeAttribute('first-name')
console.log(el.firstName) // logs null

Had we defined a different initial value,

	@attribute firstName = 'Batman'

then removing the attribute would have set the JS property back to that non-null value:

const el = document.querySelector('cool-element')

el.setAttribute('first-name', 'Superman')
console.log(el.firstName) // logs "Superman"

el.removeAttribute('first-name')
console.log(el.firstName) // logs "Batman"

This is great because the intial values that you see in the class definition are always the expected values when the element has no attributes or when all attributes are removed, making the outcome predictable and consistent.

For TypeScript, you'll want the property type to be at least string | null if the initial value is null, as removing the attribute will set the JS property back to null.

import {Element, element, attribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@attribute firstName: string | null = null
	// ...
}

For TypeScript, if the initial value is a string, then no type annotation is needed because it will always receive a string (f.e. even when the attribute is removed) and the type will be inferred from the initial value:

import {Element, element, attribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@attribute firstName = 'Batman' // always a `string`
	// ...
}

You can of course make a broader type that accepts a string from the element attribute, but also other types via the JS property directly:

import {Element, element, attribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@attribute firstName: string | number = 'Batman'
	// ...
}

const el = document.querySelector('cool-element')

el.firstName = 123 // ok

[!Note] Assigning any other value will work because @attribute does not do any coercion, it just accepts values as-is. But properties with specific attribute type decorators, f.e. @numberAttribute below, will coerce values, and their type should be defined accordingly.

`@stringAttribute`

The @stringAttribute decorator is effectively the same as the @attribute decorator.

`@numberAttribute`

A decorator that defines an attribute that accepts a number. Any value the attribute receives will be passed to the JS property. The JS property will convert a null value (attribute removed) to the default value defined by the initial property value, and will convert any string into a number (it will be NaN if the string is invalid).

import {Element, element, numberAttribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@numberAttribute age = 10
	// ...
}

const el = document.querySelector('cool-element')

el.setAttribute('age', '20')
console.log(el.age) // logs 20
console.log(typeof el.age) // logs "number"

el.removeAttribute('age')
console.log(el.age) // logs 10
console.log(typeof el.age) // logs "number"

el.age = '30' // assign a string
console.log(el.age) // logs 30
console.log(typeof el.age) // logs "number"

For TypeScript, you don't need a type annotation if the initial value is a number. Add a type annotation only if you use a non-number initial value, f.e. number | SomeOtherType, but that's not a recommended practice:

import {Element, element, numberAttribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@numberAttribute age: 'ten' | number = 'ten'
	// ...
}

const el = document.querySelector('cool-element')

el.setAttribute('age', '20')
console.log(el.age) // logs 20
console.log(typeof el.age) // logs "number"

el.removeAttribute('age')
console.log(el.age) // logs "ten"
console.log(typeof el.age) // logs "string"

`@booleanAttribute`

A decorator that defines a boolean attribute. Any value the attribute receives will be passed to the JS property. The JS property will convert a null value (attribute removed) to the default value defined by the initial property value, and will convert any string into boolean. All string values except "false" result in true, and the string "false" results in false. Additionally, a null value (attribute removed) results in false.

To mimick the same behavior as boolean attributes on built-in elements where the presence of the attribute is true, and absence of the attribute is false, start with an initial value of false:

import {Element, element, booleanAttribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@booleanAttribute hasPizza = false
	// ...
}

If the attribute value exists, the JS property will receive true, except if the value of the attribute is explicitly "false":

const el = document.querySelector('cool-element')

el.setAttribute('has-pizza', '')
console.log(el.age) // logs true
console.log(typeof el.age) // logs "boolean"

el.setAttribute('has-pizza', 'blah blah')
console.log(el.age) // logs true
console.log(typeof el.age) // logs "boolean"

el.removeAttribute('has-pizza')
console.log(el.age) // logs false
console.log(typeof el.age) // logs "boolean"

// A literal "false" value is special and is treated as false.
el.setAttribute('has-pizza', 'false')
console.log(el.age) // logs false
console.log(typeof el.age) // logs "boolean"

Here is the equivalent example in HTML describing the values of has-pizza:

<!-- hasPizza == true -->
<cool-element has-pizza></cool-element>

<!-- hasPizza == true -->
<cool-element has-pizza="true"></cool-element>

<!-- hasPizza == true -->
<cool-element has-pizza="blah blah"></cool-element>

<!-- hasPizza == false -->
<cool-element></cool-element>

<!-- hasPizza == false -->
<cool-element has-pizza="false"></cool-element>

If you start with an initial value of true, then when the attribute is removed or never existed, the JS property will always be true, which again is useful for predictability of default state.

import {Element, element, booleanAttribute} from '@lume/element'

@element('cool-element')
class CoolElement extends Element {
	@booleanAttribute hasPizza = true
	// ...
}

In this case, only an attribute value of "false" can set the JS property to false:

const el = document.querySelector('cool-element')

el.setAttribute('has-pizza', '')
console.log(el.age) // logs true
console.log(typeof el.age) // logs "boolean"

el.setAttribute('has-pizza', 'blah blah')
console.log(el.age) // logs true
console.log(typeof el.age) // logs "boolean"

el.removeAttribute('has-pizza')
console.log(el.age) // logs true
console.log(typeof el.age) // logs "boolean"

// Only a literal value of "false" will cause the JS property to be false in this case.
el.setAttribute('has-pizza', 'false')
console.log(el.age) // logs false
console.log(typeof el.age) // logs "boolean"

Equivalent HTML:

<!-- hasPizza == true -->
<cool-element has-pizza></cool-element>

<!-- hasPizza == true -->
<cool-element has-pizza="foo"></cool-element>

<!-- hasPizza == true -->
<cool-element has-pizza="blah blah"></cool-element>

<!-- hasPizza == true -->
<cool-element></cool-element>

<!-- hasPizza == false -->
<cool-element has-pizza="false"></cool-element>

:bulb:Tip:
Avoid attribute values like has-pizza="blah blah", because they are not semantic. Use the form has-pizza for true or no attribute for false when the default/initial value is false. Use the form has-pizza="true" and has-pizza="false" to be explicit especially when the initial value is true.

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