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@lume/element

Easily and concisely write Custom Elements with simple templates and reactivity.

Use the custom elements on their own in plain HTML or vanilla JavaScript, or in Vue, Svelte, Solid.js, Stencil.js, React, and Preact, with full type checking, autocompletion, and intellisense in all the template systems of those frameworks, in any IDE that supports TypeScript such as VS Code.

Write your elements once, then use them in any app, with a complete developer experience no matter which base component system your app uses.

npm install @lume/element

💡Tip:

If you are new to Custom Elements, first learn about the basics of 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 that is needed.

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.

JSX can be used for the template of an element, 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 web apps into separately and sometimes 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.
A more detailed feature description:
  • 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.
    • Decorators 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 browsers 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 variables 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 step.
    • 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 (expensive) 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.
  • 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.

Install and Setup

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

CDN method (easiest, no compiler or command line needed)

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).

Local install with build

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

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

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

npm install @lume/element

In order to use decorators today (recommended), we 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'd like to use the HTML-like markup inside JavaScript known as "JSX", instead of Solid's html template tag which requires no build, the babel-preset-solid package will also be needed:

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 compiling decorators with TypeScript, Babel is still needed for Solid JSX because TypeScript does not compile JSX into Solid.js format, only into React format. Either compile decorators with TypeScript and have Babel compile JSX in a second step, or 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. The Decorators section has details on each decorator available.

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
  eventAttribute, // A property decorator that causes values from the attributes/properties to be set as an event listener for the respective event, just like built-in "onclick" attributes/properties.
  css, // A no-op identity template tag function (useful to enable CSS syntax highlighting in various text editors)

  // Decorators for defining specific attributes types (string values are coerced to the respective JS type):
  stringAttribute,
  numberAttribute,
  booleanAttribute,
} 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'

  // Specific attribute types (the JS property will always be of the specified
  // type):
  @stringAttribute someString = ''
  @numberAttribute someNumber = 123
  @booleanAttribute someBoolean = false

  // Define event properties to specify which events the element dispatches.
  // Besides being useful for type definitions in JSX, these properties work
  // like the builtin event properties such as "onclick" (JS property or DOM
  // attribute code string).
  //
  // For example, a user can write `el.onhello = event => {...}` just like
  // they can do with builtin event properties like `el.onclick = event =>
  // {...}`.
  @eventAttribute onhello = null

  // 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 this 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 the element's content due to ShadowRoot style encapsulation.
  static css = css`
    :host {
      background: skyblue;
    }
    div {
      color: pink;
    }
  `

  // For instance-specific styling, use the 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)

    // The element may dispatch events.
    setTimeout(() => this.dispatchEvent(new Event('hello')), 3000)
  }

  // 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" onhello={() => console.log(event.target.firstName, 'says hello')}></greeting-card>

Example on CodePen (without decorators, with Solid's 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

Lume Element is built on Solid.js, so we can also use Solid.js at the top level of a module for example. 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 is a `<div>` element! For example,
// we can call regular DOM APIs like `setAttribute` on it.
el.setAttribute('foo', 'bar')

// Append the element to the body of the page, and now we'll see a
// continually-updating message on the 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 plain Solid.js, and functional components (Solid.js components) can be used 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>
)

// The `Greeting` function only needs to be called once, and it will return a
// reference to an element or multiple elements. The `Greeting` function does
// NOT need to be called over and over to re-render like in some other libraries
// (for example React). 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` will be a reference to an actual `<section>` element that the
// `Greeting` function returned.
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 (with Solid's html template tag instead of JSX)

Using 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, with Solid's html template tag instead of JSX)

Functional components vs custom elements

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

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 are not compatible with HTML, React, Vue, Angular, Svelte, or all the other web libraries and frameworks. For portability across applications and frameworks, this is where custom elements shine.

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.

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 we 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 there are 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)

💡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']
  // ...
}

Note! Although static observedAttributes works, it is recommended to use the static observedAttributeHandlers property instead:

static observedAttributeHandlers

This is an alternative to attribute decorators (recommended, see the Decorators docs below), and will be removed after decorators are supported natively in JS engines.

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.

static observedAttributeHandlers is an object where each key is a property name to be associated with an attribute, and each value is an object with 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 The `to` handler currently does nothing. In the future, if there is demand
  // for it, this will be for property-to-attribute reflection.
  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.
   *
   * **Default when omitted:** `value => value`
   */
  from?: (AttributeValue: string) => T

  /**
   * A side effect to run when the value is set on the JS property. It also
   * runs on with the initial value. Avoid this if you can, and instead use
   * effects. One use case of this is to call addEventListener with event
   * listener values, just like with native `.on*` properties.
   *
   * **Default when omitted:** `() => {}` (no sideeffect)
   */
  sideEffect?: (instance: Element, prop: string, propValue: T) => void

  /**
   * @deprecated - Define a field with the initial value instead of providing
   * the initial value here. When decorators land in browsers, this will be
   * removed.
   *
   * The default value that the respective JS property should have when the
   * attribute is removed.
   *
   * If this is not specified, and the respective class field is defined, it
   * will default to the initial value of the class field.  If this is
   * specified, it will take precedence over the respective field's initial
   * value. This should generally be avoided, and the class field initial
   * value should be relied on as the source of the default value.
   *
   * When defined, an attribute's respective JS property will be set to this
   * value when the attribute is removed. If not defined, then the JS property
   * will always receive the initial value of the respective JS class field or
   * `undefined` if the field was not defined (that's the "initial value" of
   * the field), when the attribute is removed.
   *
   * **Default when omitted:** the value of the respective class field, or
   * `undefined` if the field was not defined.
   */
  default?: T

  /**
   * Whether to convert the property name to dash-case for the attribute name.
   * This option is ignore if the `name` option is set.
   *
   * The default is `true`, where the attribute name will be the same as the
   * property name but dash-cased (and all lower case). For example, `fooBar`
   * becomes `foo-bar` and `foo-bar` stays `foo-bar`.
   *
   * If this is set to `false`, the attribute name will be the same as the
   * property name, but all lowercased (attributes are case insensitive). For
   * example `fooBar` becomes `foobar` and `foo-bar` stays `foo-bar`.
   *
   * Note! Using this option to make a non-standard prop-attribute mapping
   * will result in template type definitions (f.e. in JSX) missing the
   * customized attribute names and will require custom type definition
   * management.
   *
   * **Default when omitted:** `true`
   */
  dashcase?: boolean

  /**
   * The name of the attribute to use. Use of this options bad practice to be
   * avoided, but it may be useful in rare cases.
   *
   * If this is not specified, see `dashcase` for how the attribute name is
   * derived from the property name.
   *
   * Note! Using this option to make a non-standard prop-attribute mapping
   * will result in template type definitions (f.e. in JSX) missing the
   * customized attribute names and will require custom type definition
   * management.
   *
   * **Default when omitted:** the attribute name derived from the property
   * name, converted to dash-case based on the `dashcase` option.
   */
  name?: string

  /**
   * Whether to suppress warnings about the attribute attribute name clashes
   * when not using default `dashcase` and `name` settings. This is
   * discouraged, and should only be used when you know what you're doing,
   * such as overriding a property that has `dashcase` set to `false` or
   * `name` set to the same name as the attribue of another property.
   *
   * **Default when omitted:** `false`
   */
  noWarn?: boolean
}

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 decorator support is present (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 static observedAttributeHandlers object and in the class fields, and if we miss one or the other then things might not work as expected.

Each of the available decorators are detailed further below.

Decorators, and the static 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
  }
  // ...
}

They also work with "auto accessors", which creates a prototype getter/setter:

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

  // ...
}

It may be redundant to write accessor repeatedly for each property when the alternative non-accessor format works too. The accessor format can be a fallback in very rare cases where a performance boost is needed (for example thousands of objects with many non-accessor properties being instantiated all at once). Most likely there will be other performance issues at the point in which we have thousands of elements being instantiated at once causing an any issues.

events with static observedAttributeHandlers

This is an alternative for the @eventAttribute decorator (recommended, see the @eventAttribute docs below), and will be removed after native support for decorators lands in JS engines.

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

const SomeEl = element('some-el')(
  class extends Element {
    static observedAttributeHandlers = {
      onjump: attribute.event(),
    }

    // Also define the property explicitly (here with an optional type definition).
    /** @type {EventListener | null} */
    onjump = null

    connectedCallback() {
      super.connectedCallback()

      // This element dispatches a "jump" event every second:
      setInterval(() => this.dispatchEvent(new Event('jump')), 1000)
    }
  },
)

const el = new SomeEl()

el.onjump = () => console.log('jump!')
// or, as with "onclick" and other built-in attributes:
el.setAttribute('onjump', "console.log('jump!')")

document.body.append(el)

// "jump!" will be logged every second.

Note that for TypeScript JSX types (TSX), we want to also define event properties on the class, for example onjump in the last example. Any properties that start with on will be mapped to on-prefixed JSX props for type checking. See the TypeScript section for more info.

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 DOM tree branches and elements that will not accept any slotted children and will only have template content as their children.

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:

@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).

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)

💡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 is we can write logic based on signals, without worrying about disconnectedCallback, and we'll rest assured things clean up properly. Cleanup logic is co-located with the pieces they are relevant to, which opens the door to powerful compositional patterns...

Decorators

Using decorators (if available in your build, or natively in your JS engine) instead of static observedAttributeHandlers or static events 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 as well. 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 we 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 more predictable and consistent.

For TypeScript, if the initial value is a string and we're using @attribute (or @stringAttribute), 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:

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

You could of course make the string type more specific,

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

but note that this does not prevent any string value being set via the attribute.

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, but you'd generally want to avoid this, unless you're using a getter/setter to coerce setter values into a single consistent type that the getter always returns (like how the builtin el.style= can accept a string but the return value of el.style is always an object), or the user's input is always unchanged and mapped separately to internal structures:

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

The @attribute decorator is also useful for defining custom handling of attributes. For example, the following shows how we can define an attribute that can accept JSON string values by providing an AttributeHandler definition, using the from option to define how string values from the attribute are coerced when they are assigned to the JS property:

// Here, `attribute` is not called as a decorator. When `attribute` is given an
// argument that defines how to handle an attribute, it will return a new
// decorator function.
const jsonAttribute = attribute({from: str => JSON.parse(str)})

Now we can use the new jsonAttribute decorator in an element class:

@element('cool-element')
class CoolElement extends Element {
  @jsonAttribute someValue = {foo: 123}
  // ...
}

Now in HTML it can accept a JSON string:

<cool-element id="el" some-value='{"foo": 456}'></cool-element>
<script>
  console.log(el.someValue) // logs the object {foo: 456}
  el.setAttribute('foo', '{"foo": 789}')
  console.log(el.someValue) // logs the object {foo: 789}
</script>

Note that we could have used attribute result as a decorator directly,

@element('cool-element')
class CoolElement extends Element {
  @attribute({from: str => JSON.parse(str)}) someValue = {foo: 123}
  // ...
}

but then the result would not have been saved into a re-usable jsonAttribute variable, and the class field definition would have been a little messier to read.

What new attribute decorators will you make? A @stringEnumAttribute that accepts only certain string values otherwise throws an error? A @cssColorAttribute that accepts only CSS-format color strings otherwise throws an error? A @threeColorAttribute that coerces CSS color values into Three.js Color objects? The sky is not the limit!

@stringAttribute

The @stringAttribute decorator is effectively the same as the @attribute decorator, but without the ability to accept arguments to define new attribute decorators. See the previous section.

This is preferable over plain @attribute for keeping the class definition semantic and clear. Prefer using @attribute for custom attribute types that are not supported out of the box.

@numberAttribute

A decorator that defines an attribute that accepts a number. Any value the attribute receives will be passed to the JS property, which is then coerced into a number with parseFloat. 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 (if the string is invalid the property value will result in NaN).

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 (type error in TypeScript)
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 is not recommended:

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"

el.age = 'ten'
console.log(el.age) // logs "NaN", which is confusing (hence, avoid doing this).
console.log(typeof el.age) // logs "number"

@booleanAttribute

A decorator that defines a boolean attribute. Any value the attribute receives will be passed to the JS property, which is then coerced into a boolean. 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 the boolean true, and the string "false" results in the boolean 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>

The purpose of treating "false" as explicitly false is that this makes it possible to have the attribute be present while still being able to express both values,

<cool-element has-pizza="true"></cool-element> <cool-element has-pizza="false"></cool-element>

while also having the option to express the same thing using only attribute presence:

<cool-element has-pizza></cool-element> <cool-element></cool-element>

If you start with an initial value of true, then when the attribute is removed or never existed, the JS property will 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>

In this form, with the property initial value as true, then the following two are identical (the JS property is true in either case),

<cool-element has-pizza></cool-element> <cool-element></cool-element>

and expressing both true and false side by side would require explicit values:

<cool-element has-pizza="true"></cool-element> <cool-element has-pizza="false"></cool-element>

💡Tip:

Avoid attribute values like has-pizza="blah blah", because they are not semantic. When the default JS property value is false, always use the form has-pizza="false" or no attribute for false, and has-pizza or has-pizza="true" for true. When the default JS property value is true, always use the form has-pizza="false" for false, and has-pizza, has-pizza="true", or no attribute, for true.

@eventAttribute

Use this decorator to create event listener attributes/properties, the same as with built-in event attributes/properties such as "onclick".

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

@element('some-el')
class MyEl extends Element {
  /** @type {EventListener | null} */
  @eventAttribute onjump = null

  connectedCallback() {
    super.connectedCallback()

    // This element dispatches a "jump" event every second:
    setInterval(() => this.dispatchEvent(new Event('jump')), 1000)
  }
}

const el = new SomeEl()

el.onjump = () => console.log('jump!')
// or, as with "onclick" and other built-in attributes:
el.setAttribute('onjump', "console.log('jump!')")

document.body.append(el)

// "jump!" will be logged every second.

Note that besides the event properties working in JS, the attributes also work in plain HTML as with native event attributes such as onclick:

<body>
  <my-el onjump="console.log('jump!')"></my-el>
</body>

@signal

This is from classy-solid for creating signal properties, but because @element is composed with classy-solid's @reactive class decorator, a non-attribute signal property can be defined without also having to use classy-solid's @reactive decorator on the class:

import {Element, element, booleanAttribute} from '@lume/element'
import {reactive, signal} from 'classy-solid'

// Non element class, requires `@reactive` for fields decorated with `@signal`:
@reactive
class Something {
  @signal foo = 123 // This will be reactive
}

// An element class decoratorated with `@element` (or passed to `element()` when
// not using decorators) does not also need to be decorated with `@reactive`:
@element('cool-element')
class CoolElement extends Element {
  // hasPizza will be reactive but an attribute will not be observed for this
  // property, and the property can only be set via JS.
  @signal hasPizza = false

  // This property *does* get updated when a `has-drink` attribute is updated, and is also reactive.
  @booleanAttribute hasDrink = false

  // ...
}

@noSignal

Once in a blue moon you might need to define an attribute property that is not reactive, for some reason. Avoid it if you can, but you can do it with @noSignal:

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

@element('cool-element')
class CoolElement extends Element {
  // This property gets updated when a `has-drink` attribute is updated, but it is not reactive.
  @booleanAttribute @noSignal hasDrink = false
}

This is more useful on a getter/setter where you may implement your own reactivity for the property:

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

@element('cool-element')
class CoolElement extends Element {
  #hasDrink = false

  // This property gets updated when a `has-drink` attribute is updated, and
  // it is not reactive due to the `@booleanAttribute` decorator but due to a
  // custom implementation in the getter/setter (for example, maybe the
  // getter/setter reads from and writes to a Solid signal.
  @booleanAttribute @noSignal get hasDrink() {
    // ...
    return this.#hasDrink
  }
  @booleanAttribute @noSignal set hasDrink(value) {
    // ...
    this.#hasDrink = value
  }
}

Note

Make sure the @noSignal decorator is listed after the attribute decorator. This will not work:

class CoolElement extends Element {
  // This won't work because the attribute decorator will run before the
  // noSignal decorator so the attribute decorator will miss the signal that
  // it should skip the signal (pun intended!).
  @noSignal @booleanAttribute hasDrink = false
}

Runtime Type Checking

The from handler of a newly-defined attribute decorator (defining new attribute decorators is described in the @attribute doc above) can throw an error when an invalid string is encountered. Expanding the previous jsonAttribute example:

const jsonAttribute = attribute({
  from(str){
    const result = JSON.parse(str)
    if (/* some condition not met with result */) throw new Error('...describe the error...')
    return result
  }
})

This error handling will work regardless if setting an attribute, or setting a string via the JS property.

An alternative approach is to throw an error in the setter of an @attribute-decorated property, which can be useful for existing code that might already exist where the @attribute decorator is being added:

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

  @attribute get someValue() {
    return this.#foo
  }
  @attribute set someValue(value) {
    if (/* some condition not met with value */) throw new Error('...error description...')
    this.#foo = value
  }
}

TypeScript

Attribute property types

Here are the recommended types for properties depending on the type of attribute being defined, with non-null initial values:

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

@element('cool-element')
class CoolElement extends Element {
  @attribute firstName: string = 'John'
  @stringAttribute lastName: string = 'Doe'
  @numberAttribute age: number = 75
  @booleanAttribute likesPizza: boolean = true
  // ...
}

If properties are initialized with null values, add | null to each type:

@element('cool-element')
class CoolElement extends Element {
  @attribute firstName: string | null = null
  @stringAttribute lastName: string | null = null
  @numberAttribute age: number | null = null
  @booleanAttribute likesPizza: boolean | null = null
  // ...
}

All attribute properties can technically accept strings too, as this is how attribute string values get coerced in case of non-string attributes. Although it is not recommended, this aspect of the properties can be exposed if needed:

@element('cool-element')
class CoolElement extends Element {
  @attribute firstName: string = 'John'
  @stringAttribute lastName: string = 'Doe'
  @numberAttribute age: `${number}` | number = 75
  @booleanAttribute likesPizza: `${boolean}` | boolean = true
  // ...
}

const el = new CoolElement()

el.age = '80' // no type error here

console.log(el.age, typeof el.age) // logs "80 number"

el.age = 'blah' // type error, 'blah' is not assignable to a string with number format defined by `${number}`.

It is nice to not include the string types, especially because the string values are always coerced by the attribute from handler, and JS/TS users can set the actual values directly (f.e. numbers or booleans). You will always receive a number when reading a JS property decorated with @numberAttribute, for example, so including the string type could make things confusing and less ideal. For example, when reading the value of a @numberAttribute property, the following may be redundant and annoying when reading the value, especially in TypeScript:

if (typeof el.age === 'number') { // this check is unnecessary/annoying, required for types to check out
  const n: number = el.age
}
// or
const n: number = el.age as number // unnecessary/annoying cast in this case

if (typeof el.age === 'string') console.log('this will never be logged')

Solid.js JSX expressions

(Note this section is only for Solid.js, as other frameworks like React or Preact do not have DOM-returning JSX expressions.)

Load the required JSX types in one of two ways:

  1. 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 */

  2. 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 example from Easily create and manipulate DOM for TypeScript, it would look like the following.

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.

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.

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:

// 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 in frameworks

(For type definitions for function components, see Solid.js docs).

In Solid JSX (in Lume Elements)

Example: 'kitchen-sink-tsx'

First set up jsxImportSource as mentioned above.

To give our Custom Elements type checking for use with DOM APIs, and type checking in Solid JSX, we can add the element type definition to HTMLElementTagNameMap and JSX.IntrinsicElements. Use the ElementAttributes helper to specify which attributes/properties should be exposed in the JSX type (we do not want to expose methods for example, or we may want to skip exposing some properties that are implementation details such as those prefixed with underscores to represent that they are internal, etc):

import type {ElementAttributes} from '@lume/element'
import {Element, element, stringAttribute, numberAttribute, eventAttribute} from '@lume/element'

// List the properties that should be picked from the class type for JSX props.
// Note! Make sure that the properties listed are either decorated with
// attribute decorators, or that they are on* event properties.
export type CoolElementAttributes = 'coolType' | 'coolFactor' | 'oncoolness'

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

  // For any given event our element will dispatch, define an event prop with
  // the event name prefixed with 'on', and that accepts an event listener
  // function or null.
  @eventAttribute oncoolness: ((event: CoolnessEvent) => void) | null = null

  // This property will not appear in the JSX types because it is not listed in
  // the `CoolElementAttributes` that are passed to `ElementAttributes` below.
  notJsxProp = 123

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

/** This an event that our element dispatches, for example. */
class CoolnessEvent extends Event {
  constructor() {
    super('coolness', {...})
  }
}

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

// Hook up the type for use in Solid JSX templates
declare module 'solid-js' {
	namespace JSX {
		interface IntrinsicElements {
			'cool-element': ElementAttributes<CoolElement, CoolElementAttributes>
		}
	}
}

Now when we 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-type="hair" // ok
    cool-factor="200" // ok
    oncoolness={() = console.log('coolness happened')} // ok
	></cool-element>
)

In React JSX

Example: 'kitchen-sink-react19'

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 {ReactElementAttributes} from '@lume/element/dist/framework-types/react.js'

// ... the same CoolElement class and HTMLElementTagNameMap definitions as above ...

// Hook up the type for use in React JSX templates
declare module 'react' {
  namespace JSX {
    interface IntrinsicElements {
      'cool-element': ReactElementAttributes<CoolElement, CoolElementAttributes>
    }
  }
}

Now when we 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
    coolType="hair" // ok
    coolFactor="200" // ok
    oncoolness={() = console.log('coolness happened')} // ok
	></cool-element>
)

Note

You may want to define React JSX types for your elements in separate files than the Solid JSX types, and have React users import those separate 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 in that case, contributions welcome!).

In Preact JSX

Example: 'kitchen-sink-preact'

The definition is exactly the same as the previous section for React JSX. Define the element types with the same ReactElementAttributes helper as described above.

In our TypeScript compilerOptions we should make sure to 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/"]
    }
  }
}

Note

A default Preact app created with npm init preact will already have this set up.

The rest is the same as with defining types in a React app.

In Angular

Example: 'kitchen-sink-angular'

Register the element type for Angular like so:

// ... the same CoolElement class definition as above ...

// Type checking in angular is currently limited to only knowing the custom
// element tag names (from HTMLElementTagNameMap), but there is no way to
// provide types of template props for type checking. For now, this is the best
// we can do. See:
// https://github.com/angular/angular/issues/58483
declare global {
  interface HTMLElementTagNameMap {
    'cool-element': CoolElement
  }
}

In Vue

Example: 'kitchen-sink-vue'

Register the element type for Vue like so:

import type {VueElementAttributes} from '@lume/element/dist/framework-types/vue.js'

// ... the same CoolElement class and HTMLElementTagNameMap definitions as above ...

// Hook up the type for use in Vue templates
declare module 'vue' {
  interface GlobalComponents {
    'cool-element': VueElementAttributes<CoolElement, CoolElementAttributes>
  }
}

In Svelte

Example: 'kitchen-sink-svelte'

Register the element type for Svelte like so:

import type {SvelteElementAttributes} from '@lume/element/dist/framework-types/vue.js'

// ... the same CoolElement class and HTMLElementTagNameMap definitions as above ...

// Hook up the type for use in Svelte templates
declare module 'svelte/elements' {
  interface SvelteHTMLElements {
    'cool-element': SvelteElementAttributes<CoolElement, CoolElementAttributes>
  }
}

In Stencil.js JSX

Example: 'kitchen-sink-stencil'

Register the element type for Stencil.js like so:

import type {StencilElementAttributes} from '@lume/element/dist/framework-types/stencil.js'

// ... the same CoolElement class and HTMLElementTagNameMap definitions as above ...

// Hook up the type for use in Svelte templates
declare module '@stencil/core' {
  export namespace JSX {
    interface IntrinsicElements {
      'cool-element': StencilElementAttributes<CoolElement, CoolElementAttributes>
    }
  }
}

Setter types in framework templates

Given a custom element definition like so,

@element('my-el')
class MyEl extends Element {
  #position = new Vec3()

  get position(): Vec3 {
    return this.#position
  }

  set position(value: Vec3 | `${number} ${number} ${number}` | [number, number, number]) {
    const {x, y, z} = parseValue(value)
    this.#position.set(x, y, z)
  }
}

using it in JSX or other framework templates will show type errors for a use cases like this one,

function MyComponent() {
  return (
    <>
      <my-el position={[1, 2, 3]} />
      <my-el position={'1 2 3'} />
    </>
  )
}

despite the fact that [1, 2, 3] and "1 2 3" are both valid values that the element's position setter can accept.

We can fix this by using a __set__-prefixed property that matches the getter/setter name to define the setter type that will appear in JSX/framework templates:

@element('my-el')
class MyEl extends Element {
  #position = new Vec3()

  get position(): Vec3 {
    return this.#position
  }

  set position(value: this['__set__position']) {
    const {x, y, z} = parseValue(value)
    this.#position.set(x, y, z)
  }

  // Add a note telling people not to use this property, it is for template type definition only.
  /** @deprecated Do not use this directly. */
  declare __set__position: Vec3 | `${number} ${number} ${number}` | [number, number, number]
}

Note that we re-used the __set__position type for the setter to avoid having to write tye type definition twice. With this additional type-only property defined, the previous MyComponent JSX example will not have type errors and will allow the values.

Resources

See https://solid.js.com, https://primitives.solidjs.community, and https://github.com/lume/classy-solid for APIs that are useful with @lume/element.

Also see Custom Element (i.e. Web Component) systems that are alternative to @lume/element:

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Fast and simple custom elements.

lume.io

Topics

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Mar 6, 2025
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