@zeix/ui-element/README.md

UIElement - a HTML-first library for reactive Web Components

# UIElement

Version 0.13.3

**UIElement** - a HTML-first library for reactive Web Components

UIElement is a set of functions to build reusable, loosely coupled Web Components with reactive properties. It provides structure through components and simplifies state management and DOM synchronization using declarative signals and effects, leading to more organized and maintainable code without a steep learning curve.

Unlike SPA frameworks (React, Vue, Svelte, etc.) UIElement takes a HTML-first approach, progressively enhancing sever-rendered HTML rather than recreating (rendering) it using JavaScript. UIElement achieves the same result as SPA frameworks with SSR, but with a simpler, more efficient approach. It works with a backend written in any language or with any static site generator.

## Key Features

- 🧱 **HTML Web Components**: Build on standard HTML and enhance it with encapsulated, reusable Web Components. No virtual DOM – UIElement works directly with the real DOM.
- 🚦 **Reactive Properties**: Define reactive properties for fine-grained, efficient state management (signals). Changes automatically propagate only to the parts of the DOM that need updating, avoiding unnecessary re-renders.
- 🧩 **Function Composition**: Declare component behavior by composing small, reusable functions (attribute parsers and effects). This promotes cleaner code compared to spaghetti code problems that commonly occur when writing low-level imperative code.
- 🛠️ **Customizable**: UIElement is designed to be easily customizable and extensible. You can create your own custom attribute parsers and effects to suit your specific needs.
- 🌐 **Context Support**: Share global states across components without prop drilling or tightly coupling logic.
- 🪶 **Tiny footprint**: Minimal core (~4kB gzipped) with tree-shaking support, adding only the necessary JavaScript to enhance your HTML.
- 🛡️ **Type Safety**: Get early warnings when types don't match, improving code quality and reducing bugs.

UIElement uses [Cause & Effect](https://github.com/zeixcom/cause-effect) internally for state management with signals and for scheduled DOM updates. But you could easily rewrite the `component()` function to use a signals library of your choice or to produce something else than Web Components.

## Installation

```bash
# with npm
npm install @zeix/ui-element

# or with bun
bun add @zeix/ui-element
```

## Documentation

The full documentation is still work in progress. The following chapters are already reasonably complete:

- [Introduction](https://zeixcom.github.io/ui-element/index.html)
- [Getting Started](https://zeixcom.github.io/ui-element/getting-started.html)
- [Components](https://zeixcom.github.io/ui-element/components.html)
- [Styling](https://zeixcom.github.io/ui-element/styling.html)
- [Data Flow](https://zeixcom.github.io/ui-element/data-flow.html)
- [About](https://zeixcom.github.io/ui-element/about.html)

## Basic Usage

### Counter

Server-rendered markup:

```html
<basic-counter count="5">
  <button type="button">💐 <span>5</span></button>
</basic-counter>
```

UIElement component:

```js
import { asInteger, component, on, setText } from '@zeix/ui-element'

export default component(
  'basic-counter',
  {
    count: asInteger(), // Get initial value from count attribute
  },
  (el, { first }) => [
    // Update count display when state changes
    first('span', setText('count')),

    // Handle click events to change state
    first(
      'button',
      on('click', () => {
        el.count++
      }),
    ),
  ],
)
```

Example styles:

```css
basic-counter {
  & button {
    border: 1px solid var(--color-border);
    border-radius: var(--space-xs);
    background-color: var(--color-secondary);
    padding: var(--space-xs) var(--space-s);
    cursor: pointer;
    color: var(--color-text);
    font-size: var(--font-size-m);
    line-height: var(--line-height-xs);
    transition: background-color var(--transition-short) var(--easing-inout);

    &:hover {
      background-color: var(--color-secondary-hover);
    }

    &:active {
      background-color: var(--color-secondary-active);
    }
  }
}
```

### Tab Group

An example demonstrating how to create a fully accessible tab navigation.

Server-rendered markup:

```html
<module-tabgroup>
  <div role="tablist">
    <button
      type="button"
      role="tab"
      id="trigger1"
      aria-controls="panel1"
      aria-selected="true"
      tabindex="0"
    >
      Tab 1
    </button>
    <button
      type="button"
      role="tab"
      id="trigger2"
      aria-controls="panel2"
      aria-selected="false"
      tabindex="-1"
    >
      Tab 2
    </button>
    <button
      type="button"
      role="tab"
      id="trigger3"
      aria-controls="panel3"
      aria-selected="false"
      tabindex="-1"
    >
      Tab 3
    </button>
  </div>
  <div role="tabpanel" id="panel1" aria-labelledby="trigger1">
    Tab 1 content
  </div>
  <div role="tabpanel" id="panel2" aria-labelledby="trigger2" hidden>
    Tab 2 content
  </div>
  <div role="tabpanel" id="panel3" aria-labelledby="trigger3" hidden>
    Tab 3 content
  </div>
</module-tabgroup>
```

UIElement component:

````js
import { component, on, setProperty, show } from '@zeix/ui-element'
import { manageArrowKeyFocus } from './manage-arrow-key-focus'

export default component('module-tabgroup', {
	selected: '',
},
(el, { all, first }) => {
	el.selected =
		el.querySelector('[role="tab"][aria-selected="true"]')
			?.getAttribute('aria-controls') ?? ''
	const isSelected = target => el.selected === target.getAttribute('aria-controls')
	const tabs = Array.from(el.querySelectorAll<HTMLButtonElement>('[role="tab"]'))
	let focusIndex = 0

	return [
		first('[role="tablist"]',
			on('keydown', manageArrowKeyFocus(tabs, focusIndex)),
		),
		all('[role="tab"]',
			on('click', e => {
				el.selected =
					e.currentTarget.getAttribute('aria-controls') ?? ''
				focusIndex = tabs.findIndex(tab => isSelected(tab))
			}),
			setProperty('ariaSelected', target => String(isSelected(target))),
			setProperty('tabIndex', target => isSelected(target) ? 0 : -1),
		),
		all('[role="tabpanel"]',
			show(target => el.selected === target.id),
		),
	]
})

Auxiliary function:

```js
export const manageArrowKeyFocus = (elements, index) => e => {
	if (!(e instanceof KeyboardEvent))
		throw new TypeError('Event is not a KeyboardEvent')
	const handledKeys = [
		'ArrowLeft',
		'ArrowRight',
		'ArrowUp',
		'ArrowDown',
		'Home',
		'End',
	]
	if (handledKeys.includes(e.key)) {
		e.preventDefault()
		switch (e.key) {
			case 'ArrowLeft':
			case 'ArrowUp':
				index = index < 1 ? elements.length - 1 : index - 1
				break
			case 'ArrowRight':
			case 'ArrowDown':
				index = index >= elements.length - 1 ? 0 : index + 1
				break
			case 'Home':
				index = 0
				break
			case 'End':
				index = elements.length - 1
				break
		}
		if (elements[index]) elements[index].focus()
	}
}
````

Example styles:

```css
module-tabgroup {
  display: block;
  margin-bottom: var(--space-l);

  > [role='tablist'] {
    display: flex;
    border-bottom: 1px solid var(--color-gray-50);
    padding: 0;
    margin-bottom: 0;

    & button {
      border: 0;
      border-top: 2px solid transparent;
      border-bottom-width: 0;
      font-family: var(--font-family-sans);
      font-size: var(--font-size-s);
      font-weight: var(--font-weight-bold);
      padding: var(--space-s) var(--space-m);
      color: var(--color-text-soft);
      background-color: var(--color-secondary);
      cursor: pointer;
      transition: all var(--transition-short) var(--easing-inout);

      &:hover,
      &:focus {
        color: var(--color-text);
        background-color: var(--color-secondary-hover);
      }

      &:active {
        color: var(--color-text);
        background-color: var(--color-secondary-active);
      }

      &[aria-selected='true'] {
        color: var(--color-primary-active);
        border-top: 3px solid var(--color-primary);
        background-color: var(--color-background);
        margin-bottom: -1px;
      }
    }
  }

  > [role='tabpanel'] {
    font-family: sans-serif;
    font-size: var(--font-size-m);
    background: var(--color-background);
    margin-block: var(--space-l);
  }
}
```

### Lazy Load

An example demonstrating how to use a custom attribute parser (sanitize an URL) and a signal producer (async fetch) to implement lazy loading.

```html
<module-lazy src="/module-lazy/snippet.html">
  <card-callout>
    <div class="loading" role="status">Loading...</div>
    <div class="error" role="alert" aria-live="polite"></div>
  </card-callout>
</module-lazy>
```

UIElement component:

```js
import {
  UNSET,
  component,
  computed,
  dangerouslySetInnerHTML,
  setText,
  show,
  state,
  toggleClass,
} from '@zeix/ui-element'
import { asURL } from './as-url'

export default component(
  'module-lazy',
  {
    src: asURL,
  },
  (el, { first }) => {
    const error = state('')

    const content = computed(async abort => {
      const url = el.src.value
      if (el.src.error || !url) {
        error.set(el.src.error ?? 'No URL provided')
        return ''
      }

      try {
        error.set('')
        el.querySelector('.loading')?.remove()
        const response = await fetch(url, { signal: abort })
        if (response.ok) return response.text()
        else error.set(response.statusText)
      } catch (err) {
        const errorMessage = err instanceof Error ? err.message : String(err)
        error.set(errorMessage)
        return ''
      }
    })

    return [
      dangerouslySetInnerHTML(content),
      first(
        'card-callout',
        show(() => !!error.get() || content.get() === UNSET),
        toggleClass('danger', () => !error.get()),
      ),
      first('.error', setText(error)),
    ]
  },
)
```

Custom attribute parser:

```js
// Attribute Parser uses current element to detect recursion and set error message
export const asURL = (el, v) => {
  let value = ''
  let error = ''
  if (!v) {
    error = 'No URL provided'
  } else if (
    (el.parentElement || el.getRootNode().host)?.closest(
      `${el.localName}[src="${v}"]`,
    )
  ) {
    error = 'Recursive loading detected'
  } else {
    try {
      // Ensure 'src' attribute is a valid URL
      const url = new URL(v, location.href)

      // Sanity check for cross-origin URLs
      if (url.origin === location.origin) value = String(url)
      else error = 'Invalid URL origin'
    } catch (err) {
      error = String(err)
    }
  }
  return { value, error }
}
```

## Contributing & License

Feel free to contribute, report issues, or suggest improvements.

License: [MIT](LICENSE)

(c) 2025 [Zeix AG](https://zeix.com)