🔗 Defining a Component

UIElement builds on Web Components, extending HTMLElement to provide built-in state management and reactive updates.

A UIElement creates components using the component() function:

component("my-component", {}, () => [
	// Component setup
]);

Every UIElement component must be registered with a valid custom element tag name (two or more words joined with -) as the first parameter.

🔗 Using the Custom Element in HTML

Once registered, the component can be used like any native HTML element:

<my-component>Content goes here</my-component>

🔗 Component Lifecycle

UIElement manages the Web Component lifecycle from creation to removal. Here's what happens.

🔗 Component Creation

In the constructor() reactive properties are initialized. You pass a second argument to the component() function to defines initial values for component states.

component("my-component", {
	count: 0, // Initial value of "count" signal
	isEven: el => () => !(el.count % 2), // Computed signal based on "count" signal
	value: asInteger(5), // Parse "value" attribute as integer defaulting to 5
	name: consume("display-name") // Consume "display-name" signal from closest context provider
}, () => [
	// Component setup
]);

In this example you see all four ways to define a reactive property:

• A static initial value for a State signal (e.g., count: 0)
• A signal producer that derives an initial value or a callback function from other properties of the element (e.g., isEven: el => () => !(el.count % 2))
• An attribute parser that may provide a fallback value (e.g., value: asInteger(5))
• A context consumer that emits a ContextRequestEvent (e.g., name: consume("display-name"))

🔗 Mounted in the DOM

Runs when the component is added to the page (connectedCallback()). This is where you:

• Access sub-elements
• Set up event listeners
• Apply effects
• Emit custom events
• Provide context

UIElement expects you to return an array of partially applied functions to be executed during the setup phase. The order doesn't matter, as each function targets a specific element or event. So feel free to organize your code in a way that makes sense to you.

Each of these functions will return a cleanup function that will be executed during the disconnectedCallback() lifecycle method.

component("my-component", {
	count: 0,
}, el => [
	emit("update-count", el.count), // Emit custom event
	provide("count"), // Provide context
	first(".increment",
		on("click", () => { el.count++ }) // Add click event listener
	),
	first(".count",
		setText("count") // Apply effect to update text
	)
]);

🔗 Removed from the DOM

Runs when the component is removed (disconnectedCallback()). UIElement will run all cleanup functions returned by event listeners and effects during the setup phase (connectedCallback()). This will unsubscribe all signals the component is subscribed to, so you don't need to worry about memory leaks.

If you added event listeners outside the scope of your component or subscribed manually to external APIs, you need to return a cleanup function:

component("my-component", {}, el => [
	// Setup logic
	() => {
		const observer = new IntersectionObserver(([entry]) => {
			// Do something
		})
		observer.observe(el);

		// Cleanup logic
		return () => observer.disconnect();
	}
]);

🔗 Observed Attributes

UIElement automatically observes and converts attributes with an associated parser function in the init block and updates them whenever the attribute changes (attributeChangedCallback()).

🔗 Managing State with Signals

UIElement manages state using signals, which are atomic reactive states that trigger updates when they change. We use regular properties to access or update them:

console.log("count" in el); // Check if the signal exists
console.log(el.count); // Read the signal value
el.count = 42; // Update the signal value

🔗 Characteristics and Special Values

Signals in UIElement are of a static type and non-nullable. This allows to simplify the logic as you will never have to check the type or perform null-checks.

• If you use TypeScript (recommended), you will be warned that null or undefined cannot be assigned to a signal or if you try to assign a value of a wrong type.
• If you use vanilla JavaScript without a build step, setting a signal to null or undefined will log an error to the console and abort. However, strict type checking is not enforced at runtime.

Because of the non-nullable nature of signals in UIElement, we need two special values that can be assigned to any signal type:

• RESET: Will reset to the server-rendered version that was there before UIElement took control. This is what you want to do most of the times when a signal lacks a specific value.
• UNSET: Will delete the signal, unsubscribe its watchers and also delete related attributes or style properties in effects. Use this with special care!

🔗 Initializing State from Attributes

The standard way to set initial state in UIElement is via server-rendered attributes on the component that needs it. No props drilling as in other frameworks. UIElements provides some bundled attribute parsers to convert attribute values to the desired type. And you can also define your own custom parsers.

component("my-component", {
	count: asInteger(), // Bundled parser: Convert '42' -> 42
	date: (_, v) => new Date(v), // Custom parser: '2025-02-14' -> Date object
}, () => [
	// Component setup
]);

🔗 Bundled Attribute Parsers

The pre-defined parsers asInteger(), asNumber() and asString() allow to set a custom fallback value as parameter.

The asEnum() parser requires an array of valid values, while the first will be the fallback value for invalid results.

The asJSON() parser requires a fallback object as parameter as {} probably won't match the type you're expecting.

🔗 Accessing Sub-elements

Before adding event listeners, applying effects, or passing states to sub-elements, you need to select them using a function for element selection:

// Select the first ".increment" button and apply effects on it
first(".increment",
	// Fx functions
)

// Select all <button> elements and apply effects on them
all("button",
	// Fx functions
)

The first() function expects the matched element to be present at connection time. If not, it will silently ignore the call.

On the other hand, the all() function creates a dynamic array of elements that will be updated whenever the matching elements are added or removed from the component's DOM branch. UIElement will apply the given setup functions to added elements and run the cleanup functions on removed elements.

🔗 Adding Event Listeners

Event listeners allow to respond to user interactions. They are the cause of changes in the component's state.

component("my-component", {
	active: 0,
	value: ''
}, (el) => [
	all("button",
		on("click", (e) => {
			// Set "active" signal to value of data-index attribute of button
			const index = parseInt(e.target.dataset['index'], 10);
			el.active = Number.isInteger(index) ? index : 0;
		})
	),
	first("input",
		on("change", (e) => {
			// Set "value" signal to value of input element
			el.value = e.target.value;
		})
	)
]

🔗 Synchronizing State with Effects

Effects automatically update the DOM when signals change, avoiding manual DOM manipulation.

🔗 Applying Effects

Apply one or multiple effects in the setup function (for component itself) or in element selector functions:

() => [
	// On the component itself
	setAttribute("open", "isOpen"), // Set "open" attribute according to "isOpen" signal

	// On first element matching ".count"
	first(".count",
		setText("count"), // Update text content according to "count" signal
		toggleClass("even", "isEven") // Toggle "even" class according to "isEven" signal
	)
];

Again, the order of effects is not important. Feel free to apply them in any order that suits your needs.

🔗 Bundled Effects

🔗 Simplifying Effect Notation

For effects that take two arguments, the second argument can be omitted if the signal key matches the targeted property name, attribute, class, or style property.

When signal key matches property name:

first(".count", toggleClass("even"))

Here, toggleClass("even") automatically uses the "even" signal.

🔗 Using Local Signals for Protected State

Local signals are useful for storing state that should not be exposed to the outside world. They can be used to manage internal state within a component:

component("my-component", {}, () => {
	const count = state(0);
	const double = count.map(v => v * 2);
	return [
		first(".increment", on("click", () => {
			count.update(v => ++v);
		})),
		first(".count", setText(count)),
		first(".double", setText(double))
	];
});

Outside components cannot access the count or double signals.

🔗 Using Functions for Ad-hoc Derived State

Instead of a signal key or a local signal, you can pass a function that derives a value dynamically:

component("my-component", {
	count: 0
}, el => {
	const double = computed(() => el.count * 2);
	return [
		first(".count", toggleClass("even", () => !(el.count % 2)))),
		first(".double", setText(() => String(double.get())))
	];
});

🔗 Efficient & Fine-Grained Updates

Unlike some frameworks that re-render entire components, UIElement updates only what changes:

• No virtual DOM – UIElement modifies the DOM directly.
• Signals propagate automatically – No need to track dependencies manually.
• Optimized with a scheduler – Multiple updates are batched efficiently.

🔗 Single Component Example: MySlider

Bringing all of the above together, you are now ready to build your own components like this slider with prev / next buttons and dot indicators, demonstrating single-component reactivity.

🔗 Next Steps

Now that you understand the basics, explore:

• Styling Components – Learn techniques to apply styles to components.
• Data Flow – Learn about passing state between components.