Functional component hooks

Lightweight hooks and context primitives for signal-based render trees.
Works anywhere a render function can be scoped, with Lit or similar templating libraries.
Uses the TC39 Signal Proposal for its backend.

[!CAUTION]
NOT FOR PRODUCTION!
It's an experiment to see how far we can go.

🧠 Why

Web development is increasingly polarized around two powerful patterns:

• Immutable, top-down render trees
• Reactive primitives (known as "Signals"), which enable fine-grained updates

It's a simplification. In practice, there is overlap, and not all systems were designed with Signals as their foundation. Also, both approachs are valid and boils down to developer preference, context, and technical limitations.

Templating libraries like lit-html historically gives you efficient top-down rendering via tagged template literals, while signal-based systems give you surgical reactivity.
In the original lit/lit-html model, Custom HTML Element acts as "relays", where you can stuff all the reactive primitives, methods, and any intricate stuff you need to build an app. lit-html templates are "stateless", deterministic (theoretically).
That's was a feature, not a limitation.

However, a recent breakthrough was the integration of the TC39 Signal Proposal in a Lit friendly @lit-labs/signals package, which unlock an hybrid solution. You can still use the familiar, and easy to mindmap vertical data stream, but also avoid endless prop. drilling when needed, thanks to Signal aware template expressions.

That being said, you'll soon be tempted to do something LitElement was invented for, which is adding self-contained state to pieces of reusable unit ("Web Component" / "Custom Element" / "React FC"…):

const count = signal(0); // <-- Outside of the render tree. You're fine :)

function Counter() {
  const count = signal(0); // <-- Inside the render tree. Oh no :(

  return html`
    <button @click=${count.set(count.get() + 1)}>Clicked ${count} times</button>
  `;
}

html`<div>${Counter()}</div>`;

You don't want to do that. The official way to create a Signal is outside of a the render tree, otherwise it will be recreated on each render call. It's not like Solid-js, which was designed solely with signal primitives and JSX pre-compilation in mind.

Also, at the time of writing, official examples are featuring LitElement as a "Signal Host" thanks to a SignalWatcher Mixin.
While you can use the good old render function for Light DOM with a SignalHost and pure lit-html (no required Custom Element host), it's less straightforward than relying on the @lit-labs package, off-the-shelf Mixin. Moreover, you might encounter bugs with stratified SSR scenarios. It's still an experimental package after all, and the Lit team is collecting feedback.

So what do we need to author those self-contained "Component", à-la React, Preact, Vue-Vine… you name it?

This is where @gracile-labs/functional is coming into play.
This library is providing the ergonomics of useState, createEffect, and useContext without locking you into a full framework. It's mostly a facade for the Signal polyfill and the Lit Signals helpers (mixin, html tag…), or any custom, signal-aware templating implementation.

It works with:

• Custom JSX runtimes (e.g. JSX + tagged templates)
• Template literal systems like lit-html.
• Returned in Lit’s render() method (shadow DOM or light DOM)
• Anything else that follows the render(host, fn), top-down model

All that by avoiding heavy or redundant machinery. It just collect and offload the reactivity work the the Signals scheduler, in a colocated fashion.

The API is purposefully kept as minimal as possible.

🧪 How it works

@gracile-labs/functional scopes state and side-effects to the current render host — any object passed to withFunctional(host, renderFn). This lets you:

• Reuse useState() and useContext() safely across renders
• Scope reactive effects to a host's lifecycle
• Test hook logic without needing DOM APIs or HTMLElement interfaces
• Compose functional components with consistent reactivity

🚀 Quickstart (w. LitElement + Lit Signals)

import { WithFunctional, useState } from '@gracile-labs/functional';
import { SignalWatcher } from '@lit-labs/signals';
import { html, LitElement } from 'lit';

@customElement('counter-button')
export class CounterButton extends WithFunctional(SignalWatcher(LitElement)) {
  render() {
    const [count, setCount] = useState(0);

    return html`
      <button @click=${() => setCount(count.get() + 1)}>Count: ${count}</button>
    `;
  }
}

🧱 API

withFunctional(host, () => TemplateResult)

Establishes a render scope where useState, createEffect, and provideContext work.

WithFunctional(BaseClass)

Mixin for Lit-style components — auto-wraps render() with withFunctional(this, ...).

useState(initial: T): [Signal.State<T>, (value: T) => void]

Reactive local state. Value is a Signal.State<T>, compatible with Lit’s reactivity system.

createMemo(fn: () => T): Signal.Computed<T>

Signal-aware memoization — returns a computed signal. Recomputes when tracked signals change.

createEffect(fn: () => void | () => void)

Runs after render. Cleanup is run on re-render or unmount. Equivalent of useEffect().

onMount(fn), onCleanup(fn)

Shorthands for post-render effect logic.

createContextProvider<T>(defaultValue: T): [Provider, useContext]

Creates a scoped context system.

🧩 Advanced Use (Light DOM)

<FunctionalComponents>
  {() => (
    <ThemeProvider>
      <Child />
    </ThemeProvider>
  )}
</FunctionalComponents>

Light DOM reactivity in production builds is experimental due to current limitations in @lit-labs/signals:

• watch() directives may fail when code is minified (private names)
• Custom SignalHost objects may not be recognized in production

Use CE + Shadow DOM for now as a safe baseline.

🧪 Testing Hooks

You can test everything without Lit, DOM, or browser:

const host = {};
const result = withFunctional(host, () => {
  const [count, setCount] = useState(1);

  setCount(2);

  return count.get();
});

📦 Install

pnpm i @gracile-labs/functional

In-depth

The hook set API philosophy aligns with React's use*, but it could have been Solid's create*.

It looks like React but behave closer to Solid in places.

…

🙏 Acknowledgements

• TC39 Signals
• @lit-labs/signals