kerf-app

name	kerf-app
description	Build UIs in the kerf reactive framework (https://github.com/brianwestphal/kerf). Use this skill whenever the user is writing or modifying code that imports `kerfjs`, asks to add a feature to a kerf app, or asks "how do I do X in kerf?". Use it proactively the moment you spot a kerf import in the file you're editing.
kerf-skill-version	1.2.0

Building apps with kerf

Drop this file into your ~/.claude/skills/kerf-app/SKILL.md (or your project's .claude/skills/kerf-app/SKILL.md) so Claude Code activates it whenever you work on a kerf app.

kerf is a ~11 KB reactive UI framework (~12 KB with arraySignal): signals + DOM morphing + JSX → HTML strings. No virtual DOM, no compiler, no scheduler. The whole public surface fits in 15 exports.

Setup

• Install: npm install kerfjs
• tsconfig.json: "jsx": "react-jsx", "jsxImportSource": "kerfjs"
• Vite / esbuild need no extra config.
• Recommended companion: npm install --save-dev eslint-plugin-kerfjs and add kerfjs.configs.recommended to the project's eslint config. Enforces five of the hard rules below (no inline JSX event handlers, require data-key in each(), capture delegate() disposers, no nested mount(), prefer module JSX augmentation) at edit time — useful as a self-correction signal when authoring kerf code.

Public API — one import path

import {
  signal, computed, effect, batch,    // reactivity
  defineStore, resetAllStores,        // stores
  mount, morph, each,                 // render (reactive + one-shot) + keyed list
  delegate, delegateCapture,          // events
  toElement,                          // direct JSX → DOM Element (or DocumentFragment for multi-root)
  SafeHtml, isSafeHtml, raw, Fragment,
} from 'kerfjs';

// Optional, only when you need granular collection updates:
import { arraySignal } from 'kerfjs/array-signal';

Export	Use
signal(initial)	atomic reactive state; .value get/set
computed(fn)	derived value, read-only
effect(fn)	side effect on signal change; returns disposer
batch(fn)	coalesce multiple writes into one re-run
defineStore({initial, actions})	named multi-consumer state
resetAllStores()	reset every store (test teardown)
mount(el, render)	bind reactive render to a DOM element; returns disposer
morph(liveRoot, template)	one-shot reconcile against a populated element (SSR hydration, page-refresh diffs). Template = Element, SafeHtml, or HTML string
each(items, render, cacheKey?)	keyed list iteration; per-row memoization on identity (+ optional cacheKey — a passive comparator for external state). Distinct from data-key on the rendered element
delegate(root, type, sel, h)	one listener at the root; closest(selector) walk from target
delegateCapture(root, type, sel, h)	capture-phase escape hatch; target.matches() strict match
attr(name, value)	pre-computed AttrSpec<N,V> — .selector for delegate(), .attrs to spread into JSX (rename-safe)
attr(name)	dynamic factory — attr<N,V=string>(name) returns (value: V) => { readonly [name]: V }; both generics off → N inferred, V defaults to string; specify both to constrain values
toElement(jsx)	parse JSX into a DOM node (SVG-aware). Single-root → Element; multi-root (<><svg/> label</>, two icons side by side) → DocumentFragment that appendChild/replaceChildren/append inlines into the parent.
raw(html)	inject pre-escaped HTML
arraySignal(initial?)	granular keyed-list signal (subpath kerfjs/array-signal); each() reconciles in O(patches)

Hard rules — every AI assistant gets these wrong at least once

1. JSX renders to HTML strings, not DOM nodes. Don't pass DOM nodes as JSX children — the runtime throws. Need a ref? Build the JSX, then querySelector after mount() / toElement().
2. Diff keys: id first, then data-key. Lists MUST set data-key={item.id} per item — otherwise the diff matches by position and you lose focus, cursor, and identity on insert/delete.
3. Three escape hatches for the morph:
   • data-morph-skip — element AND subtree preserved verbatim. For library-owned hosts (Monaco, xterm, D3).
   • data-morph-skip-children — attrs on the host morph, subtree preserved. For client-hydrated slots whose loading/state classes need to flow through.
   • data-morph-preserve — element survives the trailing-removal pass even when the new template doesn't emit it. For imperatively-injected children (autoplay video, tooltip overlay, analytics pixel). Does NOT block a keyed-match move.
4. Never addEventListener inside a mount()-managed tree unless under data-morph-skip. A morph re-render may discard the node. Use delegate / delegateCapture instead.
5. Capture the delegate() / delegateCapture() disposer whenever the registration's scope is shorter than the page. Both helpers return () => void; the listener closure pins rootEl, handler, and everything the handler closes over (stores, signals, app state). Discarding the disposer on a transient root (modal, route view, mount swap, dynamic widget) leaks the listener AND the app graph it captures; re-mount cycles stack listeners linearly. mount()'s own disposer does NOT remove delegates for you. Safe to discard only when the registration is truly page-lifetime (root is document.body or equivalent, attached once at startup, never torn down).
6. One mount() per root. Don't nest mount() calls. Compose with plain functions returning JSX.
7. Components are plain functions. <MyComponent props /> works — the JSX runtime calls MyComponent(props) and uses the returned JSX — but there's no hook system, no lifecycle, and no per-instance state. State lives in module-scope signals or stores, never in component closures.
8. Signal reads must happen INSIDE the render function to be tracked. const x = count.value; mount(el, () => <span>{x}</span>) does NOT re-render. Move the read inside.
9. Store actions take (set, get), not (state). set(next) replaces state; mutating get() does nothing.
10. Use data-action attributes, not inline onClick. Inline handlers are NOT supported by the JSX → string runtime; delegate from the root.
11. arraySignal is opt-in for long keyed lists where most updates are pointwise. For short lists / filter+sort pipelines, plain signal + each(items.value, ...) is simpler and equally fast.
12. Custom-element types: declaration-merge into kerfjs/jsx-runtime, NOT into a global JSX namespace. Pattern: declare module 'kerfjs/jsx-runtime' { namespace JSX { interface IntrinsicElements { 'my-tag': KerfCustomElement & { foo?: string } } } }.
13. Each each() row must produce exactly one top-level element. Multi-root or empty rows throw a row-precise error. Wrap multiple roots in one parent.
14. each() is for DYNAMIC lists. Use .map() for static structural arrays (constant COLUMNS / TABS / settings sections) whose row render reads signals. each() memoizes per-item HTML by object identity; constant items never change identity, so the cache hits forever, the row render is never re-invoked, and signal reads inside it silently stop tracking. Outer .map() for the static frame + inner each() for the dynamic sub-list is the idiomatic shape.

Decision-making axes

When deciding which primitive to reach for, work down the axes:

Events.

• Originates inside the mount tree → delegate(rootEl, type, sel, handler). Originates outside (window-level keyboard, online/offline, beforeunload) → native window.addEventListener at module top-level.
• Gesture that needs to follow an element after press (drag, draw, resize) → at the start event, el.setPointerCapture(e.pointerId). Subsequent pointermove / pointerup redirect to the captured element and delegate(rootEl, 'pointermove', '[data-card]', …) still picks them up. Don't reach for window.addEventListener for in-mount-tree gestures.
• Well-known non-bubbler (focus, blur, scroll, load, error, mouseenter, mouseleave) → still delegate(); it auto-promotes to capture. Custom non-bubblers or strict element-match → delegateCapture().

Lists.

• Items change across renders (todos, chat messages, table rows) → each(items, render).
• Static structural enumeration whose row render reads signals → STATIC.map(item => <jsx/>). Inner each(item.children, …) still gets keyed reconcile.
• Long list with point-wise mutations → arraySignal + each(arraySig, render) for O(patches) updates.

Side effects / imperative DOM.

• Library-owned subtree survives across renders → data-morph-skip on host.
• Host attributes morph but subtree preserved → data-morph-skip-children.
• Imperatively-injected element survives the trailing-removal pass → data-morph-preserve.
• Focused input / contenteditable caret survives re-renders → automatic; no opt-in.

Raw HTML.

• User-controlled HTML → sanitize first (DOMPurify) then raw(sanitized).
• Author-controlled trusted HTML → raw(html) directly.

Canonical patterns

// Pattern 1: signal + mount + delegate
const count = signal(0);
const ACTIONS = { inc: attr('data-action', 'inc') } as const satisfies Record<string, AttrSpec<'data-action'>>;

mount(document.getElementById('app')!, () => (
  <div>
    <button {...ACTIONS.inc.attrs}>+</button>
    <span>{count.value}</span>
  </div>
));
delegate(rootEl, 'click', ACTIONS.inc.selector, () => { count.value += 1; });

// Pattern 2: keyed list with per-row memoization
mount(listEl, () => (
  <ul>
    {each(rows.value, (row) => <li data-key={row.id}>{row.label}</li>)}
  </ul>
));

// Pattern 3: store with reset
const cart = defineStore({
  initial: () => ({ items: [] as string[] }),
  actions: (set, get) => ({
    add:   (id: string) => set({ items: [...get().items, id] }),
    clear: ()           => set({ items: [] }),
  }),
});
// access: cart.state.value.items, cart.actions.add('x'), cart.reset()

// Pattern 4: one-shot reconcile (no signals, no effect)
morph(liveCard, '<article class="card">…</article>');

Diagnosing common errors

Error / symptom	Root cause	Fix
JSX: DOM elements cannot be passed as children	passed a toElement() result inside JSX	Build the whole tree in JSX; refs via querySelector after rendering
Focus / cursor lost on every keystroke	list items lack data-key	Add data-key (or id) to each list item
Click handler stops firing after re-render	el.addEventListener was used	Replace with delegate(rootEl, 'click', ACTIONS.foo.selector, ...) (or a string literal for ad-hoc cases)
Render fn never re-runs	signal was read outside the render fn	Move signal.value read inside the render fn
SVG renders as broken / namespaceless markup	innerHTML used directly	Use mount or toElement (SVG-aware)
Library widget destroyed on every render	host reachable by the morph	Wrap host in data-morph-skip; mount the library imperatively after first render
<my-tag> fails to typecheck	declaration merging targeted global JSX	Use declare module 'kerfjs/jsx-runtime' { namespace JSX { … } } instead
each(): row render at index N produced K top-level elements	row returned multiple sibling elements or zero	Wrap them in one parent so the row renders exactly one element
Drag/drop / state change has no visible effect; only elements outside each() update	Used each(STATIC_ARRAY, …) whose row render reads signals. Items never change identity → cache hits forever → row render never re-invoked → signal reads stop tracking	Replace outer with STATIC_ARRAY.map(...); keep inner each() for the dynamic sub-list. See Hard Rule 14

Workflow guidance

When the user asks you to add a feature to a kerf app:

1. Check what state already exists. Is there a signal / store you should reuse? Don't create a new one for derived data — use computed.
2. Decide where state lives. Module-scope signal for ephemeral UI state; defineStore for state shared across mounts or that needs reset() for tests.
3. Decide who fires the action. A handler on a DOM event → delegate with a data-action attribute. A signal change → effect().
4. Render output is JSX returning SafeHtml. No JSX-as-DOM-node, no inline handlers. Lists get data-key.
5. Test with kerfjs/testing's clearStoreRegistry() between unit tests if you used defineStore.

When you spot user code that violates any of the hard rules above, fix it inline AND explain the rule briefly so the user learns the pattern.

Server / SSR

SafeHtml.toString() returns the HTML string. JSX works in Node with no DOM. mount, morph, delegate, toElement all require a DOM and run client-side.

Where to look next

• API reference: https://brianwestphal.github.io/kerf/api/
• Full AI guide: https://github.com/brianwestphal/kerf/blob/main/docs/ai/usage-guide.md
• llms.txt index: https://github.com/brianwestphal/kerf/blob/main/llms.txt
• Example apps: https://brianwestphal.github.io/kerf/examples/