kerfjs

Kerf

The smallest cut.

brianwestphal.github.io/kerf — docs · examples · live demo

Introducing Kerf. The smallest cut.

~12 KB. No virtual DOM. No compiler. No magic. Reactive UI that touches only the bytes that changed.

import { signal, mount } from 'kerfjs';

const count = signal(0);

mount(document.getElementById('app')!, () => (
  <div>
    <button data-action="inc">+</button>
    <span>{count.value}</span>
  </div>
));

That's it. Your JSX renders to HTML strings, kerf's native diff applies the minimum DOM mutations to make the live tree match, and signals re-run the render only when something they read actually changed.

Here's the whole development loop — write a component, run the dev server, click around, edit, watch the browser pick it up:

Why Kerf

• Small bundle. ~12 KB minified + gzipped including @preact/signals-core (~13 KB with arraySignal). One runtime dependency. No virtual DOM, no scheduler, no concurrent-mode machinery. On the official krausest js-framework-benchmark — where kerf is a listed entry, measured on the same reference machine as every competitor (local mirror) — kerf is in the same cluster as Vue, vanjs, and Lit on most operations; Solid's compiler leads the update-path benchmarks (notably partial update), which kerf doesn't try to match by design — no compiler.

• No virtual DOM, no compiler. JSX → HTML strings → native diff. DevTools shows the real DOM because it is the DOM.

• Values bind, structure re-renders. Hand a signal itself into a JSX hole — class={selectedId} or {status} — and kerf binds that one node directly: when the signal changes, only that attribute or text node updates, with no render re-run and no list reconcile. A selection flip on a 10,000-row table touches exactly one class. Taken to its logical end: a mount whose render reads no .value at all runs exactly once, forever — every subsequent update flows through the per-hole bindings. Read .value in the render only when the structure depends on it (conditionals, list shape).

• Focus, selection, listeners survive re-renders — even mid-list. The reconciler morphs instead of rebuilding, so caret position, selection range, IME composition, and delegated listeners survive every re-render. Keyed lists get the same treatment: same-identity rows are updated in place rather than recreated, so a row reorder or a single-cell edit no longer blows away focus, scroll, or an in-flight animation the way node replacement does.

• Safe by default. Text and attribute values are HTML-escaped automatically, URL attributes are scheme-screened (javascript: / script-carrying data: dropped), inline on* handlers are rejected outright, and the same screening covers the fine-grained bound path — so untrusted data stays inert even when kerf is dropped into someone else's page. The URL screen fails loudly at your desk (throws in development) and degrades safely in the field (warns and drops in production). raw() is the explicit, auditable opt-out.

• JSX typed against HTML, not against React. Tags and attributes are checked at compile time — <diiv> and <input typo /> don't build. The attribute types are derived from the HTML standard rather than another framework's property table, and that distinction has teeth: draggable and spellcheck are enumerated attributes that take the strings "true" / "false", so kerf rejects draggable={true} rather than quietly emitting markup that means the opposite. Custom elements and web components slot in with one declaration merge.

• Small public API. ~17 exports from the main barrel (plus arraySignal and the html tagged template on their own subpaths). No hooks, no lifecycle, no per-instance state. Components are plain functions that return JSX.

• Plain TS, plain JSX, plain ESM. Drops into anything using esbuild / Vite / tsup. No plugin chain. And with the html tagged template (import { html } from 'kerfjs/html' — identical runtime semantics to JSX), a CDN / importmap project needs no build step at all.

• Grown-up tooling around a tiny core. An ESLint plugin that enforces the hard rules at edit time, an opt-in family of KERF_DEV_WARN_* runtime warnings that catch the classic mistakes in development (with zero production cost), a create-kerf-component scaffold for publishable component packages, drop-in AI-assistant configs, and side-by-side migration guides for a dozen-plus frameworks — none of which grows the core runtime past ~12 KB.

When to use Kerf

• Hybrid desktop apps (Tauri / Electron) — small bundle, predictable diff, debuggable runtime; ideal for the embedded webview.
• Embedded widgets — chat bubbles, comment boxes, dashboards dropped into someone else's page.
• Server-rendered apps with islands — Rails / Phoenix / Django / Hono. mount per island; delegate survives turbo-frame swaps.
• Admin panels & internal tools — reactivity without 200 KB of framework + state lib + router.
• Replacing jQuery — incremental migration; same delegation mental model, modern primitives.
• Prototyping — entire mental model on a postcard.

When to reach for something else

• Need a full ecosystem (router + forms + data + SSR streaming) → Next.js / Remix / SolidStart.
• Building a deeply componentised design-system app → React / Solid / Svelte.
• Need React Native / cross-platform mobile → React (Kerf + Tauri/Electron also covers many of these cases).
• Building a static site → Astro (we use it for this project's site).
• Already invested in a framework where switching cost outweighs the bundle size gain.

Quick tour

import { signal, computed, defineStore, mount, each, delegate } from 'kerfjs';

// 1. A signal — single piece of reactive state.
const count = signal(0);

// 2. A computed — auto-derived from other signals.
const doubled = computed(() => count.value * 2);

// 3. A store — multi-consumer state with named actions and reset semantics.
const cart = defineStore({
  initial: () => ({ items: [] as { id: string; name: string }[] }),
  actions: (set, get) => ({
    add: (id: string, name: string) => set({ items: [...get().items, { id, name }] }),
    remove: (id: string) => set({ items: get().items.filter((i) => i.id !== id) }),
  }),
});

// 4. Mount JSX to a DOM element. Re-renders only when read signals change.
const root = document.getElementById('root')!;

mount(root, () => (
  <div>
    <h1>Cart ({cart.state.value.items.length})</h1>
    <ul>
      {each(cart.state.value.items, (item) => (
        <li data-key={item.id}>
          {item.name}
          <button data-action="remove" data-id={item.id}>×</button>
        </li>
      ))}
    </ul>
    <p>Doubled count: {doubled.value}</p>
  </div>
));

// 5. Event delegation — one listener per event type, dispatched by data-action.
delegate(root, 'click', '[data-action="remove"]', (_e, btn) => {
  cart.actions.remove((btn as HTMLElement).dataset.id!);
});

The stringly-typed '[data-action="remove"]' pair above can be made rename-safe with the attr() helper — declare the attribute once and use it on both sides:

import { attr } from 'kerfjs';

const REMOVE = attr('data-action', 'remove'); // pre-escaped name/value/selector

<button {...REMOVE.attrs} data-id={item.id}>×</button>;          // in JSX
delegate(root, 'click', REMOVE.selector, (_e, btn) => { /* … */ }); // in delegation

Fine-grained updates: bind a signal into a hole

Inside a mount(), hand a signal itself (not its .value) into an attribute or text position and kerf wires that hole straight to the signal — the render function never re-runs and the list reconciler never walks:

const status = signal('idle');

mount(root, () => (
  <div class={status}>       {/* class attribute bound to the signal */}
    Status: {status}         {/* text node bound to the signal */}
  </div>
));

status.value = 'saving';     // updates the class + the text node directly — no re-render

The headline use is external state driving one spot: a selectedId flipping a single row's class inside a 10,000-row each() list touches exactly that one node, no reconcile. Works in static content and inside each() rows (a row's binding is torn down with the row); outside a mount() (SSR / SafeHtml.toString()) a bound signal just snapshots its current value.

This is kerf's guiding idiom — values bind, structure re-renders: pass the signal itself wherever a hole is just a value, and read .value in the render function only where the JSX structure depends on it. A render that reads no .value runs exactly once; from then on every update is a direct write to the node it concerns. See docs/2-reactivity.md §2.9.

Long keyed lists: arraySignal

For lists where most updates are pointwise (single-row edits, append-to-end, selection flips on individual rows), reach for arraySignal from the kerfjs/array-signal subpath. Mutators emit typed patches that each() applies in O(patches), not O(N):

import { arraySignal } from 'kerfjs/array-signal';

const rows = arraySignal<{ id: number; label: string }>([]);

mount(root, () => (
  <ul>{each(rows, (r) => <li data-key={r.id}>{r.label}</li>)}</ul>
));

rows.push({ id: 1, label: 'a' });               // 1 insert patch
rows.update(0, (r) => ({ ...r, label: 'A' }));  // 1 update patch
rows.move(0, 1);                                // 1 move patch

The class lives in its own subpath so apps that don't need it shed ~1 KB. Reads on rows.value are tracking, so computed(() => rows.value.filter(...)) works as expected. See docs/2-reactivity.md §2.6.

One-shot reconcile: morph

mount() wraps effect() so the render re-runs on signal changes. Sometimes you have a freshly-built template and an already-populated element and you just want to reconcile them once — no subscription, no re-render loop. That's morph:

import { morph, raw } from 'kerfjs';

morph(liveCard, freshlyBuiltCardEl);                     // Element template
morph(liveCard, '<article class="card">…</article>');    // raw HTML string
morph(liveCard, raw(htmlFromServer));                    // SafeHtml

Same algorithm mount() uses internally — data-morph-skip, data-morph-skip-children, data-morph-preserve, focused-input value + selection preservation, the <details> / <dialog> user-agent-owned open rule all carry over. Use it for SSR-fragment hydration, page-refresh diffs, third-party widget remounts. See docs/4-render.md §4.4.3.

No build step at all: the html tagged template

"No compiler" isn't just a JSX story. The html tagged template from kerfjs/html has identical runtime semantics to JSX — escaping, boolean/nullish attribute rules, URL screening, on* rejection, fine-grained signal bindings, each() composition — with no transform, so a plain <script type="module"> on a CDN / importmap page is a complete kerf app:

<script type="module">
  import { signal, mount, each } from 'https://esm.sh/kerfjs@4';
  import { html } from 'https://esm.sh/kerfjs@4/html';

  const items = signal([{ id: 1, label: 'no build step' }]);

  mount(document.getElementById('app'), () => html`
    <ul>${each(items.value, (i) => html`<li id="${i.id}">${i.label}</li>`)}</ul>
  `);
</script>

Nothing is self-hosted — kerfjs is on npm, so every ESM CDN (esm.sh, jsDelivr, unpkg) mirrors it automatically. esm.sh works with a direct import as shown; jsDelivr / unpkg want an importmap so the internal @preact/signals-core import resolves. Pin to a major (@4, as shown — the latest 4.x) rather than floating on latest, or an exact version (@4.1.0) for full reproducibility. Attribute names are written verbatim (class, not className), and holes are only legal in text positions or as a complete attribute value — anything ambiguous throws with an actionable message. See docs/6-jsx-runtime.md §6.11 (§6.11.1 for the full CDN / importmap recipes) — or the live-poll example, a complete app served exactly as authored: no bundler ever touches it.

Install

npm install kerfjs

// tsconfig.json
{
  "compilerOptions": {
    "jsx": "react-jsx",
    "jsxImportSource": "kerfjs"
  }
}

Optional: eslint-plugin-kerfjs

A companion ESLint plugin enforces kerf's hard rules at edit time. Eight rules total: four error-level AST rules catch hard-rule violations — inline JSX event handlers, missing data-key in each(), nested mount(), and global JSX.IntrinsicElements augmentation — and four warn-level rules cover delegate-disposer capture, attr() selector rename-safety, raw() XSS audit trails, and AI-assistant config hygiene. The plugin is AST-only (no parser-services dependency), so it works with any TypeScript-ESLint setup.

npm install --save-dev eslint-plugin-kerfjs

// eslint.config.js (flat config, ESLint v9+)
import kerfjs from 'eslint-plugin-kerfjs';
export default [kerfjs.configs.recommended];

Full docs at brianwestphal.github.io/kerf/docs/eslint-plugin/ — legacy .eslintrc config, per-rule examples, and the rationale for which violations get lint rules vs. dev-warns vs. strict TS.

Optional: create-kerf-component

Building a reusable component package? Scaffold one that already follows kerf's hard packaging rules (kerfjs as a peer dependency and external in the build, ESM + .d.ts, jsxImportSource: "kerfjs", subpath exports) plus an example component showing per-instance state via a factory and a wire(root) delegation disposer:

npm create kerf-component@latest my-widgets

See docs/13-component-packages.md for the full authoring guide.

Links

• Site: brianwestphal.github.io/kerf
• Docs: docs/ — overview · reactivity · stores · render · events · jsx · svg · API reference
• Migrating: coming from another framework? — side-by-side TodoMVC translations + per-framework gotchas
• AI guide: docs/ai/usage-guide.md — reference for AI tools fetching kerf docs (linked from llms.txt)
• ESLint plugin: brianwestphal.github.io/kerf/docs/eslint-plugin/ — eslint-plugin-kerfjs; eight rules (four hard-rule errors + four warns: require-delegate-disposer, prefer-attr-selector, no-raw-with-dynamic-arg, ai-assistant-configs) at edit time (source: eslint-plugin/)
• Component scaffold: npm create kerf-component@latest <dir> — create-kerf-component; generates a publishable component package with the hard packaging rules pre-wired (source: create-kerf-component/)
• Demo: live demo — nine sections exercising every primitive (counter, store-backed cart, focus survival, keyed list, morph-skip, SVG render, Tier-2 capture, arraySignal patches, fine-grained signal bindings)
• Repo: github.com/brianwestphal/kerf

Why "kerf"?

A kerf is the narrow strip of material a saw blade removes when cutting — the smallest possible cut. The framework's job is the same: apply the smallest possible mutation to update your DOM.

(And yes, kerformance → performance jokes were written. They were also rejected.)

Status

Stable — the public API follows semver. See CHANGELOG.md for the current version and what's shipped.

Sponsor

If kerf saves you time on a project you ship, sponsoring on GitHub keeps it actively maintained. Any amount is appreciated.

License

MIT