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@wcstack/defined is a headless custom-element readiness component for the wcstack ecosystem.

It is not a visual UI widget. It is an async primitive node that turns "are these custom elements registered yet?" into reactive state — the way @wcstack/permission turns a browser grant into reactive state.

With @wcstack/state, <wcs-defined> can be bound directly through path contracts:

• input surface: tags, mode, timeout
• output state surface: defined, pending, missing, count, total, error

This means readiness-aware UI — loading gates, skeletons, lazy-load failure fallbacks — can be expressed declaratively in HTML, without writing customElements.whenDefined() chains and timeout glue in your UI layer.

@wcstack/defined follows the CSBC (Core / Shell / Binding Contract) architecture:

• Core (DefinedCore) waits on whenDefined() for each tag, aggregates per mode, and drives the timeout
• Shell (<wcs-defined>) connects that state to DOM attributes and lifecycle
• Binding Contract (static wcBindable) declares observable properties (and, deliberately, no commands)

CSS already solves flash-of-unstyled-content for undefined elements, declaratively and with zero JS:

my-widget:not(:defined) { visibility: hidden; }

So if all you need is to hide an un-upgraded element, use CSS — you do not need this package. <wcs-defined> earns its place by doing what :defined cannot:

• Timeout-based failure detection. A custom element loaded via a dynamic import (e.g. @wcstack/autoloader) whose module fails to load leaves whenDefined() pending forever. CSS can only keep hiding it. With a timeout, the tag drops into missing, so a load failure becomes observable state (missing.length > 0) you can show a real error for.
• Multi-tag aggregation. Wait for all tags (mode="all") or any tag (mode="any") with one element.
• Readiness as reactive state. Drive conditional rendering, gates, and progress (count / total) — not just styling.

<wcs-defined> is a one-way element → state monitor: it observes registration, it never defines anything. Like <wcs-permission>, it has no commands at all — command-token does not apply, only event-token. The signal is monotonic (a tag, once defined, stays defined), so the state is terminal: it settles once every tag resolves, or once the timeout elapses.

Companion to the autoloader. In a real app the watched tags come from an Import Map + @wcstack/autoloader (@components/ prefix). <wcs-defined> is how you know when those lazily-imported components are ready — and, via the timeout, when one failed to arrive.

npm install @wcstack/defined

<script type="module" src="https://esm.run/@wcstack/state/auto"></script>
<script type="module" src="https://esm.run/@wcstack/defined/auto"></script>

<wcs-state>
  <script type="module">
    export default { ready: false };
  </script>
</wcs-state>

<wcs-defined tags="my-chart,my-grid" data-wcs="defined: ready"></wcs-defined>

<div data-wcs="hidden: ready">Loading components…</div>
<div data-wcs="hidden: ready|not"><my-chart></my-chart><my-grid></my-grid></div>

<wcs-state>
  <script type="module">
    export default {
      ready: false,
      missing: [],
      get hasFailed() { return this.missing.length > 0; },
    };
  </script>
</wcs-state>

<!-- If a tag has not registered within 5s, it moves to `missing`. -->
<wcs-defined tags="my-chart" timeout="5000"
  data-wcs="defined: ready; missing: missing"></wcs-defined>

<div data-wcs="hidden: hasFailed">…spinner / content…</div>
<div data-wcs="hidden: hasFailed|not">A component failed to load. Please reload.</div>

<wcs-state>
  <script type="module">
    export default {
      anyReady: false, loaded: 0, total: 0,
      get progress() { return `${this.loaded} / ${this.total}`; },
    };
  </script>
</wcs-state>

<!-- mode="any": defined flips true as soon as the first tag registers. -->
<wcs-defined tags="a-card,b-card,c-card" mode="any"
  data-wcs="defined: anyReady; count: loaded; total: total"></wcs-defined>

<span data-wcs="textContent: progress"></span>

See examples/defined-loader for the full demo (readiness gate + timeout failure + late promotion).

Attributes are read at connect time, not observed (see Notes).

All six derive from the single wcs-defined:change event, whose detail is the full snapshot. At every dispatch the invariant total === count + pending.length + missing.length holds; pending and missing partition the not-yet-defined tags, split by the timeout.

None. There is no imperative action to "define" a tag — only observation. <wcs-defined> is a pure monitor (event-token only).

<wcs-defined> reflects two boolean output states onto its ElementInternals CustomStateSet, so you can style it directly from CSS with the :state() pseudo-class — no data-wcs binding or extra class toggling required.

pending / missing / count / total are not reflected — they are not booleans, and count-like values are excluded from :state() reflection by design (see docs/custom-state-reflection-design.md §3.2).

wcs-defined:state(defined) ~ .content  { display: block; }
wcs-defined:state(defined) ~ .skeleton { display: none; } /* default */

form:has(wcs-defined:state(error)) .banner { display: block; }

Unlike attributes or classes, :state() cannot be written from outside the element, so there is no risk of confusing this output state with an input.

Browser support (:state(x) syntax): Chrome/Edge 125+, Safari 17.4+, Firefox 126+. In older browsers the states are simply never set — :state() selectors never match, but <wcs-defined> itself keeps working normally (graceful degradation, never-throw).

SSR: :state() cannot be serialized into HTML, so server-rendered markup never carries these states on first paint (@wcstack/server is unaffected). If you need to style the pre-hydration gap, pair your rule with wcs-defined:not(:defined) instead.

Custom states are invisible in DevTools' Elements panel and attachInternals() cannot be called twice, so there is no console way to inspect them directly. Two debug-only aids are provided for that:

• el.debugStates — a snapshot array of the currently-on state names (e.g. ["defined"]). It is not part of wc-bindable (not a bind target) and its shape is not a guaranteed contract — use it for debugging only.

• The debug-states attribute (opt-in, default off) mirrors state changes onto data-wcs-state-defined / data-wcs-state-error attributes on the element, so the Elements panel highlights them as they toggle:

  <wcs-defined tags="my-chart,my-grid" debug-states></wcs-defined>

Write your CSS against :state(), not data-wcs-state-*. The mirrored attributes exist purely to make state changes visible while debugging with DevTools open; they are not a supported styling hook.

• Monotonic and terminal. whenDefined() never reverts: once a tag is defined it stays defined. The state settles once every tag resolves or the timeout fires. After a timeout, a tag that registers late is promoted out of missing back into the count (so defined can still flip true afterwards).
• Invalid names fail softly. A tag name that is not a valid custom element name (no hyphen, etc.) yields a rejected whenDefined(); it is recorded in error and placed in missing, never thrown. Other valid tags keep being watched (never-throw).
• Attributes are read at connect time, not observed. <wcs-defined> does not implement observedAttributes / attributeChangedCallback. tags / mode / timeout are fixed when the element connects; to watch a different set, use a separate element (or re-connect).
• Reconnect re-watches. Removing and re-inserting the element runs connectedCallback again. A watch in flight when the element disconnects is invalidated, so a rapid disconnect→reconnect cannot leak a stale callback.
• SSR (@wcstack/server). Declares static hasConnectedCallbackPromise = true and exposes connectedCallbackPromise, so the server renderer waits for readiness before snapshotting. Specify a timeout for SSR — without one, an unresolved tag leaves the promise pending forever. The pending-forever risk is sharpest under SSR with timeout="0" (or unset) on an autoloaded tag: the render hangs awaiting a registration that may never happen. Always pair SSR + autoloaded tags with a finite timeout.
• Array getters return fresh copies. pending and missing (and the event detail arrays) are new arrays on every read/dispatch, so external mutation cannot corrupt internal state. The flip side: do not rely on referential equality between reads — compare contents, not identity.

The Core has no DOM dependency and can be used directly with bind() from @wc-bindable/core:

import { DefinedCore } from "@wcstack/defined";

const gate = new DefinedCore(["my-chart", "my-grid"], "all", 3000);
gate.addEventListener("wcs-defined:change", (e) => {
  const snap = (e as CustomEvent).detail;
  console.log(snap.defined, snap.count, snap.total, snap.missing);
});

await gate.ready;        // every tag resolved, or the timeout fired
console.log(gate.defined, gate.missing);

// later, when done:
gate.dispose();          // clear the timeout and stop watching

MIT