@zoijs/resource

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@zoijs/resource is an optional package. Add it only when you fetch data. It builds on the core's public API, so the core stays small and unchanged.

You can learn the whole thing in about 5 minutes.

Install

npm install @zoijs/core @zoijs/resource

Or with no install, from a CDN:

import { resource } from "https://esm.sh/@zoijs/resource@0.1";

What a resource is

A resource wraps a function that returns a promise. It runs that function once, and gives you three reactive readers plus a refresh:

import { html, mount } from "@zoijs/core";
import { resource } from "@zoijs/resource";

function Profile() {
  const user = resource(() => fetch("/api/user").then((r) => r.json()));

  return html`
    ${() =>
      user.loading() ? html`<p>Loading…</p>`
      : user.error() ? html`<p>Couldn't load your profile.</p>`
      : html`<h1>Hello, ${user.data().name}</h1>`}
  `;
}

mount(Profile, "#app");

That's it. No cache, no client, no provider to wrap your app in.

The API

resource(fetcher) returns four functions:

Function	What it gives you
data()	The loaded value, or undefined before the first success (reactive)
loading()	true while a load is in flight (reactive)
error()	The error, or null when there is none (reactive)
refresh()	Load again

The three readers are reactive — read them inside a binding (${() => ...}) and the DOM updates by itself when the data arrives.

Loading state

loading() is true immediately (the fetch starts as soon as you create the resource) and flips to false when it settles:

${() => user.loading() ? html`<p>Loading…</p>` : html`<p>${user.data().name}</p>`}

Error state

A rejected promise — or a function that throws — becomes error():

${() => user.error() ? html`<p class="err">${user.error().message}</p>` : ...}

Refreshing

refresh() runs the fetcher again. The old data stays readable while the new load is in flight, so the UI doesn't flash empty:

const posts = resource(() => fetch("/api/posts").then((r) => r.json()));

html`
  <button onclick=${() => posts.refresh()}>Reload</button>
  ${() => (posts.loading() ? html`<p>Refreshing…</p>` : null)}
  <ul>
    ${each(() => posts.data() ?? [], (p) => p.id, (p) => html`<li>${() => p.title}</li>`)}
  </ul>
`;

If you call refresh() again before the previous load finishes, only the most recent result is applied — a slow request can't overwrite a newer one.

Common mistakes

• Reading data() before checking loading(). data() is undefined until the first success — guard with loading()/error() first, or use posts.data() ?? [].
• Calling readers outside a binding. Like all Zoijs reactivity, wrap them in an arrow to make them live: ${() => user.loading()}, not ${user.loading()}.
• Expecting a cache. There isn't one. Each resource(...) is its own state; creating two does two fetches. That's intentional — it stays predictable.
• Passing arguments to refresh(). It just re-runs your fetcher; capture any inputs in the fetcher closure instead.

Server rendering ({ initial })

Pass { initial } to start a resource already-settled with a value instead of auto-loading. This is how a server-rendered resource hands its data to the client without a refetch (or a loading flash): the server renders with the real value and serializes it (see @zoijs/ssr's serialize); on hydration you create the resource with that same value.

// client, hydrating server-rendered HTML
const user = resource(() => fetch("/api/user").then((r) => r.json()),
                      { initial: window.__DATA__.user });

user.loading(); // false — already settled, no auto-load
user.data();    // the server's value, immediately
user.refresh(); // still loads on demand

The presence of the initial key (not its value) skips the load, so { initial: null } is a valid "settled with null" state.

What this is not

By design, to stay tiny: no query client, cache, request dedupe, mutations, suspense, optimistic updates, or infinite queries. (Basic SSR hand-off is supported via { initial } above; per-request data loaders are not.) If you need those, reach for a dedicated data library — this is the 90%-case helper.

License

MIT Zoijs contributors