sveltekit-temporal

Temporal may soon be supported in all major browsers, but your users may not update for some time.

This sv add-on wires the Temporal API polyfill into a SvelteKit project with conditional loading so browsers and server runtimes that ship Temporal natively (Chrome 144+, Firefox 139+, Node.js 26+) pay zero bytes but you can count on users with older browsers/runtimes getting the polyfill.

Usage

# When creating a new project
npx sv create my-app --add sveltekit-temporal

# Into an existing project
npx sv add sveltekit-temporal

It will prompt you to choose a polyfill:

• @js-temporal/polyfill — official, ~100 KB gzipped, spec-conservative.
• temporal-polyfill — smaller (~40 KB gzipped), same API.

Then create / update the following files:

• src/lib/temporal.{ts,js} — the conditional bootstrap module.
• src/hooks.{ts,js} — prepends import '$lib/temporal', covering both client and server (preserves existing content).
• src/app.d.ts — adds global Temporal type and Date.toTemporalInstant() augmentation (TypeScript only).

Usage in your app

After running, reference Temporal directly anywhere (+page.svelte, +page.server.ts/js, +page.ts/js,+layout.svelte, Components etc.) — no imports needed:

<script lang="ts">
	const today = Temporal.Now.plainDateISO();
	const inAWeek = today.add({ days: 7 });
</script>

<p>One week from today: {inAWeek.toString()}</p>

Idempotency

Re-running is safe. Files written by the add-on are tagged with a // sveltekit-temporal: managed block marker. On subsequent runs:

• Matching content → skipped.
• Marker present but content drifted (e.g. you switched polyfill) → managed block replaced; surrounding code left intact.
• File exists without our marker → left untouched, reported as skipped.

Runtime support

| Runtime | Native Temporal | Notes | | ------------ | -------------------------------------- | ----------------------------------------------------------------------- | | Node.js ≤ 25 | No | Polyfill required | | Node.js 26+ | Yes (behind --experimental-temporal) | Polyfill still safe to include — the conditional check skips loading it | | Bun | No | Polyfill required; Bun tracks V8 but has not shipped Temporal yet | | Deno | No | Polyfill required; Deno tracks V8 but has not shipped Temporal yet | | Chrome 144+ | Yes | Polyfill skipped automatically | | Firefox 139+ | Yes | Polyfill skipped automatically | | Safari | No | Polyfill required |

The conditional bootstrap in src/lib/temporal.ts checks typeof Temporal !== 'undefined' before loading the polyfill, so upgrading your runtime later requires no code changes.

Switching polyfills later

Re-run sv add sveltekit-temporal and pick the other option. Only src/lib/temporal.* and src/app.d.ts will change.