Server & API

API endpoints with +server.js

A +server.js file is a route with no UI — a standalone HTTP endpoint. Export functions named for the HTTP method; each returns a Response. Use it for JSON APIs, webhooks, file downloads, or anything a non-browser client calls.

import { json, error } from '@sveltejs/kit';

export async function GET({ url }) {
  const page = Number(url.searchParams.get('page') ?? '1');
  return json(await getPosts(page));      // json() sets the header + serializes
}

export async function POST({ request }) {
  const body = await request.json();
  if (!body.title) error(400, 'title required');   // throw a typed HTTP error
  return json(await createPost(body), { status: 201 });
}

Endpoint vs form action: use a form action when a page’s own form submits to the server (it integrates with use:enhance and the form prop). Use a +server.js endpoint for a reusable API consumed by fetch, a mobile app, or a third party.

Hooks: middleware for every request

src/hooks.server.js exports handle, which runs on every request before it reaches a route — the place for authentication, logging, and populating locals.

export async function handle({ event, resolve }) {
  const token = event.cookies.get('session');
  event.locals.user = token ? await getUser(token) : null;  // available to load/actions
  return resolve(event);   // continue to the route
}

Whatever you set on event.locals is available in every server load and action for that request (via locals) — this is how the current user flows through the app.

flowchart LR
  req["incoming request"] --> handle["hooks: handle()
set locals.user"]
  handle --> route["route: load / action / +server"]
  route --> resp["response"]

A request passes through handle, then the route

Render mode per route, and adapters

Each route can choose how it renders, with page options exported from +page.js/+page.server.js:

export const prerender = true — render to a static file at build time (great for pages that are the same for everyone).
export const ssr = false — skip server rendering (client-only page).
export const csr = false — ship no client JS (fully static output for that page).

Finally, an adapter (set in svelte.config.js) packages your built app for a target platform:

adapter-auto — detects common platforms (the default in a new project).
adapter-node — a standalone Node server.
adapter-cloudflare — a Cloudflare Worker/Pages.
adapter-static — a fully prerendered static site (no server needed).

You choose the adapter for where you deploy; the same app code runs across them.