`, not directly in ``.
- **`Cannot read properties of undefined (reading 'request')`** — the handler is missing `event` destructuring or you forgot to declare `export async function POST(event)` / `GET({ request, url })`.
- **Types complain in JSDoc** — ensure your `tsconfig.json` `extends` `./.svelte-kit/tsconfig.json`.
## Gotchas
- `+page.js` runs on BOTH server and client. Don't put secrets or DB calls there — use `+page.server.js`.
- Data returned from `+page.server.js` is serialized with `devalue`; functions / class instances / `Map` / `Set` need explicit conversion.
- `+server.js` files are NOT wrapped by `+layout.svelte`. They emit raw responses. Use `handle` hook for cross-cutting concerns.
- `+error.svelte` is NOT used for errors thrown in `+server.js` or `handle` hook. Use `src/error.html` for that fallback.
- `csr = false` removes ALL JavaScript on the page — including the client router. Links become full reloads.
- `prerender = true` at the root layout prerenders ALL routes. Override per-page with `prerender = false`.
- `
` to force a server navigation.
- `URLSearchParams.get()` returns `null` (not `undefined`) when the key is missing.
- `event.url` in `+server.js` is a full URL; on the client `page.url` is also full; relative URLs work in `load`'s special fetch.
- Dynamic segments match across `/` boundaries ONLY if you use rest params `[...rest]`.
- Browser extensions injecting elements into `` directly can break hydration — keep `%sveltekit.body%` inside a ``.
## FAQ
**Q: Do I need TypeScript?**
No. `npx sv create` can scaffold a JS-only project. JSDoc types still work and provide type safety.
**Q: Can I use Svelte 5 runes in SvelteKit?**
Yes. SvelteKit 2.20+ supports Svelte 5. Use `$state`, `$derived`, `$props`, `$effect` as usual.
**Q: How do I call my own backend from a `+page.server.js`?**
Use `fetch` (the SvelteKit special one inside `load` preserves cookies and accepts relative URLs). Outside `load`, pass `cookie`/`authorization` headers explicitly.
**Q: Can `+page.js` and `+page.server.js` coexist?**
No. Use one or the other for the same route. Prefer `+page.server.js` for sensitive data.
**Q: Where do I put API keys?**
Server-only env vars in `$env/static/private` (inlined at build time) or `$env/dynamic/private` (read at runtime). Never expose private env to client code.
**Q: What's the difference between `error(404)` and `throw error(404)`?**
`error()` returns a special object — DON'T throw it. Just call `error(404, 'msg')` and SvelteKit handles the rest.
**Q: How do I get query params in a page?**
`$page.url.searchParams.get('foo')` (Svelte 4 store) or `page.url.searchParams.get('foo')` (Svelte 5 via `$app/state`).
**Q: Can I make a static-only site?**
Yes — install `adapter-static` and set `prerender = true` in the root layout (or per-page). Build output is plain HTML.
**Q: How does SvelteKit differ from Next.js / Nuxt?**
File-based router, SSR-then-CSR default, no built-in API style — you write `+server.js` for endpoints. Adapters are official and well-maintained.
## Examples & References
### Examples
| File | Description |
|---|---|
| [examples/README.md](examples/README.md) | Index of all examples |
| [examples/project-creation.md](examples/project-creation.md) | `npx sv create`, --template, --types, --add, --install |
| [examples/project-types.md](examples/project-types.md) | Each project type with adapter + config snippet |
| [examples/project-structure.md](examples/project-structure.md) | `src/`, routes, static, `package.json`, `svelte.config.js` |
| [examples/svelte-config.md](examples/svelte-config.md) | Common `svelte.config.js` setups (preprocess, kit, alias) |
| [examples/web-standards.md](examples/web-standards.md) | fetch with credentials, Request, Response, FormData, streams, URL |
| [examples/routing-pages.md](examples/routing-pages.md) | +page.svelte, +page.js, +page.server.js, params, layouts |
| [examples/routing-server.md](examples/routing-server.md) | +server.js GET/POST/PUT/DELETE, content negotiation, fallback |
### References
| File | Description |
|---|---|
| [references/README.md](references/README.md) | Index of all references |
| [references/project-types-reference.md](references/project-types-reference.md) | Full decision matrix for all project types |
| [references/project-structure-reference.md](references/project-structure-reference.md) | Every file in the standard project layout |
| [references/web-standards-reference.md](references/web-standards-reference.md) | Fetch / Request / Response / FormData / Streams / URL / Web Crypto in SvelteKit |
| [references/routing-reference.md](references/routing-reference.md) | +page, +layout, +server, +error, $types reference |