Svelte Fix, Crash & Optimization Guide

Svelte is a compiler-based JavaScript framework that shifts work to build time, producing small, fast vanilla JS with no virtual DOM. SvelteKit is its official application framework, adding routing, SSR, and adapters on top of Svelte.

Install / First Setup

Scaffold a SvelteKit app with the official CLI:

npx sv create my-app
cd my-app
npm install
npm run dev

For a plain Svelte (non-Kit) SPA, use Vite: npm create vite@latest my-app -- --template svelte. The dev server defaults to http://localhost:5173 (Vite) or http://localhost:5173 for SvelteKit. Node 18+ is recommended for the toolchain.

Common Issues & Fixes

"Cannot find module" / component import without extension

Cause: Importing a .svelte file without its extension, or a wrong path. Fix: Import with the extension: import Card from './Card.svelte'. Vite/SvelteKit resolves .svelte files when the extension is included.

Reactive statement (`$:) not updating

Cause: The dependency isn't referenced in the reactive block, so Svelte doesn't track it. Fix: Reference the exact variables inside the $: block; Svelte re-runs it only when referenced dependencies change. Avoid mutating arrays/objects in place — assign a new reference.

Hydration mismatch in SvelteKit SSR

Cause: Rendering browser-only or time-dependent values during server render. Fix: Move that logic into onMount or browser checks (import { browser } from '$app/environment'), so it runs only on the client.

Build fails on missing adapter

Cause: SvelteKit needs an adapter (@sveltejs/adapter-auto, adapter-node, adapter-static) to produce output. Fix: Install the adapter and set it in svelte.config.js (e.g. import adapter from '@sveltejs/adapter-node'). For static sites use adapter-static and prerender routes.

Vite port conflict

Cause: Another dev server holds the default Vite port (5173). Fix: npm run dev -- --port 5174, or stop the conflicting process.

Performance & Optimization

• Low-End (4–8 GB RAM): Svelte compiles to minimal JS, so runtime cost is low. Run a single Vite dev server; the default setup is already lightweight.
• Mid (16 GB): Use SvelteKit's automatic code-splitting per route, and the production build (npm run build) for tree-shaken output. Lazy-load heavy components with dynamic import().
• Workstation (32 GB+): Prerender/SSR static routes (adapter-static / prerender) to cut client work, and enable fine-grained reactivity (Svelte 5 runes like $state/$derived) for efficient updates. Profile bundle with the Vite build analyzer.

Because Svelte compiles away the framework, bundle sizes are naturally small — keep it that way by importing only used utilities and avoiding large all-in-one libraries.

Version & Compatibility Notes

• Svelte 5 introduced runes ($state, $derived, $effect) as the modern reactivity model, while the older let + $: syntax remains supported for migration.
• SvelteKit requires an adapter to deploy; adapter-auto picks a platform automatically, with adapter-node/adapter-static/adapter-vercel for specific targets.
• Node 18+ is recommended for the build toolchain. For exact version support, consult official release notes.

FAQ

Q: Why isn't my $: reactive statement updating? A: Svelte re-runs it only when variables referenced inside change. Reference the real dependency, and assign new references for objects/arrays instead of mutating in place.

Q: Do I need the .svelte extension in imports? A: Yes — import components with their extension, e.g. ./Card.svelte, so the bundler resolves them.

Q: How do I fix SvelteKit hydration errors? A: Run browser-only/time-dependent code in onMount or guard with the browser flag from $app/environment.

Q: What adapter should I use for SvelteKit? A: adapter-auto for supported hosts; adapter-node for a Node server; adapter-static for fully static sites (with prerendering).

Q: What are Svelte 5 runes? A: Runes are the new reactivity primitives: $state for state, $derived for computed values, and $effect for side effects. The older syntax still works during migration.

Q: How do I change the dev server port? A: Pass --port 5174 to the dev command (e.g. npm run dev -- --port 5174).

Related Guides

• Vue Fix & Optimization Guide
• React Fix & Optimization Guide
• Dev RAM Calculator

Accuracy Note

Commands and paths reflect common, real-world setups as of 2026-08. Always verify against your installed version and OS. When in doubt, consult the official Svelte documentation.