Svelte 5: Cannot export state from a module if it is reassigned
In a .svelte.js or .svelte.ts module, $state works because the compiler rewrites every read and write of that variable inside the file. Other files that import it only get the value it had, so if the module ever reassigns it (count = 0, user = null) importers would be left holding a stale copy, and Svelte refuses to compile. Export an object and change its properties, or keep the variable private and export functions that read and set it.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# The compiler names the file and line
npx svelte-check --output human
// counter.svelte.js. This fails, because count is reassigned:
// export let count = $state(0);
// export function reset() { count = 0; }
// Option 1: export an object and mutate its properties
export const counter = $state({ count: 0 });
export function reset() { counter.count = 0; }
// Option 2: keep the variable private and export accessors
// let count = $state(0);
// export const getCount = () => count;
// export const setCount = (value) => { count = value; };
// Option 3: a class with $state fields
// export class Counter { count = $state(0); reset() { this.count = 0; } }How to diagnose Svelte errors
Svelte errors mostly arise at the compiler's boundaries: the $ store prefix only works on real stores in component context, and SvelteKit's load functions run in two different environments with different capabilities. A 500 from a load function usually means server-only code (a database client, a secret, fs) reached a universal load that also runs in the browser.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Check whether the file is a universal load (
+page.js) or a server-only load (+page.server.js). Anything touching secrets or the filesystem belongs in the latter. - Read the full server-side stack trace in the terminal, not just the browser's 500 page: SvelteKit hides details in production for security.
- For
$prefix errors, confirm the value is an actual store with asubscribemethod and that you are inside a component, not a plain module. - Use
$env/static/privateand$env/dynamic/privatefor secrets so the compiler enforces the server-only boundary for you. - Reproduce with
vite build && vite preview; dev-mode behaviour differs meaningfully from the production build.
Tools worth reaching for
vite build && vite previewSvelteKit terminal logssvelte-check$env/static/private
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
Related Svelte errors
- Svelte 5: `$:` is not allowed in runes mode, use `$derived` or `$effect` insteadAs soon as a component uses any rune ($state, $props, $derived), Svelte compiles the whole…
- Svelte 5: `$effect` can only be used inside an effectEffects have to be created while a component is initialising, so calling `$effect` from a…
- Svelte 5: `$props()` can only be used at the top level of componentsRunes are compiled away rather than called, so `$props()` has to appear exactly once as the…
- Svelte 5: `export let` is not allowed in runes mode, use `$props()` insteadA component is in runes mode, where props arrive through $props() rather than exported let…
- Svelte 5: $state / $derived can only be used inside a .svelte or .svelte.js fileRunes are compiler features, not runtime functions. They only work in files the Svelte…
- Svelte 5: effect_update_depth_exceeded (Maximum update depth exceeded)An effect wrote to state that it also reads, so running it schedules it again and Svelte…
- Svelte 5: State referenced in its own scope will never updateReading a `$state` variable into a plain `const`, or passing it to a function, copies the…
- Svelte 5: Updating state inside a derived is forbiddenA `$derived` expression re-runs whenever its inputs change, so writing state from inside one…
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Something missing or wrong?
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