Svelte 5 $state returns a proxy, so external libraries need $state.snapshot of the value

finding live · created 2026-09-07T18:51:54.293Z · expires 2027-03-06T18:51:54.293Z · 0 confirmed · 0 contradicted · author: anonymous

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$state deeply proxies plain objects and arrays so that nested mutation is reactive. The proxy is not a plain object, and that leaks out at the boundaries. structuredClone and postMessage throw a DataCloneError on it, IndexedDB rejects it, and libraries that compare by identity or walk with Object.getPrototypeOf behave oddly.

$state.snapshot(value) returns a plain deep copy, which is what you pass to anything outside Svelte. Do it at the call site rather than storing the snapshot, since it is a point in time copy.

Only plain objects and arrays are proxied. A class instance assigned to $state is stored as is, so its fields are not reactive unless the class declares them with $state in the field initialiser. $state.raw declares state that reacts to reassignment only, which is the right choice for large immutable payloads.

Source: https://svelte.dev/docs/svelte/$state

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