Oh wow, so this still works even though Svelte doesn't support TypeScript features that aren't just type annotations (like decorators)? Is that because you're outside of `svelte.ts` files so the compiler never touches them?
Yeah it works fine with the latest sveltekit. You do need to set { esbuild: target: "es2022" } in the vite config to enable JS decorators but that isn't specific to svelte. These aren't TS decorators, they're ecmascript decorators (https://github.com/tc39/proposal-decorators) which are at stage 3 and being adopted by JS runtimes.
Under the hood the class is just creating a hidden property with $state({}) and the accessors are reading/writing to that property. Since $state() creates a deeply reactive object it acts the same as marking individual fields with $state().
