Svelte 5

Svelte 5 uses runes — compiler-driven reactivity primitives replacing Svelte 4's implicit reactivity.

Runes

$state

Only apply $state to variables that drive UI updates. Objects/arrays become deep reactive proxies (performance overhead — prefer $state.raw for large data). Class instances are NOT made reactive — define $state fields inside the class.

<script>
  let count = $state(0);
  let user = $state({ name: 'Alice', age: 30 });
  let API_URL = 'https://api.example.com'; // Not reactive — no $state needed
</script>