svelte-core-bestpractices

$state

Only use the $state rune for variables that should be reactive — in other words, variables that cause an $effect, $derived or template expression to update. Everything else can be a normal variable.

Objects and arrays ($state({...}) or $state([...])) are made deeply reactive, meaning mutation will trigger updates. This has a trade-off: in exchange for fine-grained reactivity, the objects must be proxied, which has performance overhead. In cases where you're dealing with large objects that are only ever reassigned (rather than mutated), use $state.raw instead. This is often the case with API responses, for example.

$derived

To compute something from state, use $derived rather than $effect:

// do this
let square = $derived(num * num);

// don't do this
let square;

$effect(() => {
	square = num * num;