Often, you will need to derive state from other state. For this, we have the $derived rune:

let numbers = $state([1, 2, 3, 4]);
let total = $derived(numbers.reduce((t, n) => t + n, 0));

We can now use this in our markup:

<p>{numbers.join(' + ')} = {total}</p>

The expression inside the $derived declaration will be re-evaluated whenever its dependencies (in this case, just numbers) are updated. Unlike normal state, derived state is read-only.