Often, you will need to derive state from other state. For this, we have the $derived rune:
App
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:
App
<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.
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<script>
let numbers = $state([1, 2, 3, 4]);
function addNumber() {numbers.push(numbers.length + 1);
}
</script>
<p>{numbers.join(' + ')} = ...</p><button onclick={addNumber}>Add a number
</button>