Question

When should you use $derived.by instead of $derived?

let total = $derived.by(() => {
  let sum = 0;
  for (const item of cart) sum += item.price * item.qty;
  return sum;
});

Accepted Answer

When the computation needs several statements; $derived takes a single expression

Answer

When the value must be computed on the server

Answer

When the value is a promise

When should you use `$derived.by` instead of `$derived`?

Explanation