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# $derived Definition **Definition:** Creates computed values that automatically update when dependencies change **Syntax:** `$derived<T>(expression: T): T` **Parameters:** - `expression` - The expression to derive from **Returns:** Computed value that updates when dependencies change **Variants:** - `$derived.by<T>(fn: () => T): T` - Complex derivation using function ## Examples ```js // Basic derived value let count = $state(0); const doubled = $derived(count * 2); // Complex derivation with function const expensive = $derived.by(() => { return heavyComputation(count); }); // Multiple dependencies let firstName = $state('John'); let lastName = $state('Doe'); const fullName = $derived(`${firstName} ${lastName}`); ``` ## Related - `$state` - For reactive state that derived values depend on - `$effect` - For side effects responding to derived changes - `$derived.by` - For complex derivation logic