$derived.by for Complex Derivations
Compute derived values with arbitrary logic using the callback form of $derived.
$derived.by for Complex Derivations is a free Sveltejs Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Sveltejs Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Multi-Statement Derivations
For derived values needing more than one expression, use the callback form.
let result = $derived.by(() => {
if (count > 10) return "high";
if (count > 5) return "medium";
return "low";
});Same Reactivity
The callback tracks dependencies the same way as a single-expression $derived.
Better Readability
For complex logic, the .by form is clearer than packing into one expression.
Loops and Branches
You can use for-loops, if-else, and let inside the callback.
Pure Functions
Like $derived, .by must be pure. No side effects allowed.
Memoization
The result is cached until dependencies change.
Avoid Heavy Work
If the computation is expensive, consider caching outside the derivation.
Typed Inference
TypeScript infers the return type from the callback.
Combine with $effect
Use $derived.by for computed values; $effect for side effects. Keep them separate.
Migration
Convert nested $: blocks into one $derived.by for cleaner code.
Real World
Useful for filtering/sorting lists, computing aggregate metrics, conditional formatting.
Quick Check
When do you use $derived.by?
Recap
Use $derived.by(() => ...) for multi-step computed values with branches or loops.
Frequently asked questions
Is the “$derived.by for Complex Derivations” lesson free?
Yes — the full text of “$derived.by for Complex Derivations” is free to read here on the web, and the Sveltejs Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO.
What will I learn in “$derived.by for Complex Derivations”?
Compute derived values with arbitrary logic using the callback form of $derived. You practise Sveltejs Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Sveltejs Academy?
No prior experience is required. Sveltejs Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “$derived.by for Complex Derivations” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Sveltejs Academy lesson?
Yes. Every Sveltejs Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- $state.raw for Non-Reactive State
- $derived.by for Complex Derivations
- $effect.root for Manual Cleanup
- Migrating from Svelte 4 to 5