$derived: Declarative Computed Values
Compute values that automatically update when their dependencies change.
$derived: Declarative Computed Values is a free Sveltejs Academy lesson on CoddyKit. This is lesson 3 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, and your progress syncs across the web and the CoddyKit app. The Sveltejs Academy course includes 4 lessons in total.
Computed State
Use $derived(expr) to compute a value from reactive sources.
let count = $state(0);
let doubled = $derived(count * 2);Auto Tracked
Svelte tracks dependencies of the expression automatically. The derived updates when any source changes.
Read-Only
Derived values cannot be assigned directly; they always reflect the computation.
Use in Markup
Read derived values like any state.
<p>Doubled: {doubled}</p>Lazy Evaluation
Derived values compute lazily when read; if nothing reads, the cost is skipped.
Caching
The result is cached until a source changes — no repeated computation per read.
Complex Logic
For multi-statement logic, use the $derived.by(() => { ... }) form.
Compared to $:
$derived replaces $: x = .... More explicit and works outside .svelte files.
Type Inference
TypeScript infers types from the expression automatically.
No Side Effects
Derived must be pure. For side effects use $effect.
Performance
Memoized; recomputes only when dependencies change.
Quick Check
What does $derived return?
Recap
$derived is the modern, explicit way to declare computed values. Use $derived.by for multi-line logic.
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Frequently Asked Questions
Is the “$derived: Declarative Computed Values” lesson free?
Yes — the full text of “$derived: Declarative Computed Values” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO. The Sveltejs Academy course includes 4 lessons in total.
What will I learn in “$derived: Declarative Computed Values”?
Compute values that automatically update when their dependencies change. 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, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.
How long does the “$derived: Declarative Computed Values” 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
- Why Runes: Moving Beyond the Compiler Magic
- $state: Fine-Grained Reactivity
- $derived: Declarative Computed Values
- $effect: Side Effects