Computed Values: $: reactive declarations
Derive values automatically from reactive state using the $: label syntax.
Computed Values: $: reactive declarations is a free Sveltejs Academy lesson on CoddyKit — lesson 4 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.
The $: Label
Prefix any statement with $: to make it reactive. It re-runs whenever the referenced variables change.
$: doubled = count * 2;Auto-Tracking
Svelte scans the right-hand side for variable references. You do not list dependencies manually.
Use in Markup
Use the derived value in templates like any other reactive variable.
<p>Double: {doubled}</p>Block Form
The reactive label can wrap a block of statements for side effects.
$: {
console.log("count changed to", count);
localStorage.setItem("count", count);
}Multiple Reactive Statements
Declare as many $: statements as needed. Svelte orders them by dependency.
$: doubled = count * 2;
$: quadrupled = doubled * 2;Reactive if
You can combine $: with control flow for conditional side effects.
$: if (count >= 10) { alert("limit reached"); }Equivalent to useEffect
Think of $: as a more ergonomic useEffect. No dependency arrays.
Avoid Heavy Work
Reactive statements should be fast. For expensive computations consider memoization or moving work to a store.
Async Inside
You can await inside, but be careful: the statement may re-trigger before the previous awaits complete.
When Not to Use
For pure functions of inputs, $: x = f(y) is great. For real side effects, prefer lifecycle hooks like onMount.
Top-Level Only
The $: label must be at the top level of the script block, not inside a function or nested block.
Quick Check
What does $: do?
Recap
$: declares reactive computed values and side effects. Dependencies are tracked automatically.
Frequently asked questions
Is the “Computed Values: $: reactive declarations” lesson free?
Yes — the full text of “Computed Values: $: reactive declarations” 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 “Computed Values: $: reactive declarations”?
Derive values automatically from reactive state using the $: label syntax. 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 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Computed Values: $: reactive declarations” 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
- The script template style Structure
- Reactive Variables with let
- Interpolation: {expression}
- Computed Values: $: reactive declarations