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. This is 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, and your progress syncs across the web and the CoddyKit app. The Sveltejs Academy course includes 4 lessons in total.
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.
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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. 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 “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, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.
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