Reactive Statements: $: block
Run imperative side-effect code whenever its reactive dependencies change.
Reactive Statements: $: block is a free Sveltejs Academy lesson on CoddyKit. This is lesson 1 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
The reactive label runs its statement whenever its dependencies change.
$: doubled = count * 2;Statements vs Declarations
It can be a simple assignment, a function call, or a block.
Block Form
Wrap multiple statements in {} for side-effect blocks.
$: {
console.log(count);
localStorage.setItem("count", count);
}Conditional Side Effects
Combine with if for guarded actions.
$: if (count > 10) showAlert();Multiple Reactive Statements
Declare as many as you want; Svelte orders them by dependency.
Top-Level Only
The label must be at the top level of the script. Nested labels are syntax errors.
No Manual Dependencies
Svelte derives dependencies from the right-hand-side references statically.
Heavy Work
Avoid expensive computations in reactive statements. They re-run often.
Pair With Stores
Reactive statements work seamlessly with $store values too.
Order of Evaluation
Svelte topologically sorts statements: later ones can depend on earlier ones safely.
Equivalent of useEffect
Think of $: as a dependency-tracking effect with cleaner syntax.
Quick Check
Can you use $: inside a function?
Recap
$: declares reactive statements at the top level. Use blocks for side effects and conditions.
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Frequently Asked Questions
Is the “Reactive Statements: $: block” lesson free?
Yes — the full text of “Reactive Statements: $: block” 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 “Reactive Statements: $: block”?
Run imperative side-effect code whenever its reactive 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 1 of 4.
How long does the “Reactive Statements: $: block” 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
- Reactive Statements: $: block
- Reactive Dependencies and Ordering
- When to Use Stores vs Local State
- Avoiding Stale Closures