$effect: Side Effects
Run side effects that re-execute whenever their reactive dependencies change.
$effect: Side Effects 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.
Side Effect Rune
Use $effect(() => { ... }) for side effects that respond to reactive changes.
$effect(() => {
console.log("count is", count);
});Auto Tracked
Svelte tracks dependencies inside the callback automatically.
Cleanup Function
Return a function to clean up before the next run or on destroy.
$effect(() => {
const id = setInterval(tick, 1000);
return () => clearInterval(id);
});Runs After Mount
The effect runs after the component mounts and on every dependency change.
No SSR
$effect callbacks do not run during SSR. Use them for browser-only logic.
Pre-Mount Effects
$effect.pre runs before DOM updates; useful for measure-and-apply patterns.
Root Effects
$effect.root creates an effect outside component lifecycle for advanced cases.
Compared to onMount
onMount runs once; $effect re-runs on dependency changes. Often $effect replaces both onMount + reactive patterns.
Avoid Setting State
Mutating reactive state inside an effect can cause loops. Prefer $derived for computed values.
Untracked Reads
Use untrack() inside an effect to read a value without registering a dependency.
Real Use Cases
Logging, syncing to localStorage, third-party library integration, manual DOM tweaks.
Quick Check
How do you clean up an effect?
Recap
$effect handles side effects, tracks dependencies, and supports cleanup via returned functions.
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Frequently Asked Questions
Is the “$effect: Side Effects” lesson free?
Yes — the full text of “$effect: Side Effects” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO. The Sveltejs Academy course includes 4 lessons in total.
What will I learn in “$effect: Side Effects”?
Run side effects that re-execute whenever their 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 4 of 4.
How long does the “$effect: Side Effects” 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