$state: Fine-Grained Reactivity
Declare reactive state with the $state rune for granular DOM updates.
$state: Fine-Grained Reactivity is a free Sveltejs Academy lesson on CoddyKit. This is lesson 2 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.
Declare State
Use $state(initial) to declare a reactive variable.
let count = $state(0);Reassign Normally
Update with regular assignment. Svelte re-renders dependent UI.
count = count + 1;Objects and Arrays
$state with objects/arrays creates deep proxies; mutations are reactive.
let user = $state({ name: "Ada" });
user.name = "Bea"; // triggers updateNo More items = items
You no longer need to reassign to trigger updates — proxies catch mutations.
Granular Updates
Only the parts of the DOM using the changed property re-render.
Function Returns
You can return $state-backed objects from functions for reusable state slices.
function counter() {
let n = $state(0);
return {
get value() { return n; },
increment() { n++; }
};
}Use in Libraries
$state works in .ts/.js files too if they use the .svelte.ts extension.
Untracked
Use untrack() to read a state value without registering a dependency.
Raw State
For huge non-reactive data, use $state.raw to skip the proxy.
Frozen Snapshot
Use $state.snapshot(obj) to get a plain non-proxied copy.
Compared to let
$state is the modern replacement for plain let with implicit reactivity.
Quick Check
What happens when you mutate $state object?
Recap
$state creates fine-grained reactive variables, including deep-proxied objects and arrays.
Learn HTML with an AI tutor — free
Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.
- Courses
- 41
- Lessons
- 163
Frequently Asked Questions
Is the “$state: Fine-Grained Reactivity” lesson free?
Yes — the full text of “$state: Fine-Grained Reactivity” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO. The Sveltejs Academy course includes 4 lessons in total.
What will I learn in “$state: Fine-Grained Reactivity”?
Declare reactive state with the $state rune for granular DOM updates. You practise Sveltejs Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Sveltejs Academy?
No prior experience is required. Sveltejs Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.
How long does the “$state: Fine-Grained Reactivity” 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