Store Contracts and Interoperability
Understand what makes any object a valid Svelte store and use third-party stores.
Store Contracts and Interoperability 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 Contract
Any object implementing subscribe(fn) that returns an unsubscribe function is a Svelte store.
Why It Matters
The simple contract means many third-party libraries are usable as Svelte stores out of the box.
RxJS Observables
An RxJS Observable's subscribe method is compatible if you wrap it slightly.
Atomic Libraries
Libraries like nanostores and zustand work with adapters or directly.
Building Compatible Stores
For your own custom stores, just implement subscribe correctly.
function myStore() {
let val = 0;
const subs = new Set();
return {
subscribe(fn) { fn(val); subs.add(fn); return () => subs.delete(fn); }
};
}Use with $
If subscribe is compatible, the $ shorthand works automatically.
Async Sources
The subscribe callback can be called any time, including asynchronously, with new values.
Equality Checks
The contract does not require equality checks; Svelte calls subscribers every time.
Multiple Subscribers
Your store must notify all current subscribers. Use a Set or Array.
Cleanup
Return a real cleanup function that removes the subscriber. Otherwise you leak.
Interop Libraries
Use libraries like svelte-rxjs or @nanostores/svelte for richer ecosystems.
Quick Check
What returns from subscribe()?
Recap
Stores are just objects with a subscribe method. The contract makes interop with other libraries easy.
Frequently asked questions
Is the “Store Contracts and Interoperability” lesson free?
Yes — the full text of “Store Contracts and Interoperability” 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 “Store Contracts and Interoperability”?
Understand what makes any object a valid Svelte store and use third-party stores. 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 “Store Contracts and Interoperability” 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
- derived() for Computed Stores
- Custom Store Pattern with subscribe/set/update
- Readable from Writable
- Store Contracts and Interoperability