Not all application state belongs inside your application's component hierarchy. Sometimes, you'll have values that need to be accessed by multiple unrelated components, or by a regular JavaScript module.

In Svelte, we do this with stores. A store is simply an object with a subscribe method that allows interested parties to be notified whenever the store value changes. In App.svelte, count is a store, and we're setting countValue in the count.subscribe callback.

Click the stores.js tab to see the definition of count. It's a writable store, which means it has set and update methods in addition to subscribe.

Now go to the Incrementer.svelte tab so that we can wire up the + button:

function increment() {
  count.update(n => n + 1);
}

Clicking the + button should now update the count. Do the inverse for Decrementer.svelte.

Finally, in Resetter.svelte, implement reset:

function reset() {
  count.set(0);
}