Hey there, fellow Svelte enthusiasts!
A store is simply an object with a subscribe method that allows interested parties to be notified whenever the store value changes.

writeable(0)
This is a built-in function in Svelte that creates a writable store with an initial value of 0.
A writable store is a reactive variable that can be subscribed to and updated from anywhere in your Svelte component tree.
When the store's value changes, any components that are subscribed to it will automatically re-render with the new value.

subscribe This method allows you to subscribe to the store's value.
update This method allows you to update the store's value based on its current value.
set This method allows you to directly set the store's value to a new value.

The app in the previous example works, but there's a subtle bug — the store is subscribed to, but never unsubscribed. If the component was instantiated and destroyed many times, this would result in a memory leak.

Instead, You can reference a store value by prefixing the store name with $:

Auto-subscription only works with store variables that are declared (or imported) at the top-level scope of a component.

Not all stores should be writable by whoever has a reference to them. For example, you might have a store representing the mouse position or the user's geolocation, and it doesn't make sense to be able to set those values from 'outside'. For those cases, we have readable stores.

You can create a store whose value is based on the value of one or more other stores with derived. Building on our previous example, we can create a store that derives the time the page has been open

This page has been open for 0 seconds

As long as an object correctly implements the subscribe method, it's a store. Beyond that, anything goes. It's very easy, therefore, to create custom stores with domain-specific logic.

For example, the count store from our earlier example could include increment, decrement and reset methods and avoid exposing set and update:

The count is 0

+ - reset

on:click=() => update((n) => n + 1)
on:click=() => update((n) => n - 1)
on:click=() => set(0)