Stores not honoring contract described in the documentation

[mbellgardt]

Describe the bug

In https://svelte.dev/docs/svelte/stores#Store-contract the documentations states:

A store may optionally contain a .set method, which [...] synchronously calls all of the store’s active subscription functions.

This is not the behavior I observe from Svelte's built-in stores, as they seem to schedule the calls to the subscriptions till after the calling function is done. I understand that this behavior is probably intended, my main concern is with the documentation.

It would be very helpful to have a proper documentation of how Svelte's stores actually behave.

Reproduction

Consider the following example:

<script lang="ts">
	import { readonly, type Writable, writable } from 'svelte/store'
	let name = 'world';

	const select: Writable<number> = writable(0)
	const select2: Writable<number> = writable($select)
	let i = 0

	$: console.log("select ", $select)
	$: console.log("select2 ", $select2)

	select.subscribe((v) => {
		console.log('select subscribe ', v)
		select2.set(v)
		console.log('after set')
	})

	select2.subscribe((v) => {
		console.log('select 2 subscribe ', v)
		select.set(v)
	})
</script>

<button on:click={() => select.set(++i)}>assign</button>

Which (after clicking the button) results in the output:

select subscribe  1
after set
select 2 subscribe  1
select  1
select2  1

As you can see, the .set method finishes and the surrounding code is executed before the subscriptions are called.

Logs

System Info

System:
    OS: Windows 11 10.0.22621
    CPU: (16) x64 AMD Ryzen 7 PRO 4750U with Radeon Graphics
    Memory: 12.33 GB / 31.37 GB
  Binaries:
    Node: 22.14.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (127.0.2651.74)
    Internet Explorer: 11.0.22621.3527

Severity

annoyance

[Prinzhorn]

You might be confusing synchronously with immediately or instantly. I think what you're seeing is Svelte waiting for the current subscribe to finish before triggering the next subscribers.

Edit: synchronous means that this will always work:

<script>
	import { writable } from 'svelte/store'

	let store = writable('foo');
	let value;

	const unsub = store.subscribe(v => {
		value = v;
	});
	store.set('bar');
        unsub();

	console.log(value);
</script>

https://svelte.dev/playground/hello-world?version=5.32.1#H4sIAAAAAAAAA22PzW6EMAyEX8XKJUGq2Dv7I_XWd1h6SBZTRTIJSgx0hXj3TQKteujNHn8zI6_C6QFFIz6QyMPiA3WgsLOMXSXeRG8Jo2juq-DnmLksJP1wvY9jHWckzprREf_TH94xOk4x4hIfwY58a13Ldhh9YFhhCZa1IYQN-uAHkLvzFNkHlAlNMCFD2eH6yyvZey-r88991jTheedTZ2SYXJxMchRnneZcb1DNcL3BmrmWiysxc8nZ9rjDgKyk0eHoKGmq-tPgCWvyX6pk5MPldDyY3mb8ZtFwmHD7TJu2tFjXiabXFHF7AUdEhs95AQAA