Svelte 5: Mutating promise based arrays do not trigger updates

[navorite]

Describe the bug

If an awaited promise based array gets mutated, it doesn't trigger updates. To get to the correct state, one must hide the array then show the array items again. As far as I've debugged, $inspect doesn't detect the changes as well.

Reproduction

Svelte 5 REPL

Expected: Svelte 4 REPL

Logs

No response

System Info

svelte 5.0.0-next.162

Severity

blocking all usage of svelte

[trueadm]

This is because the $state rune only applies deep reactivity to plain object literals and arrays. It doesn't apply to Promises and thus any values resolved from a promise won't be automatically reactive.

[navorite]

This is because the $state rune only applies deep reactivity to plain object literals and arrays. It doesn't apply to Promises and thus any values resolved from a promise won't be automatically reactive.

Is this a limitation of proxies or is there some kind of technical limitation? And if so, shouldn't this be documented?

[trueadm]

This is because the $state rune only applies deep reactivity to plain object literals and arrays. It doesn't apply to Promises and thus any values resolved from a promise won't be automatically reactive.

Is this a limitation of proxies or is there some kind of technical limitation? And if so, shouldn't this be documented?

This isn’t really technically possible without AsyncContext, which is just a TC39 proposal. You’re crossing the async boundary, so Svelte has no knowledge of the resolved value and its relationship to state. We already document that only a plain objects and arrays work with deep reactivity.

[navorite]

This isn’t really technically possible without AsyncContext, which is just a TC39 proposal. You’re crossing the async boundary, so Svelte has no knowledge of the resolved value and its relationship to state. We already document that only a plain objects and arrays work with deep reactivity.

If I'm not mistaken, this is the line you are referring to in the docs?

Objects and arrays are made deeply reactive by wrapping them with Proxies:

This doesn't state clearly that only POJOs and arrays are made deeply reactive.

Also this should be documented in the breaking changes because it also breaks whether runes are used or not.

[trueadm]

Why is it a breaking change? Runes are only a 5.0 feature, they weren’t in 4. We can adjust the documentation to clarify.

[navorite]

Why is it a breaking change? Runes are only a 5.0 feature, they weren’t in 4. We can adjust the documentation to clarify.

Because it breaks in non-runes mode.

[trueadm]

@navorite Ah that's likely something we can fix in non-runes mode then. :)