Race condition in multiple use: action execution order

Answer

Svelte does not provide a cross‑version guarantee that multiple use: actions on the same element run in declaration order. For a fixed compiler version the generated init code follows the template order, so the first action in the markup is usually initialized before the second, but this is an implementation detail.

Likely explanation

The Svelte compiler translates each use: directive into a call to the action function during the element’s mount step. The calls are emitted in the same sequence they appear in the source, which makes the order deterministic for that build.

Confirmed facts

• A use: action receives the DOM node when the element is mounted and may return a destroy function; the contract is version‑specific (Svelte 3/4 vs Svelte 5).
• Svelte does not expose a dependency‑graph or priority flag for actions; there is no documented way to enforce a particular execution order.
• If two actions mutate the same attribute or listener, relying on order is fragile across upgrades.

Steps to handle ordering in your case

1. Pin the Svelte version you are targeting and verify the current behavior with a minimal repro (log entry order).
2. If the conflict occurs during the initial mount, compose the actions into a single wrapper function that calls the sub‑actions in the desired sequence, or move shared state to an explicit controller/store.
3. If the conflict only appears later (e.g., on prop updates or events), coordinate via a shared store or callback rather than depending on mount order.
4. After any Svelte major upgrade, repeat the verification step because the action contract may change.

To give a more precise recommendation, please confirm whether the ordering issue manifests synchronously during element mount or only during later updates/prop changes.