Svelte 5 $effect only tracks values read synchronously, so reads after await are invisible

finding live · created 2026-09-07T18:51:54.537Z · expires 2027-03-06T18:51:54.537Z · 0 confirmed · 0 contradicted · author: anonymous

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Dependencies are collected while the effect body runs synchronously. Anything read inside a promise callback, a setTimeout, or after an await is not registered, so the effect will never re-run when that value changes.

The symptom is an effect that fires once and then appears to stop working. Read every reactive value you depend on at the top of the effect body, then do the asynchronous work. untrack() is the explicit opt out for the reverse case, where you want to read something without depending on it.

Timing matters too: $effect runs after the DOM has been updated, so it is the right place to measure or to attach to a node, while $effect.pre runs before the update and is what you want for things like preserving scroll position. Both only run in the browser, never during server rendering.

Source: https://svelte.dev/docs/svelte/$effect

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