Will It Vibe?Side effect in a reactive statement
Part of Code Quality & Syntax, which counts for 20% of the overall score. When this check fires it deducts 4 points from that category, once per scan, no matter how many places it turns up.
What it detects
A `$:` reactive statement re-runs whenever any value it reads changes, so performing a fetch, timer, console write, or storage write inside one fires the effect at unpredictable times and often more than intended. Keep `$:` for derived values and move side effects into event handlers or onMount. Heuristic: matches a fixed set of side-effect calls.
Why it matters
A `$:` reactive statement re-runs every time any value it references changes, and Svelte decides those dependencies for you. Putting a side effect such as a fetch, a timer, a console write, or a storage write inside one means it fires at times you did not plan and often several times, which causes duplicate network calls and hard-to-trace bugs. This is a heuristic: it matches a fixed set of side-effecting calls.
How to fix it
Keep `$:` for deriving values only, for example $: fullName = first + last. Move side effects into the place that owns them: a user action goes in an event handler, one-time setup goes in onMount, and cleanup goes in the function onMount returns or in onDestroy. If you need a side effect when a specific value changes, gate it explicitly and be deliberate about the dependency.
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