---
title: Correctness
description: Every Correctness rule svelte-vitals runs.
---

{/* rules-index:start */}

Svelte 5 runes and lifecycle misuse: code that compiles but behaves wrong.

| Rule                                                                                    | Severity    | Summary                                                                                                                                                               |
| --------------------------------------------------------------------------------------- | ----------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [`correctness/orphan-effect`](/rules/correctness/orphan-effect)                         | 🔴 critical | An $effect created outside component initialisation throws effect_orphan at runtime.                                                                                  |
| [`correctness/orphan-lifecycle`](/rules/correctness/orphan-lifecycle)                   | 🔴 critical | onMount, getContext and friends called outside component initialisation throw lifecycle_outside_component at runtime.                                                 |
| [`correctness/server-browser-global`](/rules/correctness/server-browser-global)         | 🔴 critical | window, document, localStorage accessed in module scope or a load/handler crash SSR with a ReferenceError.                                                            |
| [`correctness/autoplay-muted`](/rules/correctness/autoplay-muted)                       | 🟡 warning  | Browsers block autoplay with audio, and a blocked autoplay does not error, so a <video autoplay> without muted silently never starts playing for real visitors. |
| [`correctness/base-path-navigation`](/rules/correctness/base-path-navigation)           | 🟡 warning  | A hardcoded root-relative link resolves against the domain root, not kit.paths.base, so under a base path it lands outside the app and 404s in production.            |
| [`correctness/checkable-bind-value`](/rules/correctness/checkable-bind-value)           | 🟡 warning  | bind:value on a checkbox or radio input binds the DOM value property, which checkbox/radio interaction never changes, so the bound state silently never updates.      |
| [`correctness/each-index-key`](/rules/correctness/each-index-key)                       | 🟡 warning  | Keying an {#each} block by its index gives items position-based identity, the same bug as no key, only masked.                                              |
| [`correctness/each-key`](/rules/correctness/each-key)                                   | 🟡 warning  | An {#each} block over dynamic data should have a key.                                                                                                       |
| [`correctness/effect-as-derived`](/rules/correctness/effect-as-derived)                 | 🟡 warning  | Use $derived instead of an $effect that only assigns state.                                                                                                           |
| [`correctness/effect-as-onmount`](/rules/correctness/effect-as-onmount)                 | 🟡 warning  | An $effect that reads no reactive value belongs in an event handler, {@attach}, or onMount instead.                                                         |
| [`correctness/instance-browser-global`](/rules/correctness/instance-browser-global)     | 🟡 warning  | A component's instance script runs on the server during SSR, so window/document reads at its top level crash the render.                                              |
| [`correctness/nonreactive-builtin-state`](/rules/correctness/nonreactive-builtin-state) | 🟡 warning  | A plain Map, Set, Date, URL, or URLSearchParams in $state is not proxied, so its mutations are invisible to reactivity and the UI silently stops updating.            |
| [`correctness/prop-mutation`](/rules/correctness/prop-mutation)                         | 🟡 warning  | Don't mutate a prop from $props() unless it is declared $bindable.                                                                                                    |
| [`correctness/stale-prop-derivation`](/rules/correctness/stale-prop-derivation)         | 🟡 warning  | A value computed from a prop without $derived is evaluated once, so the UI silently stops tracking the parent.                                                        |
| [`correctness/unmutated-state`](/rules/correctness/unmutated-state)                     | 🔵 info     | Use const (or $state.raw) for a $state that is never mutated.                                                                                                         |

{/* rules-index:end */}

See [Configuration](/guides/configuration) to change a rule's severity or turn it off.