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$effect.active docs #9598

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docs: $effect.active
Rich-Harris Nov 22, 2023
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Rich-Harris Nov 22, 2023
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18 changes: 18 additions & 0 deletions sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md
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Expand Up @@ -168,6 +168,24 @@ In rare cases, you may need to run code _before_ the DOM updates. For this we ca

Previously, you would have used `beforeUpdate`, which — like `afterUpdate` — is deprecated in Svelte 5.

## `$effect.active`

The `$effect.active` rune is an advanced feature that tells you whether or not the code is running inside an effect or inside your template ([demo](/#H4sIAAAAAAAAE3XP0QrCMAwF0F-JRXAD595rLfgdzodRUyl0bVgzQcb-3VYFQfExl5tDMgvrPCYhT7MI_YBCiiOR2Aq-UxnSDT1jnlOcRlMSlczoiHUXOjYxpOhx5-O12rgAJg4UAwaGhDyR3Gxhjdai4V1v2N2wqus9tC3Y3ifMQjbehaqq4aBhLtEv_Or893icCsdLve-Caj8nBkU67zMO5HtGCfM3sKiWNKhV0zwVaBqd3x3ixVmHFyFLuJyXB-moOe8pAQAA)):

```svelte
<script>
console.log('in component setup:', $effect.active()); // false

$effect(() => {
console.log('in effect:', $effect.active()); // true
});
</script>

<p>in template: {$effect.active()}</p> <!-- true -->
```

This allows you to (for example) add things like subscriptions without causing memory leaks, by putting them in child effects.
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I think an example link would help here. I myself am not sure from reading the docs how I would use this. For example I'm a bit confused that isActive is false at component initialization time. I thought that the idea is to use this as a way to subscribe at component initialization time (because we know it's cleaned up on destroy)


## `$props`

To declare component props, use the `$props` rune:
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