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Clarify that onMount/onCleanup are reactive-tree based #1519

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Clarify that onMount/onCleanup are reactive-tree based #1519

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8 changes: 8 additions & 0 deletions src/routes/reference/lifecycle/on-mount.mdx
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Expand Up @@ -52,6 +52,14 @@ Non-tracking function executed once on mount.
- By the time `onMount` runs, refs have already been assigned.
- Returning a function from `fn` does not register cleanup. Use [`onCleanup`](/reference/lifecycle/on-cleanup) inside `onMount` when cleanup is needed.

:::note
"Mounted" in Solid refers to when a component is rendered within the reactive tree, not when it is physically inserted into the DOM.
If you store JSX in a variable before conditionally rendering it, `onMount` runs when that JSX is evaluated, even if the elements have not yet been added to the visible page.
Similarly, [`onCleanup`](/reference/lifecycle/on-cleanup) runs based on the reactive ownership tree rather than DOM removal.

For cases where you need to detect actual DOM insertion or removal, consider using a [`ref`](/concepts/refs) callback or the [Lifecycle primitives from solid-primitives](https://github.com/solidjs-community/solid-primitives/tree/main/packages/lifecycle).
:::

## Examples

### Access a ref after mount
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