> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/sveltejs/svelte/llms.txt
> Use this file to discover all available pages before exploring further.

# onMount

> Schedule a function to run when a component is mounted to the DOM

Schedules a function to run as soon as the component has been mounted to the DOM. Like [`$effect`](/runes/effect), but the provided function only runs once.

## Signature

```ts theme={null}
function onMount<T>(
  fn: () => NotFunction<T> | Promise<NotFunction<T>> | (() => any)
): void
```

<ParamField path="fn" type="function" required>
  A function to run once the component is mounted. If a function is returned *synchronously*, it will be called when the component is unmounted.
</ParamField>

## Usage

`onMount` must be called during the component's initialisation (but doesn't need to live *inside* the component; it can be called from an external module).

```svelte theme={null}
<script>
  import { onMount } from 'svelte';

  onMount(() => {
    console.log('component mounted');
  });
</script>
```

### Cleanup function

If a function is returned *synchronously* from `onMount`, it will be called when the component is unmounted:

```svelte theme={null}
<script>
  import { onMount } from 'svelte';

  onMount(() => {
    const interval = setInterval(() => {
      console.log('tick');
    }, 1000);

    return () => clearInterval(interval);
  });
</script>
```

### Async functions

`onMount` can accept async functions, but the cleanup function must be returned synchronously if needed:

```svelte theme={null}
<script>
  import { onMount } from 'svelte';

  onMount(async () => {
    const data = await fetch('/api/data');
    console.log(await data.json());
  });
</script>
```

## Server-side rendering

`onMount` functions do not run during [server-side rendering](/api/server/render). If you need to run code on both the server and client, use `$effect` instead.

## Notes

* Must be called during component initialization
* Only runs once per component instance
* Does not run during SSR
* Can be called from external modules
* Cleanup function must be returned synchronously