In Svelte 5, the component lifecycle consists of only two parts: Its creation and its destruction. Everything in-between - when certain state is updated - is not related to the component as a whole, only the parts that need to react to the state change are notified. This is because under the hood the smallest unit of change is actually not a component, it's the (render) effects that the component sets up upon component initialization. Consequently, there's no such thing as a "before update"/"after update" hook.
## `onMount`
The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM. It 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).
`onMount` does not run inside a component that is rendered on the server.
```svelte
<script>
import { onMount } from 'svelte';
onMount(() => {
console.log('the component has mounted');
});
</script>
```
If a function is returned from `onMount`, it will be called when the component is unmounted.
> [!NOTE] This behaviour will only work when the function passed to `onMount` _synchronously_ returns a value. `async` functions always return a `Promise`, and as such cannot _synchronously_ return a function.
Schedules a callback to run immediately before the component is unmounted.
Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the only one that runs inside a server-side component.
```svelte
<script>
import { onDestroy } from 'svelte';
onDestroy(() => {
console.log('the component is being destroyed');
});
</script>
```
## `tick`
While there's no "after update" hook, you can use `tick` to ensure that the UI is updated before continuing. `tick` returns a promise that resolves once any pending state changes have been applied, or in the next microtask if there are none.
```svelte
<script>
import { beforeUpdate, tick } from 'svelte';
beforeUpdate(async () => {
console.log('the component is about to update');
await tick();
console.log('the component just updated');
});
</script>
```
## Deprecated: `beforeUpdate` / `afterUpdate`
Svelte 4 contained hooks that ran before and after the component as a whole was updated. For backwards compatibility, these hooks were shimmed in Svelte 5 but not available inside components that use runes.
```svelte
<script>
import { beforeUpdate, afterUpdate } from 'svelte';
beforeUpdate(() => {
console.log('the component is about to update');
});
afterUpdate(() => {
console.log('the component just updated');
});
</script>
```
Instead of `beforeUpdate` use `$effect.pre` and instead of `afterUpdate` use `$effect` instead - these runes offer more granular control and only react to the changes you're actually interested in.
### Chat window example
To implement a chat window that autoscrolls to the bottom when new messages appear (but only if you were _already_ scrolled to the bottom), we need to measure the DOM before we update it.
In Svelte 4, we do this with `beforeUpdate`, but this is a flawed approach — it fires before _every_ update, whether it's relevant or not. In the example below, we need to introduce checks like `updatingMessages` to make sure we don't mess with the scroll position when someone toggles dark mode.
With runes, we can use `$effect.pre`, which behaves the same as `$effect` but runs before the DOM is updated. As long as we explicitly reference `messages` inside the effect body, it will run whenever `messages` changes, but _not_ when `theme` changes.
`beforeUpdate`, and its equally troublesome counterpart `afterUpdate`, are therefore deprecated in Svelte 5.
