To run code when your component is destroyed, use onDestroy.

For example, we can add a setInterval function when our component initialises, and clean it up when it's no longer relevant. Doing so prevents memory leaks.

<script>
  import { onDestroy } from 'svelte';

  let counter = 0;
  const interval = setInterval(() => counter += 1, 1000);

  onDestroy(() => clearInterval(interval));
</script>

While it's important to call lifecycle functions during the component's initialisation, it doesn't matter where you call them from. So if we wanted, we could abstract the interval logic into a helper function in utils.js...

import { onDestroy } from 'svelte';

export function onInterval(callback, milliseconds) {
  const interval = setInterval(callback, milliseconds);

  onDestroy(() => {
    clearInterval(interval);
  });
}

...and import it into our component:

<script>
  import { onInterval } from './utils.js';

  let counter = 0;
  onInterval(() => counter += 1, 1000);
</script>

Open and close the timer a few times and make sure the counter keeps ticking and the CPU load increases. This is due to a memory leak as the previous timers are not deleted. Don't forget to refresh the page before solving the example.