You can also declare event handlers inline:

<div on:mousemove={(e) => (m = { x: e.clientX, y: e.clientY })}>
	The mouse position is {m.x} x {m.y}
</div>

In some frameworks you may see recommendations to avoid inline event handlers for performance reasons, particularly inside loops. That advice doesn't apply to Svelte — the compiler will always do the right thing, whichever form you choose.