svelte-core-bestpractices/references/bind.md

Function bindings

You can also use bind:property={get, set}, where get and set are functions, allowing you to perform validation and transformation:

<input bind:value={
    () => value,
    (v) => value = v.toLowerCase()}
/>

In the case of readonly bindings like dimension bindings, the get value should be null:

<div
    bind:clientWidth={null, redraw}
    bind:clientHeight={null, redraw}
>...</div>

[!NOTE] Function bindings are available in Svelte 5.9.0 and newer.