If you have multiple type="radio" or type="checkbox" inputs relating to the same value, you can use bind:group along with the value attribute. Radio inputs in the same group are mutually exclusive; checkbox inputs in the same group form an array of selected values.
Add bind:group={scoops} to the radio inputs...
App.svelte
<input
type="radio"
name="scoops"
value={number}
bind:group={scoops}
/>...and bind:group={flavours} to the checkbox inputs:
App.svelte
<input
type="checkbox"
name="flavours"
value={flavour}
bind:group={flavours}
/>1
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<script>
let scoops = 1;
let flavours = [];
const formatter = new Intl.ListFormat('en', { style: 'long', type: 'conjunction' });</script>
<h2>Size</h2>
{#each [1, 2, 3] as number}<label>
<input
type="radio"
name="scoops"
value={number}/>
{number} {number === 1 ? 'scoop' : 'scoops'}</label>
{/each}<h2>Flavours</h2>
{#each ['cookies and cream', 'mint choc chip', 'raspberry ripple'] as flavour}<label>
<input
type="checkbox"
name="flavours"
value={flavour}/>
{flavour}</label>
{/each}{#if flavours.length === 0}<p>Please select at least one flavour</p>
{:else if flavours.length > scoops}<p>Can't order more flavours than scoops!</p>
{:else}<p>
You ordered {scoops} {scoops === 1 ? 'scoop' : 'scoops'} of {formatter.format(flavours)}</p>
{/if}