Cyclic dependency, I think

+page.svelte

<script lang="ts" context="module">
	import { writable } from "svelte/store";

  export type ObjectValues<T> = T[keyof T];

  export const Digit = {
    Thumb: "Thumb",
    Fore: "Fore",
    Index: "Index",
    Ring: "Ring",
    Pinky: "Pinky",
  } as const;

  type Digits = Set<ObjectValues<typeof Digit>>;

  class Person {
    _leftFingersGloved: Digits = new Set();

    addSleeveToLeftGlove(d: ObjectValues<typeof Digit>) {
      this._leftFingersGloved.add(d);
    }
    hasSleeveOnLeftGlove(d: ObjectValues<typeof Digit>) {
      return this._leftFingersGloved.has(d);
    }
    deleteSleeveFromLeftGlove(d: ObjectValues<typeof Digit>) {
      this._leftFingersGloved.delete(d);
    }

    _rightFingersGloved: Digits = new Set();

    addSleeveToRightGlove(d: ObjectValues<typeof Digit>) {
      this._rightFingersGloved.add(d);
    }
    hasSleeveOnRightGlove(d: ObjectValues<typeof Digit>) {
      return this._rightFingersGloved.has(d);
    }
    deleteSleeveFromRightGlove(d: ObjectValues<typeof Digit>) {
      this._rightFingersGloved.delete(d);
    }
  }

  export const person = writable(new Person());
</script>
<script lang="ts">
	import LeftGlove from "./LeftGlove.svelte";
	import RightGlove from "./RightGlove.svelte";


</script>

<LeftGlove />
<RightGlove />

LeftGlove.svelte

<script lang="ts">
	import { Digit, person, type ObjectValues } from './+page.svelte';

	export let digitSleeves: (ObjectValues<typeof Digit>)[] = [];
	$: {
		let lSet = new Set(digitSleeves);
		for (let digit of Object.values(Digit)) {
			if (lSet.has(digit) && $person.hasSleeveOnLeftGlove(digit) === false) {
				$person.addSleeveToLeftGlove(digit);
			}
			if (lSet.has(digit) === false && $person.hasSleeveOnLeftGlove(digit)) {
				$person.deleteSleeveFromLeftGlove(digit);
			}
		}
		$person = $person;
	}
</script>

<h4>left hand glove</h4>
<div>
	{#each Object.values(Digit) as digit}
		<label><input type="checkbox" bind:group={digitSleeves} value={digit} />{digit}</label>
	{/each}
</div>

<pre>{JSON.stringify(digitSleeves, undefined, 2)}</pre>

RightGlove.svelte

<script lang="ts">
	import { Digit, person, type ObjectValues } from './+page.svelte';

	export let digitSleeves: (ObjectValues<typeof Digit>)[] = [];
	$: {
		let lSet = new Set(digitSleeves);
		for (let digit of Object.values(Digit)) {
			if (lSet.has(digit) && $person.hasSleeveOnRightGlove(digit) === false) {
				$person.addSleeveToRightGlove(digit);
			}
			if (lSet.has(digit) === false && $person.hasSleeveOnRightGlove(digit)) {
				$person.deleteSleeveFromRightGlove(digit);
			}
		}
		$person = $person;
	}
</script>

<h4>left hand glove</h4>
<div>
	{#each Object.values(Digit) as digit}
		<label><input type="checkbox" bind:group={digitSleeves} value={digit} />{digit}</label>
	{/each}
</div>

<pre>{JSON.stringify(digitSleeves, undefined, 2)}</pre>