Using RXJS Observables in Svelte with autosubscribe feature

Counter.svelte

<script lang="ts">
	import { map } from 'rxjs';
	import { count$, evenCount$, oddCount$ } from './counter';
	import { toReadable, toWritable } from './util';

	let count = toWritable(count$);
	let lastEven = toReadable(evenCount$.pipe(map(String)), 'none')
	let lastOdd = toReadable(oddCount$.pipe(map(String)), 'none')
</script>

<div class="counter">
	<button on:click={() => ($count -= 1)} aria-label="Decrease the counter by one">
		<svg aria-hidden="true" viewBox="0 0 1 1">
			<path d="M0,0.5 L1,0.5" />
		</svg>
	</button>

	<div class="counter-viewport">
		<div class="counter-digits">
			<strong>{Math.floor($count)}</strong>
		</div>
	</div>

	<button on:click={() => ($count += 1)} aria-label="Increase the counter by one">
		<svg aria-hidden="true" viewBox="0 0 1 1">
			<path d="M0,0.5 L1,0.5 M0.5,0 L0.5,1" />
		</svg>
	</button>
</div>

Last even: {$lastEven}
Last odd: {$lastOdd}

<style>
	.counter {
		display: flex;
		border-top: 1px solid rgba(0, 0, 0, 0.1);
		border-bottom: 1px solid rgba(0, 0, 0, 0.1);
		margin: 1rem 0;
	}

	.counter button {
		width: 2em;
		padding: 0;
		display: flex;
		align-items: center;
		justify-content: center;
		border: 0;
		background-color: transparent;
		touch-action: manipulation;
		color: var(--text-color);
		font-size: 2rem;
	}

	.counter button:hover {
		background-color: var(--secondary-color);
	}

	svg {
		width: 25%;
		height: 25%;
	}

	path {
		vector-effect: non-scaling-stroke;
		stroke-width: 2px;
		stroke: var(--text-color);
	}

	.counter-viewport {
		width: 8em;
		height: 4em;
		overflow: hidden;
		text-align: center;
		position: relative;
	}

	.counter-viewport strong {
		position: absolute;
		display: flex;
		width: 100%;
		height: 100%;
		font-weight: 400;
		color: var(--accent-color);
		font-size: 4rem;
		align-items: center;
		justify-content: center;
	}

	.counter-digits {
		position: absolute;
		width: 100%;
		height: 100%;
	}
</style>

counter.ts

import { BehaviorSubject, filter } from 'rxjs';

export const count$ = new BehaviorSubject(0);

export const evenCount$ = count$.pipe(filter((value) => value % 2 === 0));

export const oddCount$ = count$.pipe(filter((value) => value % 2 !== 0));

util.ts

import { Observable, type Subject } from 'rxjs';
import { get, readable, type Readable, type Writable } from 'svelte/store';

export function toReadable<T>(observable: Observable<T>, initialValue?: T): Readable<T> {
	return readable<T>(initialValue, (next) => {
		const subscription = observable.subscribe({ next });
		return () => subscription.unsubscribe();
	});
}

export function toWritable<T>(subject: Subject<T>, initialValue?: T): Writable<T> {
	const readable = toReadable(subject, initialValue);
	return {
		...readable,
		set(value) {
			subject.next(value);
		},
		update(updater) {
			subject.next(updater(get(readable)));
		}
	};
}

export function fromReadable<T>(readable: Readable<T>): Observable<T> {
	return new Observable((observer) => readable.subscribe((value) => observer.next(value)));
}