Function derived

packages/svelte/src/store/shared/index.js:129–181

(stores, fn, initial_value)

 * @returns {Readable<T>}
 */
export function derived(stores, fn, initial_value) {
	const single = !Array.isArray(stores);
	/** @type {Array<Readable<any>>} */
	const stores_array = single ? [stores] : stores;
	if (!stores_array.every(Boolean)) {
		throw new Error('derived() expects stores as input, got a falsy value');
	}
	const auto = fn.length < 2;
	return readable(initial_value, (set, update) => {
		let started = false;
		/** @type {T[]} */
		const values = [];
		let pending = 0;
		let cleanup = noop;
		const sync = () => {
			if (pending) {
				return;
			}
			cleanup();
			const result = fn(single ? values[0] : values, set, update);
			if (auto) {
				set(result);
			} else {
				cleanup = typeof result === 'function' ? result : noop;
			}
		};
		const unsubscribers = stores_array.map((store, i) =>
			subscribe_to_store(
				store,
				(value) => {
					values[i] = value;
					pending &= ~(1 << i);
					if (started) {
						sync();
					}
				},
				() => {
					pending |= 1 << i;
				}
			)
		);
		started = true;
		sync();
		return function stop() {
			run_all(unsubscribers);
			cleanup();
			// We need to set this to false because callbacks can still happen despite having unsubscribed:
			// Callbacks might already be placed in the queue which doesn't know it should no longer
			// invoke this derived store.
			started = false;
		};
	});
}

/**
 * Takes a store and returns a new one derived from the old one that is readable.