Function flatten

packages/svelte/src/internal/client/reactivity/async.js:36–115

(blockers, sync, async, fn)

 * @param {(values: Value[]) => any} fn
 */
export function flatten(blockers, sync, async, fn) {
	const d = is_runes() ? derived : derived_safe_equal;

	// Filter out already-settled blockers - no need to wait for them
	var pending = blockers.filter((b) => !b.settled);

	var deriveds = sync.map(d);

	if (DEV) {
		deriveds.forEach((d, i) => {
			// TODO this is kinda useful for debugging but a lousy implementation —
			// maybe the compiler could pass through the template string
			d.label = sync[i]
				.toString()
				.replace('() => ', '')
				.replaceAll('$.eager(() => ', '$state.eager(')
				.replace(/\$\.get\((.+?)\)/g, (_, id) => id);
		});
	}

	if (async.length === 0 && pending.length === 0) {
		fn(deriveds);
		return;
	}

	var parent = /** @type {Effect} */ (active_effect);

	var restore = capture();
	var blocker_promise =
		pending.length === 1
			? pending[0].promise
			: pending.length > 1
				? Promise.all(pending.map((b) => b.promise))
				: null;

	/**
	 * @param {Source[]} async
	 */
	function finish(async) {
		if ((parent.f & DESTROYED) !== 0) {
			return;
		}

		restore();

		try {
			fn([...deriveds, ...async]);
		} catch (error) {
			invoke_error_boundary(error, parent);
		}

		unset_context();
	}

	var decrement_pending = increment_pending();

	// Fast path: blockers but no async expressions
	if (async.length === 0) {