Function eager

packages/svelte/src/internal/client/reactivity/batch.js:1258–1306

(fn)

 * @returns {T}
 */
export function eager(fn) {
	var initial = true;
	var value = /** @type {T} */ (undefined);

	if (active_reaction === null) {
		return fn();
	}

	let parent = active_reaction;

	let version = version_map.get(parent) ?? source(0);
	version_map.set(parent, version);

	teardown(() => {
		if (parent.f & DESTROYING) version_map.delete(parent);
	});

	get(version);

	eager_effect(() => {
		if (initial) {
			// the first time this runs, we create an eager effect
			// that will run eagerly whenever the expression changes
			var previous_batch_values = batch_values;

			try {
				batch_values = null;
				value = fn();
			} finally {
				batch_values = previous_batch_values;
			}

			return;
		}

		// the second time this effect runs, it's to schedule a
		// `version` update. since this will recreate the effect,
		// we don't need to evaluate the expression here
		if (eager_versions.length === 0) {
			queue_micro_task(eager_flush);
		}

		eager_versions.push(version);
	});

	initial = false;

	return value;
}

/**
 * Mark all the effects inside a skipped branch CLEAN, so that