Subscribers must see a change before it is persisted

Problem Description

src/store.js batches state changes. Subscribers are notified on the microtask queue so the UI updates in the same tick, while persistence is deferred to a macrotask so it never blocks rendering.

That ordering is a real guarantee - a persistence hook that reads the store must never run before subscribers have observed the change - and it is entirely untested. Several changes in the same tick are also supposed to coalesce into a single notification.

Output Specification

Add src/store.test.js proving the ordering guarantee between notification and persistence, and proving that multiple changes in one tick produce one notification.

Run npm test before you finish; it must pass.

Leave src/store.basic.test.js in place.

Input Files

Extract the following files before beginning.

=============== FILE: package.json =============== { "name": "state-core", "version": "1.1.0", "private": true, "scripts": { "test": "node --test" } }

=============== FILE: src/store.js =============== 'use strict';

function createStore(initial) { let state = initial; const subscribers = []; const persisted = []; let scheduled = false;

function schedule() { if (scheduled) { return; } scheduled = true; queueMicrotask(() => { scheduled = false; for (const subscriber of subscribers) { subscriber(state); } }); setTimeout(() => { persisted.push(state); }, 0); }

return { setState(next) { state = next; schedule(); }, subscribe(fn) { subscribers.push(fn); return () => { const index = subscribers.indexOf(fn); if (index >= 0) { subscribers.splice(index, 1); } }; }, persistedValues() { return persisted.slice(); }, read() { return state; }, }; }

module.exports = { createStore };

=============== FILE: src/store.basic.test.js =============== 'use strict';

const test = require('node:test'); const assert = require('node:assert/strict'); const { createStore } = require('./store');

test('reads back the value that was set', () => { const store = createStore(0); store.setState(7); assert.equal(store.read(), 7); });