Dedup keys are kept forever in one environment and we do not know why

Problem Description

src/notificationDedup.js suppresses duplicate notification sends using a key store with a time-to-live. The TTL exists so the store does not grow without bound - we had a memory incident traced to a dedup map that was never pruned.

The store now takes a clock so its behaviour is testable without waiting. No test currently covers what happens at the TTL boundary, and none covers whether expired keys are actually released rather than merely reported as absent.

Output Specification

Add src/notificationDedup.test.js covering the suppression window: what happens just inside it, exactly at its edge, and after it, plus evidence that expired entries stop occupying the store.

Run npm test before you finish; it must pass.

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

Input Files

Extract the following files before beginning.

=============== FILE: package.json =============== { "name": "notifications", "version": "3.2.0", "private": true, "scripts": { "test": "node --test" } }

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

const DEFAULT_TTL_MS = 15 * 60 * 1000;

function createExpiringStore({ ttlMs = DEFAULT_TTL_MS, now }) { const entries = new Map();

function isStale(entry, at) { return at - entry.storedAt >= ttlMs; }

return { get(key) { const entry = entries.get(key); if (!entry) { return null; } if (isStale(entry, now())) { entries.delete(key); return null; } return entry.value; }, set(key, value) { entries.set(key, { value, storedAt: now() }); }, sweep() { const at = now(); let removed = 0; for (const [key, entry] of entries) { if (isStale(entry, at)) { entries.delete(key); removed += 1; } } return removed; }, size() { return entries.size; }, }; }

function createNotifier({ store, transport }) { return function send(notification) { const key = ${notification.userId}:${notification.kind}; if (store.get(key) !== null) { return { status: 'suppressed' }; } transport.deliver(notification); store.set(key, true); return { status: 'sent' }; }; }

function createTransport() { const delivered = []; return { deliver(notification) { delivered.push(notification); }, deliveredCount() { return delivered.length; }, }; }

module.exports = { createExpiringStore, createNotifier, createTransport, DEFAULT_TTL_MS };

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

const test = require('node:test'); const assert = require('node:assert/strict'); const { createExpiringStore, createNotifier, createTransport, } = require('./notificationDedup');

test('a repeated notification is suppressed', () => { let clock = 0; const transport = createTransport(); const store = createExpiringStore({ now: () => clock }); const send = createNotifier({ store, transport }); const notification = { userId: 'u_1', kind: 'digest' };

assert.equal(send(notification).status, 'sent'); assert.equal(send(notification).status, 'suppressed'); assert.equal(transport.deliveredCount(), 1); });