feat: working on chapter 11

Author: lmammino

11-advanced-recipes/01-local-initialization-check/README.md

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+# 01-local-initialization-check
+
+This sample showcases some simple solutions to the problem of asynchronously
+initialized modules.
+
+## Run
+
+There are 4 different examples in this folder.
+
+### 1. Wrong initialization (not calling `.connect()`)
+
+Execute it with:
+
+```
+node wrong-init-no-connect.js
+```
+
+### 2. Wrong initialization (calling `.connect()` but not waiting for it)
+
+Execute it with:
+
+```
+node wrong-init-no-await.js
+```
+
+### 3. Correct initialization (using a local initialization check)
+
+Execute it with:
+
+```
+node localInitializationCheck.js
+```
+
+### 4. Correct initialization (using the delayed startup pattern)
+
+Execute it with:
+
+```
+node delayedStartup.js
+```

11-advanced-recipes/01-local-initialization-check/db.js

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+import { setTimeout } from 'node:timers/promises'
+
+class Database {
+  connected = false
+
+  async connect() {
+    if (!this.connected) {
+      // simulate the delay of the connection
+      await setTimeout(500)
+      this.connected = true
+    }
+  }
+
+  async query(queryString) {
+    if (!this.connected) {
+      throw new Error('Not connected yet')
+    }
+    // simulate the delay of the query execution
+    await setTimeout(100)
+    console.log(`Query executed: ${queryString}`)
+  }
+}
+
+export const db = new Database()

11-advanced-recipes/01-local-initialization-check/delayedStartup.js

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+import { db } from './db.js'
+
+async function getConnectedDb() {
+  await db.connect()
+  return db
+}
+
+async function getUsers(db) {
+  await db.query('SELECT * FROM users')
+}
+
+const connectedDb = await getConnectedDb()
+await getUsers(connectedDb)

11-advanced-recipes/01-local-initialization-check/localInitializationCheck.js

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+import { db } from './db.js'
+
+async function getUsers() {
+  if (!db.connected) {
+    await db.connect()
+  }
+
+  await db.query('SELECT * FROM users')
+}
+
+await getUsers()

11-advanced-recipes/01-local-initialization-check/package.json

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+{
+  "name": "01-local-initialization-check",
+  "version": "1.0.0",
+  "description": "This sample showcases some simple solutions to the problem of asynchronously initialized modules.",
+  "type": "module",
+  "scripts": {},
+  "engines": {
+    "node": ">=23"
+  },
+  "engineStrict": true,
+  "keywords": [],
+  "author": "Luciano Mammino and Mario Casciaro",
+  "license": "MIT",
+  "dependencies": {}
+}

11-advanced-recipes/01-local-initialization-check/wrong-init-no-await.js

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+import { db } from './db.js'
+
+db.connect()
+const users = await db.query('SELECT * FROM users')
+console.log(users)

11-advanced-recipes/01-local-initialization-check/wrong-init-no-connect.js

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+import { db } from './db.js'
+
+const users = await db.query('SELECT * FROM users')
+console.log(users)

11-advanced-recipes/02-async-init-queue/README.md

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+# 02-async-init-queue
+
+This sample showcases how to use pre-initialization queues with an
+asynchronously initialized component.
+
+## Run
+
+To run the example launch:
+
+```
+node index.js
+```

11-advanced-recipes/02-async-init-queue/db.js

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+import { setTimeout } from 'node:timers/promises'
+
+class Database {
+  connected = false
+  commandsQueue = []
+
+  async connect() {
+    // simulate the delay of the connection
+    await setTimeout(500)
+    this.connected = true
+    while (this.commandsQueue.length > 0) {
+      const command = this.commandsQueue.shift()
+      command()
+    }
+  }
+
+  async query(queryString) {
+    if (!this.connected) {
+      console.log(`Request queued: ${queryString}`)
+
+      return new Promise((resolve, reject) => {
+        const command = () => {
+          this.query(queryString).then(resolve, reject)
+        }
+        this.commandsQueue.push(command)
+      })
+    }
+
+    // simulate the delay of the query execution
+    await setTimeout(100)
+    console.log(`Query executed: ${queryString}`)
+  }
+}
+
+export const db = new Database()

11-advanced-recipes/02-async-init-queue/index.js

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+import { setTimeout } from 'node:timers/promises'
+import { db } from './db.js'
+
+db.connect()
+
+async function updateLastAccess() {
+  await db.query(`INSERT (${Date.now()}) INTO "LastAccesses"`)
+}
+
+updateLastAccess()
+await setTimeout(600)
+updateLastAccess()