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Event Loop phases (timers, pending, poll, check, close). Microtasks vs macrotasks. process.nextTick vs queueMicrotask vs setImmediate. Callbacks, Promises, async/await. Promise.allSettled, Promise.any.

Node.js Core: Từ Cơ bản đến Nâng cao

Phần 1: Node.js Internals

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1. Event Loop Phases

   ┌───────────────────────────┐
┌─>│         timers            │  setTimeout, setInterval
│  └──────────┬────────────────┘
│  ┌──────────┴────────────────┐
│  │     pending callbacks     │  I/O callbacks deferred
│  └──────────┬────────────────┘
│  ┌──────────┴────────────────┐
│  │       idle, prepare       │  internal use
│  └──────────┬────────────────┘
│  ┌──────────┴────────────────┐
│  │         poll              │  I/O events (fs, net)
│  └──────────┬────────────────┘
│  ┌──────────┴────────────────┐
│  │         check             │  setImmediate
│  └──────────┬────────────────┘
│  ┌──────────┴────────────────┐
└──│      close callbacks      │  socket.on('close')
   └───────────────────────────┘

2. Microtasks vs Macrotasks

console.log('1: Script start')

setTimeout(() => console.log('2: setTimeout'), 0)       // Macrotask (timers)
setImmediate(() => console.log('3: setImmediate'))       // Macrotask (check)

Promise.resolve().then(() => console.log('4: Promise'))  // Microtask
queueMicrotask(() => console.log('5: queueMicrotask'))   // Microtask
process.nextTick(() => console.log('6: nextTick'))       // Microtask (ưu tiên cao nhất)

console.log('7: Script end')

// Output:
// 1: Script start
// 7: Script end
// 6: nextTick        ← nextTick luôn chạy trước
// 4: Promise
// 5: queueMicrotask
// 2: setTimeout      ← hoặc 3 trước (phụ thuộc event loop)
// 3: setImmediate

3. process.nextTick vs queueMicrotask

API	Queue	Thứ tự	Khi nào dùng
process.nextTick	nextTick queue	Cao nhất	Đảm bảo chạy ngay sau operation hiện tại
queueMicrotask	Microtask queue	Sau nextTick	Standards-compliant, web-compatible
Promise.then	Microtask queue	Sau nextTick	Async chaining
setTimeout(fn, 0)	Timers phase	Macrotask	Defer to next event loop iteration
setImmediate	Check phase	Macrotask	Execute sau I/O poll

4. Async Patterns Evolution

import { readFile } from 'node:fs'
import { readFile as readFileAsync } from 'node:fs/promises'

// 1. Callback (old style)
readFile('/etc/hosts', 'utf8', (err, data) => {
  if (err) throw err
  console.log(data)
})

// 2. Promise
readFileAsync('/etc/hosts', 'utf8')
  .then(data => console.log(data))
  .catch(err => console.error(err))

// 3. Async/Await (recommended)
async function main() {
  try {
    const data = await readFileAsync('/etc/hosts', 'utf8')
    console.log(data)
  } catch (err) {
    console.error(err)
  }
}

5. Promise Combinators

const tasks = [
  fetch('https://api.example.com/users'),
  fetch('https://api.example.com/posts'),
  fetch('https://api.example.com/comments'),
]

// Promise.all — fail nếu 1 reject
const [users, posts, comments] = await Promise.all(tasks)

// Promise.allSettled — không fail, trả status
const results = await Promise.allSettled(tasks)
results.forEach(r => {
  if (r.status === 'fulfilled') console.log(r.value)
  else console.error(r.reason)
})

// Promise.any — trả kết quả đầu tiên thành công
const fastest = await Promise.any(tasks)

// Promise.race — trả kết quả đầu tiên (fulfilled hoặc rejected)
const first = await Promise.race(tasks)

6. AbortController

const controller = new AbortController()
const { signal } = controller

// Timeout tự động cancel
setTimeout(() => controller.abort(), 5000)

try {
  const response = await fetch('https://api.example.com/data', { signal })
  const data = await response.json()
} catch (err) {
  if (err instanceof DOMException && err.name === 'AbortError') {
    console.log('Request was aborted')
  }
}