Node.js Event Loop 완전 정복

console.log('1: 동기');

setTimeout(() => console.log('2: setTimeout 0'), 0);
setImmediate(() => console.log('3: setImmediate'));

Promise.resolve().then(() => console.log('4: microtask'));
process.nextTick(() => console.log('5: nextTick'));

console.log('6: 동기');

// 출력 순서:
// 1: 동기
// 6: 동기
// 5: nextTick        ← nextTick 큐 (microtask보다 우선)
// 4: microtask       ← Promise 마이크로태스크 큐
// 2: setTimeout 0    ← timers 단계
// 3: setImmediate    ← check 단계

// nextTick > Promise.then > setImmediate > setTimeout
process.nextTick(() => console.log('nextTick 1'));
process.nextTick(() => console.log('nextTick 2'));

Promise.resolve()
  .then(() => console.log('promise 1'))
  .then(() => console.log('promise 2'));

setImmediate(() => console.log('immediate'));

const fs = require('fs');

fs.readFile(__filename, () => {
  setTimeout(() => console.log('timeout'), 0);
  setImmediate(() => console.log('immediate'));
  // I/O 콜백 안에서는 setImmediate가 항상 setTimeout보다 먼저 실행됩니다.
});

// ❌ 절대 금지 - Event Loop 블로킹
function heavySync() {
  let sum = 0;
  for (let i = 0; i < 1e9; i++) sum += i;
  return sum;
}

// ✅ Worker Thread로 분리
const { Worker, isMainThread, parentPort } = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename);
  worker.on('message', (result) => console.log('결과:', result));
} else {
  let sum = 0;
  for (let i = 0; i < 1e9; i++) sum += i;
  parentPort.postMessage(sum);
}