Callback hell là gì? Cách giải quyết?

Dùng async/await để giải quyết callback hell — code đọc như synchronous, try/catch natural; Promise.all() cho parallel thay vì await trong for loop.

• Callback hell (pyramid of doom) — ví dụ thực: fs.readFile('a.txt', (err, data) => { if(err) throw err; db.query(data, (err, rows) => { if(err) throw err; sendEmail(rows[0], (err) => { if(err) throw err; console.log('done') }) }) }) — 3 cấp lồng nhau, error handling lặp lại, logic khó theo dõi.
• Giải pháp Promise chaining: readFile('a.txt').then(data => db.query(data)).then(rows => sendEmail(rows[0])).catch(err => console.error(err)) — flat hơn nhưng còn awkward với multi-value.
• Giải pháp async/await: try { const data = await readFile('a.txt'); const rows = await db.query(data); await sendEmail(rows[0]); } catch(err) { console.error(err); } — đọc như synchronous code, try/catch natural.
• Error propagation khác nhau: callback phải check err mỗi cấp thủ công, một missed check = silent failure; Promise/async throw tự động bubble up đến .catch/try-catch.
• Lưu ý async/await: await trong vòng lặp for...of sẽ chạy sequential — dùng Promise.all(items.map(item => process(item))) để parallel.

Use async/await to solve callback hell — code reads like synchronous, natural try/catch; use Promise.all() for parallel operations instead of await in a for loop.

• Callback hell (pyramid of doom) — real example: fs.readFile('a.txt', (err, data) => { if(err) throw err; db.query(data, (err, rows) => { if(err) throw err; sendEmail(rows[0], (err) => { if(err) throw err; console.log('done') }) }) }) — 3 levels of nesting, repeated error handling, logic hard to follow.
• Promise chaining solution: readFile('a.txt').then(data => db.query(data)).then(rows => sendEmail(rows[0])).catch(err => console.error(err)) — flatter but still awkward with multi-value passing. async/await solution: try { const data = await readFile('a.txt'); const rows = await db.query(data); await sendEmail(rows[0]); } catch(err) { console.error(err); } — reads like synchronous code, natural try/catch.
• Error propagation differences: callbacks must manually check err at every level — one missed check means silent failure; Promise/async throws automatically bubble up to .catch/try-catch.

Pitfall: await inside a for...of loop runs sequentially — use Promise.all(items.map(item => process(item))) for parallel execution.