What is callback hell and how do you avoid it?

Deeply nested callbacks make code hard to read, debug, and maintain.

// ❌ Callback hell
getUser(id, (user) => {
  getPosts(user, (posts) => {
    getComments(posts[0], (comments) => { ... });
  });
});

// ✅ Async/await
const user = await getUser(id);
const posts = await getPosts(user);
const comments = await getComments(posts[0]);