What is callback hell and how do you avoid it?

Booking a train ticket involves steps: log in, search trains, choose seat, pay, download ticket. If each step must start inside the previous step's "when done" function, the code moves to the right with every step and ends in a pile of closing brackets, sometimes called the pyramid of doom.

The pyramid:

getUser(id, (err, user) => {
  if (err) return handle(err);
  getOrders(user.id, (err, orders) => {
    if (err) return handle(err);
    getInvoice(orders[0], (err, invoice) => {
      if (err) return handle(err);
      send(invoice);
    });
  });
});

The same flow flattened with async/await:

async function sendFirstInvoice(id) {
  try {
    const user = await getUser(id);
    const orders = await getOrders(user.id);
    const invoice = await getInvoice(orders[0]);
    send(invoice);
  } catch (err) {
    handle(err);
  }
}

Ways out, from lightest to best:

• Give callbacks names and move them out to top level.
• Return promises and chain .then().
• Use async/await, with one try/catch covering the whole flow.
• Use util.promisify to convert old callback APIs.

• Thinking promises alone fix it if you still nest .then inside .then.
• Forgetting return inside a .then, which breaks the chain.