Callback hell

It's a layout of many nested callbacks in which dependencies, errors and control flow are hard to follow.

// Outline of the problem
loadUser(id, (error, user) => {
  if (error) return handleError(error);
  loadOrders(user.id, (error, orders) => {
    if (error) return handleError(error);
    renderOrders(orders);
  });
});

Solution: I extract named functions and, where the API allows, use Promises and async/await. I keep error handling at the right level.

Pitfall: callbacks themselves aren't the problem. The problem is composition that's hard to maintain, and errors that get skipped.