Good code / bad code: bad code

badExample.js

function checkOutCart(session, cb) {                            // this is the code that executes on checkout
  getUserInfo(session.userId, function(err, user) {             // it'll get the user information
    if (err) { return cb(err); }                                // if it has any error, it will call back with it
    if (user.hasDiscountedProfile) {                            // if the user has a profile for discounts
      retrieveDiscount(user.profileId, function(err, discAmt) { // it will retrieve that discount
        if (err) { return cb(err); }                            // on error, it aborts the operation
        session.cart.addDiscount(discAmt);                      // adds the discount to the cart
        getPaymentScreenConfig(function(err, payScrCfg) {       // it retrieves the configuration from the payment screen
          if (err) { return cb(err); }                          // on error, it aborts
          cb(null, new PaymentScreen(payScrCfg, session.cart)); // creates a payment screen and returns it
        });                                                     // something finished... getPaymentsScreenConfig
      });                                                       // something finished... what was it? *looks* ah, getUserInfo. No! retrieveDiscount. I think.
    } else {                                                    // if not... if not what?
      if (err) { return cb(err); }                              // if error... from what? operation aborted.
      getPaymentScreenConfig(function(err, payScrCfg) {         // it gets the payment screen configuration... again?
        cb(new PaymentScreen(payScrCfg, session.cart));         // returns the payment screen
      });                                                       // finish get payment screen
    }                                                           // else section finished
  });                                                           // something else finished here
  trackActivity(session, ACTIONS.CHECKED_OUT);                  // tracks activity... is this in the right place?
}                                                               // everything finished... what was wrong with this in the first place?