在此函数调用上获取未定义

Question

我认为写入数据库需要时间这一事实使得此函数未定义。我对吗？如果是这样，程序员如何在数据库调用的回调中避免使用百万个嵌套函数？

function createStripeCustomer(email) {
            stripe.customers.create({
                    email: email
                },
                function (err, customer) {
                    if (err) {
                        console.log('The stripe error is ', err);
                    }
                    console.log('The created customer id is ', customer.id); // This works

                    return customer.id;   // we know customer.id works because it shows up in console 
                }
            );
}

出于某种原因，这给出了undefined：

var getCustomerId = createStripeCustomer(req.body.email);
console.log ('This should say the id but it says undefined: ', getCustomerId);