currying.js

const saveCard = ( {body:{source, amount}, user:{email, _id}}, res ) =>
                  getBundle(amount) ?
                    customers.create({email, source, metadata: {_id:_id.toString()}})
                    .then(writeCustomerId )
                    .then(createCharge(amount,_id))
                    .then(grantCredits)
                    .then(payload=>res.json(payload) )
                    .catch(e=> res.status(400).send(e))
                 : res.status(400).send('Product not found')


const bundles = [
   {category: 'Beginner', price: 1000, credits:10},
   {category: 'Reasonable', price: 2000, credits:20},
   {category: 'Beginner', price: 4000, credits:100},
 ]

const getBundle = amt => bundles.find( ({price}) => price === +amt )

const writeCustomerId = ({id, metadata:{_id}}) =>
                         User.findOneAndUpdate({_id}, {customer:id}, {upsert:1, safe: 1, new: 1})

const createCharge = (amount,_id) => ({customer}) =>
                     charges.create({amount, customer, currency:'usd', metadata:{_id:_id.toString()}})

const grantCredits = charge => User.findOneAndUpdate(
                       {_id: charge.metadata._id},
                       {credits: getBundle(charge.amount).credits},
                       {upsert:1, safe: 1,new:1})
                     .then(mergePayloads(charge))

const mergePayloads = charge => model => Object.assign( charge, model.toObject())

1. check product
2. create customer
3. write customer id in the local db
4. create charge
5. grant credits to user
6. merge stripe and db payload