functional example cart with reduce in js

functional_cart_test.js

user = {
  name: 'Joe',
  active: true,
  purchases: [],
  cart: []
}

const compose = (f, g) => { return (...args) => f(g(...args)) } // in reduce new acc/f will be a fn

// all functions are coiled up from last to first -
// last one gets n-ary arguments, the rest passes only 1 out -> 1 in
function purchaseItem(...fns) { return fns.reduce(compose) }

purchaseItem(
  emptyCart,
  buyItem,
  applyTaxToItems,
  addItemToCart
)(user, {name: 'macbook pro m1 64g', price: 5000})


function addItemToCart(user, item) {
  console.log('additemtocart')
  const cart2 = user.cart.concat(item)
  return Object.assign({}, user, {cart: cart2})
}

function applyTaxToItems(user) {
  console.log('applytax')
  const {cart} = user
  const up = cart.map( i => ({name: i.name, price: i.price*1.3}) )
  return Object.assign({}, user, {cart: up})
}

function buyItem(user) {
  console.log('buyitem')
  return Object.assign({}, user, {purchases: user.cart})
}

function emptyCart(user) {
  console.log('emptycart')
  return Object.assign({}, user, {cart: []})
}