1-withoutExtraction.js

function getRevenueOfGermanCustomers(userService, orderService) {
  return userService.getUsers()
    .filter(user => user.country === "DE")
    .flatmap(user => orderService.getOrdersByUser(user.id))
    .map(order => order.total)
    .reduce((sum, total) => sum + total)
}

2-withArrowsAtTop.js

function getRevenueOfGermanCustomers(userService, orderService) {
  // We have to define our implementation details logic before we use it, which leads to unnecessary noise.
  const isUserFromGermany = user => user.country === "DE";
  const getOrdersForUser = user => orderService.getOrdersByUser(user.id);
  const getOrderRevenue = order => order.total;
  const sum = (a, b) => a + b;
  
  return userService.getUsers()
    .filter(isUserFromGermany)
    .flatmap(getOrdersForUser)
    .map(getOrderRevenue)
    .reduce(sum)
}

3-withFunctionsAtBottom.js

function getRevenueOfGermanCustomers(userService, orderService) {
  return userService.getUsers()
    .filter(isUserFromGermany)
    .flatmap(getOrdersForUser)
    .map(getOrderRevenue)
    .reduce(sum)
  
  // This works: We have moved our implementation details out of the spotlight. However, this is verbose
  // and tools like Prettier will format each function into 3 lines instead of keeping them on a single line each.
  
  function isUserFromGermany (user) { return user.country === "DE"; }
  function getOrdersForUser (user) { return orderService.getOrdersByUser(user.id); }
  function getOrderRevenue (order) { return order.total; }
  function sum (a, b) { return a + b; }
}

4-withArrowsOutsideFunctionScope.js

function getRevenueOfGermanCustomers(userService, orderService) {
  return userService.getUsers()
    .filter(isUserFromGermany)
    .flatmap(user => getOrdersForUser(orderService, user))
    .map(getOrderRevenue)
    .reduce(sum)
}

// When defining the arrow functions out of scope, we can move them to the bottom.
// This allows us to move implementation details out of the spotlight,
// but forces us to define an additional parameter for getOrdersForUser and requires
// an additional wrapping arrow function in our declarative flow.
const isUserFromGermany = user => user.country === "DE";
const getOrdersForUser = (orderService, user) => orderService.getOrdersByUser(user.id);
const getOrderRevenue = order => order.total;
const sum = (a, b) => a + b;

5-withSingleExpressionFunctions.js

function getRevenueOfGermanCustomers(userService, orderService) {
  return userService.getUsers()
    .filter(isUserFromGermany)
    .flatmap(getOrdersForUser)
    .map(getOrderRevenue)
    .reduce(sum)
    
  function isUserFromGermany (user) = user.country === "DE";
  function getOrdersForUser (user) = orderService.getOrdersByUser(user.id);
  function getOrderRevenue (order) = order.total;
  function sum (a, b) = a + b;
}