reduce is awesome

reduce.js

// 3 ways to reduce from an array
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce?v=a)

var orders = [
  { amount: 100 },
  { amount: 200 },
  { amount: 300 },
  { amount: 400 },
]

// es5 'for' iteration
var totalAmount = 0
for (var i = 0; i < orders.length; i++) {
  totalAmount += orders[i].amount
} 

// reduce es5
var totalAmount = orders.reduce(function(sum, order) {
  return sum + order.amount
}, 0)

// reduce es6
// easy to understand, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
let totalAmount = orders.reduce((sum, order) => sum + order.amount, 0)

// invoke the function
console.log(totalAmount) // => 1000