Common use cases for JavaScript reduce built in function

js-reducer.js

//Summing an Array of Numeric Properties
const lineItems = [
  { description: 'Eggs (Dozen)', quantity: 1, price: 3, total: 3 },
  { description: 'Cheese', quantity: 0.5, price: 5, total: 2.5 },
  { description: 'Butter', quantity: 2, price: 6, total: 12 }
]

const sumReducer = (sum, val) => sum + val
lineItems.map(li => li.total).reduce(sumReducer, 0)


//Find the Maximum Value
const dates = [
  '2019/06/01',
  '2018/06/01',
  '2019/09/01', // This is the most recent date. Find it.
  '2018/09/01'
].map(v => new Date(v))

const maxDate = dates.reduce((max, d) => d > max ? d : max, dates[0])

//Grouping Values
const characters = [
  { name: 'Jean-Luc Picard', age: 59 },
  { name: 'Will Riker', age: 29 },
  { name: 'Deanna Troi', age: 29 }
]

const reducer = (map, val) => {
  if (map[val] == null) {
    map[val] = 1
  } else {
    ++map[val]
  }
  return map
}

characters.map(char => char.age).reduce(reducer, {})

//Promise chaining
const functions = [
  async () => 1,
  async () => 2,
  async () => 3,
]

// Chain the function calls in order, starting with an empty promise.
// In the end, `res` is equivalent to
// `Promise.resolve().then(fn1).then(fn2).then(fn3)`
const res = await functions.reduce((promise, fn) => promise.then(fn), Promise.resolve())
console.log(res) //3