数组的reduce是可以指定返回的值

reduce使用场景

数组和对象中的值相加

let total = [ 0, 1, 2, 3 ].reduce(
 ( previousValue, currentValue ) => previousValue + currentValue,
 0
)

//initialValue给定的初始值
let initialValue = 0
let sum = [{x: 1}, {x: 2}, {x: 3}].reduce(
   (previousValue, currentValue) => previousValue + currentValue.x
   , initialValue
)

console.log(sum) // logs 6

二维数组转换为一维数组

let flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  ( previousValue, currentValue ) => previousValue.concat(currentValue),
  []
)

计算数组中每个元素出现的次数

let names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice']

let countedNames = names.reduce(function (allNames, name) {
  if (name in allNames) {
    allNames[name]++
  }
  else {
    allNames[name] = 1
  }
  return allNames
}, {})
// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

按照属性对obj进行分类

let people = [  { name: 'Alice', age: 21 },  { name: 'Max', age: 20 },  { name: 'Jane', age: 20 }];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    let key = obj[property]
    if (!acc[key]) {
      acc[key] = []
    }
    acc[key].push(obj)
    return acc
  }, {})
}

let groupedPeople = groupBy(people, 'age')
// groupedPeople is:
// {
//   20: [
//     { name: 'Max', age: 20 },
//     { name: 'Jane', age: 20 }
//   ],
//   21: [{ name: 'Alice', age: 21 }]
// }

利用扩展运算符和reduce扩展对象中的数组

// friends - an array of objects
// where object field "books" is a list of favorite books
let friends = [{  name: 'Anna',  books: ['Bible', 'Harry Potter'],
  age: 21
}, {
  name: 'Bob',
  books: ['War and peace', 'Romeo and Juliet'],
  age: 26
}, {
  name: 'Alice',
  books: ['The Lord of the Rings', 'The Shining'],
  age: 18
}]
let allbooks = friends.reduce(function(previousValue, currentValue) {
  return [...previousValue, ...currentValue.books]
}, ['Alphabet'])
 allbooks = [ 'Alphabet', 'Bible', 'Harry Potter', 'War and peace',   'Romeo and Juliet', 'The Lord of the Rings',  'The Shining' ]

实现管道函数

// Building-blocks to use for composition
const double = x => x + x
const triple = x => 3 * x
const quadruple = x => 4 * x

// Function composition enabling pipe functionality
const pipe = (...functions) => initialValue => functions.reduce(
    (acc, fn) => fn(acc),
    initialValue
)

// Composed functions for multiplication of specific values
const multiply6 = pipe(double, triple)
const multiply9 = pipe(triple, triple)
const multiply16 = pipe(quadruple, quadruple)
const multiply24 = pipe(double, triple, quadruple)

// Usage
multiply6(6)   // 36
multiply9(9)   // 81
multiply16(16) // 256
multiply24(10) // 240

实现map

if (!Array.prototype.mapUsingReduce) {
  Array.prototype.mapUsingReduce = function(callback, initialValue) {
    return this.reduce(function(mappedArray, currentValue, currentIndex, array) {
      mappedArray[currentIndex] = callback.call(initialValue, currentValue, currentIndex, array)
      return mappedArray
    }, [])
  }
}

[1, 2, , 3].mapUsingReduce(
  (currentValue, currentIndex, array) => currentValue + currentIndex + array.length
) // [5, 7, , 10]

按顺序执行Promise

function runPromiseInSequence(arr, input) {
  return arr.reduce(
    (promiseChain, currentFunction) => promiseChain.then(currentFunction),
    Promise.resolve(input)
  )
}

// promise function 1
function p1(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 5)
  })
}

// promise function 2
function p2(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 2)
  })
}

// function 3  - will be wrapped in a resolved promise by .then()
function f3(a) {
 return a * 3
}

// promise function 4
function p4(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 4)
  })
}

const promiseArr = [p1, p2, f3, p4]
runPromiseInSequence(promiseArr, 10)
  .then(console.log)   // 1200