ES6 reduce的实现

reduce 方法对累计器和数组中的每个元素（从左到右）应用一个函数，将其简化为单个值。
reducer 函数接收4个参数:

• 1. Accumulator (acc) (累计器)
• 2. Current Value (cur) (当前值)
• 3. Current Index (idx) (当前索引)
• 4. Source Array (src) (源数组)

有默认参数

const arr = [1, 2, 3]
let result = arr.reduce(function(val, item, index, origin) {
  return val + item
}, 0)

无默认参数

result = arr.reduce(function(val, item, index, origin) {
  return val + item
})

求平均值

result = arr.reduce(function(val, item, index, origin) {
  const res = val + item
  if (index === origin.length-1) {
    return res/origin.length
  } else {
    return res
  }
})
console.log(result)

实现reduce

Array.prototype.myRedece = function(reducer, initValue){
  for (let i = 0; i < this.length; i++) {
    initValue = reducer(initValue, this[i], i, this)
  }
  return initValue
}

const res = arr.myRedece(function(val, item){
  return val + item
}, 0)

console.log(res)

实现reduceRight

Array.prototype.myRedeceRight = function(reducer, initValue){
  for (let i = this.length -1 ; i >= 0; i--) {
    initValue = reducer(initValue, this[i], i, this)
  }
  return initValue
}

const res1 = arr.myRedece(function(val, item){
  return val + item
}, 0)

console.log(res1)