JavaScript中数组reduce()方法使用详情

arr.reduce(function(tmp, value, [index]) {
    // dosomething...
},  [startValue]) // 提供初始值通常更安全

let arr = [1, 2, 3, 4]
let sum = arr.reduce((x,y) => x + y) // 求和
let mul = arr.reduce((x,y) => x * y) // 求乘积
let average = arr.reduce((x, y) => (x + y) / arr.length) // 求平均值

const list = [
    { name:'张三', age:20 },
    { name:'李四', age:15 },
    { name:'王五', age:35}
]
const newList = list.filter(item => item.age > 18).map(item => {
    return Object.assign({}, item, {
        is: '成年'
    })
})

const newList = list.reduce((tmp, item) => {
    return item.age > 18 ? tmp.concat(Object.assign({}, item, { is: '成年' })) : tmp
}, [])
console.log(newList)

let person = [
  {id: 0, name: "小明"},
  {id: 1, name: "小张"},
  {id: 2, name: "小李"},
  {id: 3, name: "小孙"},
  {id: 1, name: "小周"},
  {id: 2, name: "小陈"},
]
 
let obj = {}
person = person.reduce((prev,item)=>{
  if(obj[item.id] === undefined) {
    prev.push(item)
    obj[item.id] = 1
  }
  return prev
}, [])
console.log(person)

const result = [
  {subject: '物理', marks: 41},
  {subject: '化学', marks: 59},
  {subject: '高等数学', marks: 36},
  {subject: '应用数学', marks: 90},
  {subject: '英语', marks: 64},
]
let initialValue = {
  pass: [],
  fail: []
}
const groupedResult = result.reduce((accumulator, current) => {
  (current.marks >= 50) ? accumulator.pass.push(current) : accumulator.fail.push(current)
  return accumulator
}, initialValue)
console.log(groupedResult)