[1,2,3,4].reduce((acc,item,index,arr)=>{},initValue)
acc:累加器,初始值未initValue
item:当前的数组项
index:索引
arr:完整数组
函数体内部返回结果为下一次循环中acc的值
如果使用变量接收最终结果
let a = [1,2,3,4].reduce((acc,item,index,arr)=>{},initValue), a为循环结束时acc的值。
部分使用场景：
1.数组求和
[1,2,3,4].reduce((acc,item)=>acc+item,0) //10
2.数组去重
let arr = [1, 2, 3, 1, 5, 8, 6, 1, 3, 2, 4, 5, 6, 1, 3, 5, 7, 8, 9, 5, 1, 2, 6, 6, 1]
let sortArr = arr.sort().reduce((acc,item)=>{
if(acc.length===0||acc[length-1]!==item){
acc.push(item)
}
return acc
},[]) //[1,2,3,4,5,6,7,8,9]
3.按属性分组
const groupBy = (arr, attr) => {
return arr.reduce((acc, item) => {
let attrValue = item[attr]
if (!(attrValue in acc)) {
acc[attrValue] = []
}
acc[attrValue].push(item)
return acc
}, {})
}
let arr = [
{ name: 'a', age: 20 },
{ name: 'a', age: 20 },
{ name: 'b', age: 21 }
]
console.log(groupBy(arr, 'age'))
/*======运行结果=======*/
/*==
{
20:[
{ name: 'a', age: 20 **},
{ name: 'a', age: 20 }]，
21:[
{ name: 'b', age: 21 }]
}
*/
4.功能型管道函数
const double = x => x + x
const triple = x => 3 * x
const four = x => 4 * x
const pipe = (...functions) => input => functions.reduce((acc, func) =>func(acc), input)
const one = pipe(double, triple)
console.log(one(6)) //36
/*
const pipe = (...functions) => input => functions.reduce((acc, func) =>func(acc), input)
等价于 function pipe(...functions){
return function(input){
return functions.reduce((acc,func)=>func(acc),input)
}
}
*/
