数组reduce方法详解

数组reduce()

语法:[java]reduce(callFun,[initValue])
callFun: (previousValue,currentValue,currentIndex,arr)=>{}
initValue: previousValue初始值(可选参数)[/java]

reduce() 方法对数组中的每个元素按序执行一个由您提供的 callFun 函数，
每一次运行 callFun 会将先前的计算结果作为参数传入，最后将其结果汇总为单个返回值。

• 1. 遍历数组元素
• 2. 执行函数调用，将callFun函数计算结果作为参数传给previousValue
• 3. 最后将其结果汇总为单个返回值

应用:
– 求数组所有值的和
– 将二维数组转化为一维
– 计算数组或字符串中每个元素出现的次数
– 数组去重

[java]
/**
* 遍历元素
* 有初始值情况
*/
const test1 = () => {
let arr = [4, 6, 7, 9]
arr.reduce((previousValue, currentValue, currentIndex) => {
console.log(`previousValue:${previousValue}, currentValue:${currentValue}, currentIndex:${currentIndex}`)
return currentValue
}, 0)
}[/java]

[java]/**
* 遍历元素
* 无初始值情况
*/
const test2 = () => {
let arr = [4, 6, 7, 9]
arr.reduce((previousValue, currentValue, currentIndex) => {
console.log(`previousValue:${previousValue}, currentValue:${currentValue}, currentIndex:${currentIndex}`)
return currentValue
})
}[/java]

[java]/**
* 累加求和
*/
const test3 = () => {
let arr = [4, 6, 7, 9]
let sum = arr.reduce((previousValue, currentValue, currentIndex) => {
console.log(previousValue) //0,4,10,17
return previousValue + currentValue //4,10,17,26
}, 0)
console.log(sum)
}[/java]

[java]/**
* 累加求和
*/
const test4 = () => {
let arr = [
{ name: ‘jack’, score: 98 },
{ name: ‘rose’, score: 88 },
{ name: ‘tom’, score: 90 },
]
//计算总成绩
let total = arr.reduce((previousValue, currentValue) => currentValue.score + previousValue, 0)
console.log(total)
}[/java]

[java]/**
* 将二维数组转化为一维
*/
const test5 = () => {
const arr = [
[1, 2],
[3, 4],
[5, 6],
]
const newArr = arr.reduce((previousValue, currentValue) => […previousValue, …currentValue], [])
console.log(newArr)
}[/java]

[java]/**
* 计算数组中每个元素出现的次数
* 判断元素是否在对象中: 对象[元素]
* in语句: 元素 in 对象
*
*/
const test6 = () => {
const str = ‘helloworldjavascript’
const arr = str.split(”)
const obj = arr.reduce(
(previousValue, currentValue) =>{
// previousValue[currentValue] ? previousValue[currentValue]++ : (previousValue[currentValue] = 1)
currentValue in previousValue ? previousValue[currentValue]++ : (previousValue[currentValue] = 1)
return previousValue
},{})
console.log(obj)
}[/java]

[java]/**
* 数组去重
*/
const test7 = ()=>{
const arr = [1,2,3,1,2,4,5,3]
const newArr = arr.reduce((previousValue,currentValue)=>{
if(previousValue.indexOf(currentValue) == -1){
previousValue.push(currentValue)
}
return previousValue
},[])
console.log(newArr)
}

[/java]

[java]/**
* 检查数据类型
* 对象Object
* 数组Array
* 函数Function
*/
const test8 = () => {
// let callback = ()=>{} // [object Function]
// let callback = {} // [object Object]
// let callback = [] // [object Array]
// let callback = ” // [object String]
// let callback = 0 // [object Number]
let callback = null // [object Null]
let value = Object.prototype.toString.call(callback)
console.log(value)
}
[/java]
[java]/**
* 自定义reduce
*/
Array.prototype.myReduce = function (callback, init) {
//判断callback是否函数
if (!callback && Object.prototype.toString.call(callback) !== ‘[object Function]’) {
throw new TypeError(‘callback is not Function’)
}
//判断方法调用者是否数组
if (Object.prototype.toString.call(this) !== ‘[object Array]’) {
throw new TypeError(‘not Array’)
}
//数组不能为空
if (this.length === 0 && init === undefined) {
throw new TypeError(‘Reduce of empty arr with no inital value’)
}
let index = init === undefined ? 1 : 0 //索引号
let previousValue = init === undefined ? this[0] : init
const len = this.length

//reduce功能: 遍历数组元素,每次执行结果返回给previousValue,循环完返回previousValue
while (index < len) {
const currentValue = this[index]
previousValue = callback(previousValue, currentValue, index, this)
index++
}
return previousValue
}[/java]

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